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Installing a Genesis Carburetor

Written by Randy Fox (randysgym)   
Sunday, 06 September 2009


yamaha road star roadstar, genesis carburetor

Some Road Star owners love the stock, 40mm Mikuni, CV carburetor. Others favor their aftermarket Mikuni HSR 42mm or 45mm, while still others are sold out to the S&S Super-G, using an aftermarket manifold.

However, it is hard to argue with the incredible performance of the Genesis Big Air, 48mm carb from P.S.I. It is an enormous improvement, as others have also written. See Appendix-A for more information and my experiences with this amazing carburetor.

This is not an inexpensive project, compared to some others you could do. However, in terms of cost per power improvement, it begins to look very reasonable. Prepare for sticker shock in the neighborhood of $1,500 for the privilege of escalating yourself into the realm of fuel injection equivalent power.

Now, with my Genesis carb, I can more than keep up with the fuel injected models, without unduly sacrificing gas mileage. Frankly, when you compare the performance and price of fuel injection to the Genesis carb, it doesn't look so pricey anymore.

Here’s what I did to install my Genesis Series IV carb on my Road Star.

Note: This article applies to model years 1999-2007. If your Road Star is a model year 2008 or later, your bike is fuel injected, and has no carburetor.

Important Note: This article assumes you have some mechanical familiarity with the carburetor of your bike. If you question your skill in working with the intake systems of motorcycles you should not attempt this project (in my opinion).

First, a Comparison

Before diving into the tools and materials needed for this project, let me show what this carburetor looks like, compared to a few others, for the Road Star.

The machined, billet aluminum, powdercoated, Genesis carb body is a thing of beauty. See comparison photos below, showing from left to right: Genesis Series IV 48mm, Mikuni HSR-42mm flat-side, stock Mikuni CV 38mm. Note in the third photo that the stock Road Star carb is a rotating butterfly type air regulator, while the other two are lifting-slide types. Note also that the Genesis has no need for an accelerator pump, and no protrusions within the throat except the computer designed venturi steps. Note especially, the float bowl sizes, and of course the throat opening sizes.

yamaha road star roadstar, genesis carburetor, hsr 42, compare
Left sides: Genesis, Mikuni HSR-42, stock Road Star Mikuni CV

yamaha road star roadstar, genesis carburetor, hsr 42, compare
Right sides: Genesis, Mikuni HSR-42, stock Road Star Mikuni CV

yamaha road star roadstar, genesis carburetor, hsr 42, compare
Back sides: Genesis, Mikuni HSR-42, stock Road Star Mikuni CV

Tools Needed

Small set of hex (allen) wrenches

Small set of metric sockets & wrench, for removing the upper motor mount and fuel pump brackets

3/8" Socket wrench with about a 1-foot extension (or other suitable tool), for securing the Genesis carb to its S&S Super-G type manifold

Screwdriver and/or needle nose pliers, for removing/securing hose clamps

Hose cutter (e.g. utility knife), for cutting fuel line

Phillips ('+') screwdriver, optional, for throttle cable installation

3/32" hex (small allen) wrench, for installing the P.S.I. end-barrel, for the throttle cable -- at the carb end

Wire cutters, to cut off the excess of the inner throttle cable

8mm or 10mm Open-end wrench (depending on the manufacturer of your throttle cable/s, for tightening cable adjuster nuts. Tip: A small adjustable wrench or pliers could be used instead.

File, belt or drum sander, or grinder, for possible fine-fitting the manifold to the engine. Not commonly needed... but just in case

Soldering gun or torch, optional, for sealing cut cable ends from fraying

Anti-Seize compound, optional, for reducing cross-threading potential

Parts/Materials Needed

Genesis, 48mm, Big Air, Series IV carburetor kit. It comes with the carburetor, a manifold o-ring, a barrel end-fitting (for the throttle-cable inner cable) with tiny set-screw, a Road Star specific metering needle (EK 149-126-097C), and fuel nozzle (151.4), as well as general installation and tuning instructions.

Nemesis Super-G intake manifold for the Road Star, available through Nemesis Racing of Australia, distributed in North America by DK Powdercoating (email: powdercoatka@aol.com) in Southwest Missouri, USA. It comes with o-rings for mounting onto the engine. See photo below.

yamaha road star roadstar, genesis carburetor install nemesis manifold

3/8" (or thicker) phenolic or aluminum spacer for the carb-to-manifold connection -- S&S Super-G type. Tip: I used 7/8" (a 3/8" in tandem with a 1/2" phenolic, shown brown in the photo below) from my local Harley dealer. This spaced my breather (air cleaner) out to its original position. Others swear by the 1" aluminum spacer from Vulcan Engineering. Tip: Basically, the longer the spacer, the greater the performance of the carb -- similar to the 'ram' effect of the high-rise manifolds of the 1970s era American muscle cars. Note: The spacer also offers important insulation from the engines's heat.

yamaha road star roadstar, genesis carburetor install

Breather Box (air cleaner). Most any S&S Super-G compatible breather will do just fine. There are many dozens of styles and brands to choose from... Or you can modify your existing breather's backing plate, like I did.

Breather gasket, S&S Super-G compatible, (assuming none is provided with the breather you purchase). Or use a gasket sheet, and cut your own gasket using the template shown later in this article.

