Carburetor balancing Tool

This is a design for a simple, cheap, easy to make tool to balance carburetors. As shown it is for a two cylinder bike. It might be possible to gang three together to work with a four cylinder but that is beyond the scope of this article. The balancer should work nicely on SU carbs. I presume you already have some knowledge of balancing techniques, or can get up to speed on what you need to know. This is mainly about building the tool. Main Tube

Where my design differs from most others is that it measures the differential pressure between two sources. Okay differential vacuum, but vacuum short of a perfect one is still pressure. The greater vacuum, that is to say the lesser pressure, will move the indicator less than the lesser vacuum, greater pressure. The tool is simply a length of clear plastic tubing, I used 9/16" fuel line with a small indicator inserted in the tube. Choose an indicator that will move freely in the main tube but one whose size is close to the inside diameter of the tubing so leakage around the indicator is minimized. I cut a small section of an ordinary plastic clothes hanger and inserted it in mine. The indicator tube has smaller tubes to attach to the carburetors. On each end of the main tubing insert some sort of reducer to allow attaching smaller hoses, chosen to fit your carburetor fittings and long enough to attach to the carbs and allow the indicator to be seen easily. The reductions keep the plastic indicator from being sucked (blown) out of the main tube. I bought some brass fittings but you may be able to find cheaper reducing fittings.

To use the tool just hook the smaller tubes to the vacuum fittings on each carburetor. On my bike one carburetor fitting was capped off and the other went to the fuel off/on/reserve valve mechanism. Adjust your carbs to get the indicator to center in the main tube at your tuning speed. Just remember the indicator will go to the side with greater vacuum. You may find the indicator is very sensitive and as a result balancing is finicky. What I did was bend the main tube into a U shape so the plastic indicator has to be lifted and fight gravity to move away from center. This reduces the sensitivity. The weight of the indicator will also affect the sensitivity and you may want to use a heavier indicator such as an appropriate diameter ball bearing. Assembled I only used my tool with the bike on its stand but there is no reason short of good sense or lack of courage that you could not use the tool while riding the bike. Just to get an indication of course, not actually adjusting the carburetors, unless of course you are part monkey.

This is my own design, I have not sought a patent. As far as I am concerned I release the design into the Public Domain. Feel free to build one for your own use. There is no charge for this information and if you provide it to someone else do not be an ass, do not charge money. Remember if you give someone knowledge you still have the knowledge so you have lost nothing. The photos however are copyrighted and can not be used without permission.