Porting The Stock Manifold

Reprinted by Permission from Mr Tidy's Tech Tips

(While Waiting For Greg's Manifold) 
Its not rocket science  !
Its really very simple - if you have the proper tool !! 

You need a variable speed Dremel tool with a course sandpaper covered cylinder that is about  ½" in diameter.  I use the one that came as part of the Dremel kit. Use the low speed setting.  This tool really "Hog's" off the rubber! Take it SLOW AND EASY !!! At least the first time.

Look inside the manifold and you will see some very "Abrupt" corners where the mixture is routed left and right to the cylinders.  Stick your finger in there and feel the thickness of the rubber at the corners.  My guess is that it's about  ¾" thick. Most of this can be removed with the Dremel.  Chamfer this sharp edge so as to have approximately a 1" radius.  The radius isn't that important! Just form a nice, smooth transition around those corners.

In your mind, visualize the fuel mixture flowing into the stock manifold.  Those abrupt , sharp corners disrupt the mixture as it makes a right or left hand turn into the intakes.  You can just picture the "Dead" areas where the mixture swirls around these sharp corners and creates turbulence.
 
This turbulence is the cause of problems and must be reduced.  Now visualize the fuel flowing around the smooth corners. Much more efficient!

The two pictures give you an example of what the manifold looks like now, and what it should look like after porting.

Click on any picture to enlarge.

Your going from this

As you are grinding the rubber away , you will eventually come to the aluminum casting that feeds each cylinder. Don't panic ! This is where I usually stop, but I am sure you could go a bit farther.  Keep on checking the thickness of the rubber at those corners.  Do a bit on each side, alternating between sides.  Remember : If the rubber is cut too thin, your carburetor will fall off!  Not really, but don't go too far.

The idea here is to "Smooth" the flow of mixture as it splits left and right to each intake.  As you are grinding, constantly check that both sides are "EQUAL" in size. This is EXTREMELY important for a balanced fuel flow to each piston.  The shape and depth of cut MUST BE THE SAME on both sides !!  If not, your bike will veer left or right !!

This operation typically takes me less than 1/2 hour - being very careful to "Balance" the cuts.  Just hold a firm hand on the tool , and check often with your finger.  I hold the manifold in my left hand. You may find it easier to mount it in a vice.  Bright lighting is necessary to see where you are grinding.  I would not worry about smoothing  the surfaces too much.  It just won't buy  any performance improvement. Increasing the "Bore" diameter will not gain you any performance improvement. Just attack those sharp corners.  It's virtually impossible to mess up a manifold, as long as both sides are cut the same !!! 

Actually, nothing can be much worse than stock!

Hey guys, put another notch in your butt stock !! Just do it !! I promise you will feel the difference.
 

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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.

Comments (6)

Porting the stock manifold Written by wildwilly, on 08-16-2005 15:49 what grade of sandpaper did you use?

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