Note: The Genesis, Series IV carb comes with a self contained enricher (choke), which some owners, including me, feel is difficult to reach easily. If you want to keep your stock, Mikuni choke cable, you will need the following**:

  • The Mikuni choke adapter kit for the Genesis carb, from P.S.I.
  • Your stock, Mikuni choke cable, choke plunger, and choke spring
  • A lot of time, steady hands, and special tools to modify your plunger and cable

**Important Note: As of the date of this article, several Road Star, Genesis carb owners (including me) could not get the P.S.I. Mikuni choke adapter to operate reliably without modifying the cable (shortening), the spring (shortening), and the plunger, significantly.

Fuel filter, if you will be replacing your current one. Tip: The stock filter is too big to fit well between the cylinders. I ordered one from my auto parts store (O'Rielly’s). Here’s the Part#: Wix, 33027. Note: This filter has 5/16” male ends.

Fuel hose, 24" or more. 5/16” or 3/8” inside diameter. Note: The fuel tank’s stock petcock is 5/16”, but the Genesis’ fuel fitting is 3/8”. I used 5/16”, sprayed the carb’s fitting with dry silicone, and pushed/twisted hard. If you go with 3/8” hose, you will need to be careful to clamp the petcock adequately so that no gas can leak there. The larger diameter will increase pressure into the carb -- a good thing -- but either size seems to work equally well. Available at local auto parts stores. Tip: Be very sure you get hose made specifically for fuel.

Hose clamps (small) – (qty: 4, enough of them to clamp the petcock, the carb, and both ends of the fuel filter). Available at local hardware and auto parts stores.

3/8" x 1.25"L SAE (non-metric), standard thread bolts, (qty: 2), for mounting the carb. Available at local hardware and auto parts stores.

3/8" flat washers with small outside diameter, (qty: 2), for mounting the carb. Available at local hardware and auto parts stores.

S&S throttle cable end adapter. It's the cable casing end-fitting for the carb end. It must have a sharp bend (low profile), 90° angle fitting, otherwise the cable won't likely clear the underside of the fuel-tank. Important Tip: Also, be sure the S&S fitting is the right diameter for the hole in the top of your carb. At first, I got a short, straight (non-bent) fitting from Barnett. It was the right diameter, and fit the hole perfectly. However, I needed the 90° angle fitting because the fuel tank pushed against the cable and strained it, so I got a new fitting from a friend. It turned out to be 0.5mm larger diameter, and wouldn't fit into the hole. I ended up reeming the hole. P.S.I. says that a small number of fitting manufacturers are making S&S type fittings that are slightly over-size. If you reem the hole in carb to fit your fitting, note that there are two rubber o-rings in the hole that must be removed prior to hole reeming. Plus, be sure to reem the o-rings slightly, too. The diameter of the fitting that fit properly for my carb (before I reemed it) was approximately 5.3mm. The elbow fitting from my friend measured 5.8mm -- 0.5mm larger.

New throttle pull cable, optional. If you do replace your throttle cable, just order it with the S&S type end adapter.

Clear, plastic fuel line, optional but recommended. For Pilot-Air vent. See the end of the Fitting the Fuel Hose section following, for details. I got mine from Home Depot. It is normally for gas powered weed wackers, fuel hose. Brand: Arnold, Part#: GL-0232, Type: Fuel Line for most small 2-cycle engines, 3/32" ID (2.4mm), 3/16" OD (4.8mm).

Removing the Old Carb, Manifold, and Throttle Cable

This section describes how to remove a stock, Road Star carburetor. If you have a different carb, your removal process will differ somewhat.

Remove the gas tank (Refer to the Tank Removal article if needed).

Remove the upper motor mount from between the cylinders on the left side of the bike (2 bolts to the frame, 2 bolts to the heads). Note: If you still have a fuel pump and/or choke cable attached to the upper motor mount, the pump must be removed first, and the choke cable must be detached from its mounting bracket, as well. Refer to the Service Manual. See photo below.

yamaha road star roadstar, genesis carburetor install

Important Note: You should not use a fuel pump with the Genesis carb.

If you have a fuel pump and/or choke cable that have been relocated from their stock locations, remove the fuel pump and detach the choke cable end. Also, remove the fuel filter and fuel hoses.

Remove your breather (air cleaner) from the carb and from the cylinder heads. Also detach the idle adjuster. Refer to the Service Manual for details.

Next, detach the throttle cable/s from the carburetor. To do this, you must loosen the cable adjustment nuts (the ones along the cable, near the carb) to a large degree, then work the cable-casing end-fitting free from its bracket, then work the inner-cable end-barrel free from its pulley on the carb.

Next, disconnect the TPS. To do this, follow the wire harness from the carb to the left side of the frame. There is a TPS wiring connector along the frame at this point; disconnect it. See photo below. Tip: Do not unscrew the TPS from the carb.

yamaha road star roadstar, genesis carburetor install TPS

Note: There is no known way to connect a TPS to the Genesis carb. If you wish to keep the Road Star's TPS function, you will need to install a standalone driver. See the Fabricating a TPS Driver article for details.

Now unplug the carb heater wire from the carb. Be a little careful as you do this, as its attachment is a bit delicate. See photo below.

yamaha road star roadstar, genesis carburetor install

Finally, you’re ready to remove the carb from the bike. Loosen the big hose clamps from the manifold connection at back-side of the carb. Then wiggle the carb loose from the manifold.

Next, remove the intake manifold from between the cylinders. It is held in place with four bolts. Some aftermarket manifolds must be removed by sliding them toward the right side of the bike, rather than up and out. Coax it; wiggle it; do not force it.

