Technical Tip By Mo...

Carburetor Pump Around (read everything and understand what is happening before you start)

Reason for the Pump Around:

Karting has evolved to the point where many engines are using a float-type carburetor. While it works well for motorcycles, it can be a real pain to tune for the kart application. Mainly this is due to the fact that the fuel has the tendency to crowd up against the float bowl side wall while going thru the corners and produces a 'flat spot' or hesitation  when you step on the throttle. The pump around eliminates this by maintaining a constant level of fuel in the float bowl and not depending on the floats to keep this level accurate. The photos are of a 36mm flatslide Mikuni but the application will work on any 'Float-Type' Carburetor.

How To Do It:

First thing to do is to remove the float bowl and then proceed to remove the floats, needle plunger, and needle seat. These items are not used in this modification.

Next, find a Two-Stage Fuel Pump  or two single stage pumps can be used with a pulse Tee.

This is a Two Stage.

Get a small piece of  tubing that is the same size or slightly larger than the fuel inlet hose (either copper or aluminum). This tubing is how you maintain a constant level of fuel in your carb. Notice how it sticks into the float area (the height you want the fuel in the bowl area). In this photo, the tube is cut off on an angle. While this is not necessary it makes a nicer looking modification and it's easier to determine the fuel height. This tube is held in place with either JB Weld (least desirable), Devcon-F Putty from Loctite (most desirable), or some type of Epoxy that is not affected by the fuel you plan to use.

The next thing to do is to locate some neoprene or vinyl type sponge material. This material can not be affected by the fuel that you are planning to use. You can also use some 'Fuel Cell Material' if you can afford it or find a race builder that has a destroyed fuel tank and rob a piece from it. Cut 2 small pieces that will fit loosely on each side of the float bowl (and not interfere with anything else). Do Not Glue Them In Place. Let them fit loosely.

The next thing to do is to locate a fine screen mesh (use brass to prevent rusting). Cut a template that will fit into the bowl area. This screen does nothing more than prevent the sponges from interfering with the main jet. Again, Do Not Glue It In Place.

Last, but not least, is the venting of the float area. What we done here is to connect both vent nipples together with a common hose and just cut a small notch in the top of it. Works great.

Assemble the float bowl onto the carb, hook up the fuel lines and you are ready to go.

Small Note:
Our organization allows this type modification and even encourages this type of innovation, however, other organizations do not encourage it and will use it as a way to DQ you if it is not done correctly. Sometimes this modification must be done entirely thru the bowl area (think about it, it is possible) to keep from breaking any rules.