Installing a Vacuum Fuel Pump

A number of riders have made the change from the stock electrical pump to a vacuum pump.  The vacuum pump is used in ultralights and snowmobiles, thus it is more than reliable for our bike applications.  There are two pumps that have been used; the smaller Mikuni DF44-211 (14 L/h) and the larger Mikuni DF52-136 (31 L/h).  See http://www.mikuni.com/pdf/fuel_pumps.pdf for pics of the various models and info.  The larger pump has duel fuel outlets; for our application one of the outlets must be plugged.

Installation recommendations:

1.  As the vacuum pulse decreases distance between the pump and the carburetor increases, the distance between these two should be minimized.  I would recommend no longer than 1-foot for the smaller pump.
2.  Use thick-walled tubing for the vacuum line in order to minimize collapse and maintain a strong pulse.
3.  The vent should face down.
4.  To minimize vapour lock, place the pump in an area where it is cooled.

The following details my installation:

1.  First remove the original pump, bracket, fuel filter and lines.
2.  On the vacuum pump, remove the 4 screws on top and replace with same size bolts (Figure 1).  This is only important for this installation, as the screws cannot be removed to service the pump once the pump is in place.
3.  Remove the manifold (I have a Nemesis).
4.  On the Nemesis there is a flat spot corresponding to where the vacuum nipple is on the stock manifold (Figure 1).  At this spot I drilled a hole through the manifold the same size as the inside diameter of the vacuum nipple on the pump. 

NOTE: On the stock manifold: 1) the vacuum nipple on the pump must be removed; or 2) the vacuum nipple on the manifold must be removed; or 3) a short piece of rubber tubing could join the two nipples (test to see if this will fit when the manifold is mounted).

5.  Enlarged the same hole by drilling with a bit just a little smaller than the outside diameter of the largest part of the vacuum nipple.  All the nipples on the pump bulge out near the ends (Figure 2).  DO NOT drill all the way through the manifold; drill only about ¾ of the way down the original hole.
6.  Sand the bulge on the vacuum nipple on the pump until it fits snugly into the manifold.  Do not over sand otherwise the fit will be loose.
7.  Smear the vacuum nipple from the bulge back to the pump body with JB Weld, making sure to not to get any JB Weld from the bulge to the end of the nipple so that the hole in the nipple does not become blocked with JB Weld.  Insert the nipple into manifold until it bottoms.  Smooth any excess JB Weld around the nipple/manifold joint.
8.  Orientate the pump as shown if Figure 2.
9.  Install manifold on the bike (Figure 3).  Note the close tolerances and proximity to the engine; however, there is airflow around the pump.
10.    The original fuel filter can be used.  I replaced it with a small clear snowmobile filter that has the same size inlet/outlet as the pump.
11.    I used ¼” fuel line for all connections rather than the stock 5/16”.  It will be a tight fit to connect to the carburetor and petcock, but the smaller line does fit.
12.    Connecting the fuel line to the pump inlet is also very difficult because of the tight quarters and awkward angle.  I recommend the line be put on before the manifold is put into the engine.

Figure 4 shows an alternative method to mount the vacuum pump.  A length of vacuum line is required (weakens the vacuum pulse) and the vacuum inlet does not face down (problems with condensation or if diaphragm leaks)

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Good Luck!

Ted (DocShadow)

Editors Note:

I asked Ted to write this article as he has one of the cleanest installations I have seen for the vacuum pump.  The elimination of the vacuum tubing between the manifold and the pump is a great idea, and will lend itself to better performance and longevity.

I want to add a few notes from my own experience using the same pump that Ted used. 

First, use 1/4" fuel line as Ted has.  Using lines larger than this leads to extraordinarily long priming times when starting up with dry lines.  This is most noticeable when starting up after running out of gas, or potentially when switching to reserve.  I ran a battery down trying to start my bike up using 5/16" line.

Second, as Ted has outlined the length of vacuum line is critical.  Longer lines will have more tubewall flex and you will loose the pulse strength coming through it that runs the pump.  I installed mine just as Ted shows in the alternate installation in picture #4.

Third, if you choose to locate to any other orientation other than the one proposed by Ted, or the alternate shown in picture number 4, remember that having the pump sitting above the gas level in the tank (for example when very low on gas) can lead to even longer priming times.  The lower the better.  The installation depicted in picture 4 has the vacuum pump above the lowest level you can achieve in your tank.  It works very well, but moving the pump higher than this can cause problems.

Finally, the second pump Ted lists in his article has proven to be the pump of choice for some who were unable to get sufficient fuel supply using the smaller one.  We have not completely outlined the variables that affect fuel starvation.  Most bikes will work very well with the first.  It is also smaller and easier to work with.

Thanks to Ted for sending this article in.  The quality of work and standard of excellence he aspires to are evident in this article and its pictures.

-GRAM

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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 (3)

Written by emma12, on 03-07-2013 15:23 this worked great, what a money saver

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