Once the manifold has been removed, be very careful not to drop anything into the cylinders’ intake ports. If you do drop something into a cylinder, and you can't reach in to successfully retrieve it, the cylinder head may have to be removed (a much bigger project) BEFORE the bike can be started, or you risk complete engine failure (VERY expensive). Tip: Stuff a clean latex glove or rag loosely into each intake port of the engine to prevent accidentally dropping something into the engine. Just be sure you remove them before installing your new manifold.

Store the intake system, including the old manifold, carb and fuel pump assemblies. However, be sure to drain the gasoline from all parts, and let them sit out for several days before storing indoors. Otherwise they may stink and create a fire and/or health hazard.

Removing the Old Throttle Cable

You can skip this section if you will not be replacing your throttle cable/s.

Depending on the age and condition of your throttle cable/s, you might want to replace it/them. If you still have a throttle return cable, you may also want to consider discarding it.

Opinion: I think the throttle return cable is an outdated feature, still legally required of manufacturers, that adds no valuable functionality or safety. It made sense back in the days when carbs were made differently, return springs were prone to breaking, and cabling wasn't nearly so durable yet flexible. I eliminated my throttle return cable soon after purchasing my Roadie in 2003, and have enjoyed smoother throttle control, less handlebar clutter, and one less thing to go wrong, ever since. In fact, while the Genesis carb does accommodate a throttle return cable, it performs no mechanical function.

If you will be replacing your old throttle cable/s, you must remove it/them. To do this, you must remove the right hand-grip assembly. But, to do that, you must first remove the front brake handlebar assembly, in order to gain access to the lower screw that secures the throttle grip assembly.

Once you have removed the two hex (allen) bolts that clamp the brake assembly to the handlebar, you will need to temporarily tape, or tie the brake assembly to the handlebars (or some other handy, high place). Be careful not to stress the hydraulic hose (or scratch any chrome).

The right-hand control box and throttle control assembly is held together by two phillips ('+') screws. Be very careful removing -- and later installing -- these, as the screw-heads are easily stripped. Also, the lower screw is hidden behind a gray wiring cable. You can work the cable free of its entry guide-channel; just be careful not to stress the cable.

Once the screws are removed, the two halves of the control box may easily come apart, so be ready. Also, internal wiring for the starter and kill switches will get exposed, so a bit of care is required. Tip: The internal wiring is held in place with ties and clamps, so don't worry about a wiring mess. Just be careful not to stress the wiring (or pinch it upon reassembly).

Removing the throttle cable is easy from this point. Refer to the Service Manual for details. Just note that the phillips ('+') screw that clamps the throttle cable end-fitting to the control box is easily stripped, so be careful.

Put your new cable/s on using these instructions in reverse, although you may wish to wait until the Genesis carb in installed, as described later in this document.

Installing the New Intake Manifold

To install the Nemesis, Super-G type intake manifold, first check the fit without any rubber o-rings. Slide the manifold in from the bike's right side, little by little. Wiggle, but do not force. Too much pressure and you could damage the manifold or the heads. They are aluminum, and easily dented, scuffed, and scratched. Be careful of the mating surfaces.

If the manifold won't go in without force, try the following: File, sand, or grind the manifold along the top (upper) edge of the 'ears' of the mounting flanges -- especially for the flange associated with the front head. Every engine spacing is a little different; if filing is needed, take a little off at a time, and trial fit as you go. In any case, be very careful not to take too much off. See photo below.

yamaha road star roadstar, genesis carburetor install nemesis manifold

Important Tip: Be sure to leave a minimum of 1/16" between the manifold's o-ring groove and its upper edge.

Once you have a good fit, coat both manifold o-rings with some silicone grease, and insert them fully into the groves of the manifold. Then smear a thin coat of the silicone grease onto the mounting surfaces of the heads.

Next, carefully slide and wiggle the manifold into position. Be very sure the o-rings stay in position as you line up the manifold bolt-holes with the corresponding holes in the heads. Double check that the o-rings are seated in their grooves by examining around the manifold, and looking down the manifold's throat with a handheld light. If necessary, file a little more from the 'ears' to get the right fit. Take your time, and get this step right; it is crucial.

Then begin installing the four manifold bolts (with washers). Special care must be given that you don't cross thread a bolt. Wiggle and coax the manifold so that you can start each bolt using your fingers. If the fit seems off enough to disallow this, check the o-ring installation more thoroughly. Chances are, at least one o-ring may have jumped its groove, or you may just have to file a bit more off the manifold 'ears'. There are many close fitting surfaces coming together here. Look closely to see what the hang-up is. Don't just assume more grinding/filing will result in a looser fit.

After starting one bolt, go on to the next. Do not tighten any bolt down beyond the first couple of threads before beginning the next bolt. Oh, and feel free to wiggle/rock the manifold a bit to help each bolt get started more easily.

Tip: I like to use a little anti-seize compound on the lower third of the threads for most nuts, bolts, and screws around the engine. This is required for stainless steel fasteners, but I use it for others as well -- anywhere that aluminum, high heat, and steel come together. The anti-seize compound keeps the aluminum from sticking too hard to the steel fastener, preventing galling (thread tearing). It also helps get the first threads meshing more easily -- reducing cross-thread potential.

Once you've got all four mounting bolts started, you can begin tightening them down, but first recheck that the o-rings are properly seated; really look closely. Once you're satisfied with the o-rings, tighten the manifold bolts little by little in a criss-cross pattern. Use 7.2 ft-lbs (86.4 in-lbs) of torque -- snug, but not too tight.

At this point, reinstall the upper motor-mount bracket. Use 35 ft-lbs of torque. In other words, the bolts must be tight, but be careful not to strip or over-tighten.

Fitting the Fuel Hose

yamaha road star roadstar, genesis carburetor install

Next, decide on a path for your fuel hose. Whatever pathway you choose, use the following criteria:

  • Keep the general direction of the hose moving downward as it runs from the tank toward the carb, as much as possible.
  • Make room for a fuel filter somewhere along the line. Tip: I also wrapped a piece of old, rubber inner-tube around the filter, as thermal insulation.

To begin fitting the fuel hose, temporarily mount the fuel tank. Tip: If you'd rather not go to that trouble, just know that the tank's petcock sits 3" directly above the chrome, external oil tube connection on the front cylinder (left side of the bike, of course). Then, to install your fuel line the same way I did mine, do the following:

  • Starting at the tank petcock (or measurement point 3" above the front cylinder's left-side external oil line), guide the hose back, just under the top engine mount, between the cylinders and below the manifold.
  • Select a location for the fuel filter along this path, then mark the hose at that location. Remove the hose and use a hose cutter or sharp knife to squarely cut the hose at the mark-point.

Next, put the measured hose piece onto the upstream end of the filter. Then push the remaining hose piece onto the other end of the filter.

Install and tighten a small hose clamp onto each of the two hose pieces, at the filter. Then slip another clamp onto the tank petcock end of the hose. Tip: Check one more time that the filter is oriented in the right direction.

Now, if your tank is on, install the hose onto the petcock, and tighten the clamp. Then guide the hose and filter between the cylinders, as before. Do not further fit the hose until the carb is installed.

OK, now one more thing before mounting the carb. I'd suggest, as others have also recommended, that you make a simple modification to your new Genesis carb: Replace the tiny, short, stub of a breather-hose for the Pilot-Air vent (see the Parts/Materials Needed section on page 2 of this article for details on this). It sits right below the carb's intake throat (at least on my carb). Since it is so short, and has no filter associated with it, I feel that it may be prone to two issues:

  • Since the end of the hose points directly into the oncoming wind, when the bike is under way, it may make the pilot mixture difficult to tune. The faster the bike goes, the more air is ramed into the vent, and the leaner the mixture becomes. Although the effect may be small, the fix is simple, and at these prices, I wanted to get it right, and then forget about it.
  • It might intake dust and dirt.

The solution, for me, was to replace the original tube with a much longer one, up and over the carb's intake throat, and down the 'down-wind' side of the carb. This way, I feel that air can be asperated as needed (without any ram effect), and there is significant protection from inhaling debris. See photo below:

yamaha road star roadstar, genesis carburetor install pilot air

Mounting the Carb

The first step in mounting the Genesis carb is to remove the two screws on top of the carb, being careful of spring pressure from the large throttle return slide-spring inside. Tip: Use one hand to hold the top-plate of the carb down while you take out the top screws. This will keep the slide-spring from acting like a jack-in-the-box, and prevent any screw threads from getting stripped.

Gently pull the top-plate, the spring, the slide (including the metering needle) from the carb. The slide and needle are especially delicate. Even though they are made of tough, hardened materials, they are precision machined, and the very heart of the carbs awesome performance. Set them aside while you finish installing the carburetor body.

Next, insert the two 3/8"x 1.25"L mounting bolts, with washers, through the back side of the carb-mounting-holes of the manifold. Then slip the manifold spacer/s -- with o-ring/s installed -- onto the bolts. Be sure to orient the spacer/s with the o-ring/s facing toward the manifold.

Tip: Use a bit of silicone grease on the o-ring/s to help keep it/them in place during final positioning. This may not be necessary with some phenolic spacers that have built-in o-rings.
Tip: Before inserting the mounting bolts into the backside of the manifold, I used a little anti-seize compound on the end threads to reduce cross-thread potential and later difficulty in bolt removal.

Now smear some silicone grease on the supplied carb/manifold o-ring. Then fit it into the large o-ring groove at the back of the carb.

Next, carefully hold the carb up to the mounting bolts, and start screwing in the bolts using your fingers from around the sides of the carb, or in from the left side of the bike, using a socket with long extension. Tip: You will need to wiggle and adjust the carb as you do this, to be certain that you do not cross thread or bind-up the bolts.

Once both bolts are started, check that the o-rings of the carb and spacer/s are still in place; this is important. Then tighten the two mounting bolts to pull the carb up against the spacer/s and manifold. Torque the bolts to about 15-25 ft-lbs (fairly tight, but not so tight as to strip an aluminum thread in the carb).

Connecting the Fuel Hose

Next, guide the lower fuel hose segment to a good point along side the carb fuel inlet fitting, on the carb's left side. Tip: The fuel inlet fitting is movable so it can be better positioned. However, it has limits.

As you hold the hose along side the inlet fitting, mark the cut point on the hose. Then use a hose cutter or sharp knife to cut it at the mark-point. If you have too little space to make the cut, remove the hose, make the cut, and reinstall the hose -- or get a sharper knife. Tip: Better to cut a bit long, than too short.

Slip on a small hose clamp, work the hose fully onto the carb fuel inlet fitting, position the clamp, and then tighten it.

yamaha road star roadstar, genesis carburetor install

Connecting the Throttle Cable/s

Once the carb body is installed, ensure the throttle cable is connected to the hand grip and fed through the guide at the front of the fork-tree and back through the cable guide near the frame neck and back to the carb area. If you will be using a throttle return cable (not necessary), do the same check for this cable also.

The instructions below apply to the throttle PULL cable, not the throttle RETURN cable, except where otherwise noted.

First, loosen the cable length adjuster (at the end/s of the cable) as much as possible, allowing the most inner cable to be exposed.

Now, since the Genesis carb comes with its own cable end-barrel, you must cut the stock end-barrel off the inner cable. That's right; it's going to feel like you're destroying your throttle cable! To do this, pull the inner cable while pushing on the outer cable-casing, to be certain the inner portion is pulled out as far as it will go.

Next, measure a distance on the inner cable, 3" from the end of the outer cable-casing. Mark it. Then using sharp wire cutters, cut the inner cable off at your marked spot. See photo below.

yamaha road star roadstar, genesis carburetor cable

Next, insert the freshly cut inner cable end through the brass fitting of the carb's top plate, nearest its center. Then press the outer cable end-fitting into the carb's brass fitting. The fit should be snug; in fact, it might be very snug. Be sure the cable's end-fitting fully seats into the top plate's brass fitting.

OK, now install the inner cable's end-barrel. To do this, pull the inner cable while holding onto the top plate to be certain it is pulled through as far as it will go without excessive force. Then measure and mark a point on the cable, 2 1/4" (57mm) from the underside of the top plate. See photo below.

yamaha road star roadstar, genesis carburetor cable

Next, be sure the set screw on the end of the P.S.I. supplied cable end-barrel is loose enough so that you can slide it onto the cable. Then slide the barrel onto the cable and over the mark you made. Moderately tighten the barrel's tiny set screw using a 3/32" Allen wrench.


Now would be a good time to solder the end of the inner cable, if you want to do this. Doing so will prevent the cable end from fraying -- a good thing. Just be sure you don't leave so much solder on the cable that it becomes difficult to slide the Genesis end-barrel over it. Warning: A growing number of cable manufacturers use a special lubricant impregnated into the cable. The heat from soldering could destroy lubricant at the cable end, and perhaps also catch on fire. Use minimal heat, and work quickly.

OK, now for a cable test fit...

Install the slide spring over the end-barrel, up the cable, and onto the brass fitting on the underside of the top plate. Be sure you orient the spring so that its 'tang' end is not the end toward the top plate. See photo below.

yamaha road star roadstar, genesis carburetor slide spring

Next, compress the slide spring against the top plate. Compress it enough to expose the cable's end barrel plus about an inch of cable. This may not be as easy as it sounds; the spring is unwieldy.

With the compressed slide spring and top plate in one hand, hold the carb's slide in the other hand (or get someone to help you), and orient the slide so as to insert the cable's end-barrel -- with its set screw end away from you -- into the slide's 'barrel retainer hole' (the circle). Then lay the cable into the cable channel, and compress the spring a little further, as needed, to get it lined up with the 'slide spring guide hole'. As you do this, rotate the spring to orient its tail-end 'tang' in line with the cable channel. Finally, release the spring into the 'slide spring guide hole', while coaxing the spring's 'tang' over the end of the cable end-barrel. Whew! See photo below.

yamaha road star roadstar, genesis carburetor

Now insert the slide in its track in the carb, being careful the metering needle enters its nozzle hole without resistance. Then push the top plate down into place, and screw it fully down. No need to torque it down, yet though.

Check the slide operation, and the free-play (slack) in the throttle cable. Play with the cable adjustment nuts to remove any slack from throttle operation and achieve proper slide control. The slide should drop to its idle-stop when you release the handle grip, and the slide should rise to the roof of the carb throat when you twist the handle grip fully. There should be precious little free-play at the close-throttle (idle) position.

If the cable adjustment nuts are not able to achieve good throttle operation, you will have to adjust the end-barrel up or down as needed. If the slide does not drop all the way, the barrel must be lowered down the cable. If the slide does not raise all the way, the barrel must be raised. If the barrel has been raised or lowered, you must go through the test fitting process again.

Once you are happy with the throttle/slide operation, unscrew the top plate, and remove the slide. Then remove the spring and the cable from the slide.

Next, note/mark the final position of the cable end-barrel, and unscrew and remove the end-barrel's set screw completely. Dab a small drop of RED thread-lock liquid on the screw threads. Finally, position the barrel to its final place on the cable again, and screw in the set screw very firmly.

Now reassemble the cable and spring into the slide, and insert the slide in its track in the carb once more (being careful to guide the delicate metering needle into its nozzle). Then screw down the top plate. Torque the two top screws to 6 ft-lbs (60 in-lbs; not too tight).

Finally, if you are keeping your throttle return cable, connect it to the carb's top plate. It goes into the brass fitting that sits to the right. Note: I'm not sure how it gets secured, as I did not use it. Refer to the Genesis installation instructions from P.S.I, or call them directly.

Installing the Breather (Air Cleaner)

At this point, you have two, practical choices:

  • Modify your existing breather to accommodate the Genesis intake throat.
  • Purchase an S&S Super-G compatible style breather. There are dozens, if not hundreds, to choose from.

I chose to modify mine. I like the shape and look of it, plus I saw nothing I liked better. I also thought it offered good air flow... Not to mention that it was one less expense.

Modifying my breather involved modifying its backing plate. I have read that others have been able to simply cut a 2.2" (2 1/5", or 56mm) hole, centered around the existing mounting flange. Then they just drilled three holes for the mounting screws.

I decided to cut out a big square hole around my backing plate's mounting flange. That allowed me to make a replacement square from sheet metal, epoxy and rivet it in, then cut my hole in a slightly altered position. I wanted to reposition the breather slightly lower, and a little better centered from its original position. See photos below.

yamaha road star roadstar, genesis carburetor breather

yamaha road star roadstar, genesis carburetor breather

Tip: I made a template for the cut outs, shown below. If you wish to use the template, copy it to your computer, then print it using the sizing option in your print option control panel. Adjust the sizing up or down until the printed size of the template's scale-ruler matches your ruler.

yamaha road star roadstar, genesis carburetor breather

Tuning the Genesis Carb

Like most machines designed for extremely high performance, the Genesis carb works amazingly well when properly adjusted, and moderately well, at best, otherwise. Note: In my opinion, this carburetor is as forgiving as many others, if ill-tuned. Once it's tuned well, though, it should need no ongoing attention for normal street riding.

In fact, here's another area where the Genesis Series IV carb really shines: All normal adjustments can be made by turning knobs on the outside of the carb! No need to disassemble anything to try a different main-jet; the Power-Jet handles that, and it's adjusted with a knob near the carb's top. No need to change clips or washers for the needle; the metering needle is easily adjusted from the carb's top. It's a tuner's dream.

First, check the adjustment settings on the carb. Make sure they match the Spec Sheet that came with it. In case you have no spec sheet, the settings for the Road Star should start somewhere near the following:

  • Pilot-Fuel: 0 to 1.5 turns. Tip: Mine is (and several others are) set to 0 turns out.
  • Pilot-Air: 2 to 4 turns out
  • Needle Height: 12 clicks up/out from bottom. Note: If you have an earlier type needle adjuster screw, with 4 clicks per revolution (instead of 8), set the needle to 6 clicks up/out from bottom.
  • Power-Jet: 18 clicks out

Every bike will need unique settings. Exhaust pipes, engine mods, coil and/or ignition module changes, spark plug choices, and even altitude and ambient temperature can demand different carb adjustment settings.

All settings are measured as either clicks or as full-revolution turns out from a fully screwed-in position. As you fully seat (screw in) each adjuster, be very careful not to twist with any more pressure than needed to feel the adjuster bottom out. The adjusters are brass -- except for the needle -- and the adjuster seats are aluminum. Both are soft metals, and easily damaged. Use extreme caution.

Next, connect a fuel source to the fuel hose. If you install your gas tank as your fuel source, access to some of the adjusters will be limited, and you may need to remove the tank several times as you fine tune the carb adjustments for your bike. But that would also be necessary for adjustments to ANY carb.

Tip: Some Genesis owners mount their stock gas tank, then when they want to make a needle adjustment, they remove only the tank's rear mounting bolt. The carb's needle adjuster -- on the top-plate -- can easily be reached by lifting the rear of the tank a bit. Just be careful; the engine will be hot!

Tip: I use a 1/2 gallon test-tank made by Summit Racing (See the Build a Cheap and inexpensive Aux Fuel Tank article for details). I welded a mount for it so I can bolt it to the bike's frame for testing. This gives me full access to all adjustments. Others have modified 1-gallon plastic gas cans, and some have even modified 2-liter soft-drink containers. Just be sure to keep an eye to safety. See a photo of my mount, below.

yamaha road star roadstar, genesis carburetor, auxillary aux fuel tank

Now it's time to start the bike. I know; the breather has not yet been installed. Don’t worry.

Pull the carb's enricher (choke) knob all the way up, and then -- without touching the throttle handle -- press the starter button. If the bike doesn't start, try again with the choke off. If still no results, pull the choke up again, plus place your hand momentarily over the carb throat while you press the starter. Then take your hand away, and continue cranking the starter.

If it just won't start, it's time to stop and think things through...

If it looks or smells like the carb is allowing too much gas, you need to try to reduce fuel and/or increase air. Unless the Pilot-Fuel screw is already set to zero, try adjusting it in 1/2 turn or so. Or try adjusting the metering needle in (down) 4 clicks (1/2 turn) or so. Or try increasing (screwing out) the Pilot Air 1/2 turn. If still no go, adjust more.

Of course, if you suspect it is too lean, do the opposite. Also, if you have a too lean condition, check for air leaks at the junctions between the carb, spacers, manifold, and engine.

If all this fails, you may have the wrong needle and/or nozzle for your bike's set up -- or you may have a problem unrelated to the carb. Ask for help on the forum of this site or other popular forums that discuss the Genesis carb. Also, don't forget P.S.I. ; they built the carb, and are extremely motivated to increase its popularity. They can be a tremendous resource.

Tip: I bought a larger enricher (choke) jet. I swapped out the 0.015" jet for a 0.022" jet. P.S.I. thought it would help with easier starts in St Louis' cold weather. To replace the enricher jet (a.k.a. Pickup Tube), you will need to remove the float bowl. The float bowl is easily removed with a single, nut at the bottom-center. Just be careful of the float assembly inside, as it is only held in place by the friction of a tiny o-ring. Turns out that the larger enricher jet was too big, and I re-installed the original one.

Once you're started and idling, the first thing is to adjust the idle to a proper level -- not too slow. The adjuster is on the carb's left side (facing the installed carb). Let the bike warm up a bit before you do this -- enough so that the choke is not needed, at least.

Next, adjust the Pilot Fuel and Pilot Air mixture. It appears that most Series IV carbs, installed on Road Stars, need very little, if any, pilot fuel. Try shutting yours completely off. Just be careful you don't bottom the adjuster out very hard at all; it is brass, and easily damaged.

Pick a pilot fuel/air combo where the idle is strong and smooth, and yet there isn't any visible exhaust smoke when you nudge or blip the throttle just above idle.

Finally, it's time to go for a ride. The purpose of this ride is two fold:

  • Warm the engine thoroughly.
  • Begin adjusting the metering needle height. This is the most important adjustment for getting the Genesis to perform its best. Note: for top performance, the Genesis carb will work best when set between 8 and 14 clicks (4 to 7 quarter turns). Tip: To adjust the needle height, simply raise the slide fully by twisting the throttle grip while simultaneously pressing down on the needle adjustment knob on the top of the carb.

Note: If your carb seems to work best when adjusted outside the 'sweet-spot' range described above, the carb will likely work even better with a different needle or nozzle. Call or email P.S.I. for assistance.

Once the mid-range throttle has been adjusted completely, you can move on to the final adjustment: the Power-Jet. This adjustment is similar to changing 'main jets' on most carbs, but far easier. Turning the Power-Jet adjuster knob 4 clicks (1/3 turn) equates to one Mikuni main jet size change. The Power-Jet adjuster is located near the top, but on the front (face) of the carb.

To adjust the Power-Jet, you must either take your bike to an experienced, reliable dyno tuner, or you must do some high speed testing.

High speed testing has been around for a long time. Attempt this method only if you are skilled and comfortable riding in such conditions, and you have a place to perform this test without breaking the law. This method is potentially dangerous. Perform it at your own risk. To do high speed testing, I suggest the following:

First, thoroughly warm your bike via a 20 minute ride. Next, go to your testing track or un-congested roadway designated for legal high-speed driving. Then accelerate the bike to about 55mph in 4th gear. Tip: If you have a smaller than stock, front, drive pulley, you could start at around 50mph.

Now twist the throttle to WOT (wide open throttle), and accelerate to 75 or 80 mph (the faster the better for testing purposes, as long as it is not near the bike's 4th gear top speed). Tip: Since the Genesis carb has no accelerator-pump, you can proceed to the next step immediately.

When you hit your chosen speed, quickly back off of the throttle 1/8th turn. While doing so, pay attention to the RATE of continued acceleration. In other words, the bike will still be speeding up since it has plenty of power for higher speeds than you are going. Remember, your throttle position is still quite aggressive. Here are the possibilities and their meaning:

  1. The bike increases its rate of acceleration -- In other words, it feels like it has a bit MORE power with the throttle eased a bit. In this case, the Power-Jet is set too lean and you need to screw the adjuster out (counter-clockwise).
  2. The bike feels like you have done nothing different -- In other words, it continues accelerating at the same rate as when the throttle was wide open. This is ideal. It indicates that the Power-Jet is set very close to perfect.
  3. The bike looses some power -- In other words, you may still be accelerating, but at a slower rate. Unless you are nearing your bike's top 4th gear speed or you reduced the throttle by more than 1/8 turn, the Power-Jet may be set too rich. Try adjusting the knob in (clockwise).

Make several test runs, as follows:

  • Keep adjusting the Power-Jet OUT until you experience conditions 2 or 3 above. Note: You may not ever experience condition 3, even with an overly rich setting.
  • Then adjust the Power-Jet IN until you barely experience condition 1 above.
  • Then adjust the Power-Jet back OUT one click.


Appendix-A, Information and Experiences with the Genesis Carb From P.S.I.

My results are typical of this carb installed on Road Stars.

With other carbs, the Road Star power slowly builds as the throttle is twisted, peaks at about 2,400 rpm, and then begins to drop off at higher rpm. Yes, that's a very slow peak speed. That's why some refer to the Roady motor as a "tractor engine." It's also why you may have been left behind the pack, if you've ever tried to shift your bike at too high an rpm, while on a group ride.

However, Genesis carb -- with its computer designed, finned and stepped throat -- pretty much makes that same, max horsepower available over a huge range of rpm. I don’t know of another product that will offer anything close to that increase. In fact, it’s hard for me to imagine pouring the money into a big bore project without including one of these carbs in the budget. Without it, you’re likely cheating yourself out of half of the useable power (and smiles).

The Genesis carb rivals fuel injection efficiency, while offering do-it-yourself tuning simplicity. It is truly a breakthrough in carburetor technology. Too bad carburetors seem to be rapidly disappearing from our mechanical landscape.

Seeing the sure fate of carbureted motorcycles, I bought my Roady, in part, as an effort to own a great bike I could maintain totally myself. Note: Fuel injection makes at-home maintenance and performance upgrades very difficult.

Now, with the Genesis carb, I can keep up with the fuel injected models, without unduly sacrificing gas mileage. Frankly, when you compare the performance and price of fuel injection to the Genesis carb, it doesn't look so pricey anymore.

Appendix-B, Technical Information on Needles and Nozzles

Here's a list of needles made for this carb, as of the publish date of this article. This information is given for completeness only. It is not necessary to involve yourself in this, in most situations:

  • EK 150-127-099C, useful generally for 74-88" engines
  • EK 149-126-097C, useful generally for 95-103" engines
  • EK 149-125-095C, useful generally for 107-117" engines
  • EK 150-125-095C, useful generally for 107-117" engines
  • EK 149-124-093C, useful generally for 121-131" engines
  • EK 149-123-089C, useful generally for 139-155" engines
  • EK 149-122-085C, useful generally for 160+" engines

The numbers represent needle diamters, in thousandths of an inch. So a middle number of 126 would mean that the needle is 0.126" diameter in its central region. The first number is the Pilot (near-idle) region of the needle's (upper) diameter, and the last number is the needle's wide open throttle (lowest) diameter.

To remove the needle, remove the top of the carb (including the slide assembly), disconnect the cable and spring from the slide, Then use an allen (hex) wrench to fully unscrew and remove the needle-height adjuster screw from the slide. Then push the needle up, being careful not to force, scratch, or jam it.

As you continue pushing the needle up, it will be stopped by the adjuster-screw detent ball. At this point you must use some force to push the needle-head past this point. Just be cautious that the rest of the needle is not jamed against another part of the slide, and scratched.

Once the needle is out, the needle-spring and the tiny, plastic spring cup will come out fairly easily. The cup may need a bit of coaxing to get past the adjuster-screw detent ball, however.

Be careful not to loose any parts. They are small. See photo below:

yamaha road star roadstar, genesis carburetor slide

Fuel nozzles, stainless steel cylinders into which the needles slide, are available in sizes between 150.9 and 152.0, in 0.2 increments. The numbers represent thousandths of an inch, so a 151.4 nozzle would be 0.1514" ID. This information is given for completeness only. It is not necessary to involve yourself in this, in most situations.

Should you need to replace a nozzle, you will need to first remove the float bowl. The bowl is held on with just one, large, nut in the center of the bowl's bottom. Remove the Power-Jet fuel hose before unscrewing the bowl's center nut. Then the bowl will drop easily.

As the bowl is removed, the float assembly will usually drop out with the bowl. Be careful. The float assembly is very delicate.

If the float assembly has not dropped out with the bowl, pull down on the white plastic guard to remove the float assembly. See photo below.

yamaha road star roadstar, genesis carburetor float

Next, the main pickup tube must be removed. This tube is the big one in the center of the carb's underside. It is held in place with 3 tiny allen (hex) screws (and thread-loc). The screws use a 3/32" wrench, as I recall.

Remove the tube screws, then wiggle and coax the tube CAREFULLY out of its seat. The nozzle is held in place via an o-ring, at the base of the main tube.

Before reassembly, be sure to carefully clean off all old thread-loc around the tube's seat area, and inspect the o-ring carefully for damage or wear. Replace as needed. See photos below:

yamaha road star roadstar, genesis carburetor nozzle

yamaha road star roadstar, genesis carburetor nozzle

Also, three different slides are available: Mild, Medium, and Agressive. So far, it appears the Road Star works best with the Mild slide, regardless of the bike's modifications -- at least for the Genesis Series IV carb.

If, for some reason, you do need to purchase a different needle, nozzle, or slide, P.S.I. offers a 30 day exchange/return. This means, you could purchase several needles and/or nozzles, for example. Experiment with which combination suits you best, then return the unneeded parts (in like new condition) within 30 days for a full refund. It's a great program, especially since Genesis parts are of the highest quality, and therefore pricey.


Questions should be asked in our forum (Use discuss link below). The forum is very active and you stand a good chance of getting your questions answered there. If you would like to leave feedback for the author, or have additional information you think will benefit others, please use the comment section at the bottom of this page.

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DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.

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  Comments (10)
Good info
Written by zip1700, on 06-13-2012 22:12
I appreciate the write up and the fact that you shared your experience and opinions with us. That is what a good forum is all about, no matter how someone may feel about it, at least we now have a look into it from your eyes. Thanks again
Big Air
Written by 01HotRod, on 04-18-2012 21:36
I have been running the Big Air carb on my 01 Roadstar since 2008. My motor is a 110 c.i. Patrick Racing big bore with 10:25 pistons 2mm oversize valve, Dyna ignition, Monster Pro 2 into 1 header, with no stinking baffle. It puts 110inch Harleys on the trailer any day of the week. With stock cams it dyno's at 85hp and 115ft/lb torque. Will bury the stock 120 mph speed-o, I love it.
Written by comara, on 10-25-2011 15:36
excelente articulo,,me gustaria saber mas al respecto pues me gusta la idea para mi moto
Written by Starkruzen, on 03-29-2011 19:57
Great article, very detailed and outstanding pictures and tips. A first class white paper. For me, this is a huge project and cost prohibited. A cost and benifit analysis would indicate that, for a novice and experienced Roadie this carb is not the way to go. I have the Maxair HSR 42 on my 07 Road* and I can say that Maxair gives you a "plug and Play" system with a TPS. I got 54.7 MPG and I can stomp a fuel injected Roadie and recently a Fat boy. And the total cost which includes mounting your stock TPS, new SS cables and choke cable which is plug and play for $713.
Written by 99xv1600, on 02-22-2010 17:38
You spent 1500 on this and you can now keep up with fuel injected models? What is so special about the injection bikes your riding with? I can keep up with them without this!
Written by ironman, on 01-05-2010 17:19
GREAT article, very detailed, & well written. The price knocked me off my chair :cry :cry
Written by Rayjay, on 01-05-2010 14:58
Just looked at the website for the carb. It goes for around $1100 in black anodized. :cry  
Then you have to go for the extras. Total cost would be around $2000 by my estimate. 
carb install
Written by JYD, on 01-05-2010 02:10
Like everyone else,what is the cost of this project?
Written by texasscott1, on 01-04-2010 23:02
Excellent writeup. Might want to correct the beginning though. Road Stars come stock with a 40mm Mikuni. Also the cost would be nice to know.
Project costs?
Written by Rayjay, on 01-04-2010 22:33
It would have been nice to see the cost of this project for each component and the complete package.

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