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Synchronizing Carburetors on Twin Cylinder EnginesThis article provides information, based on my research and experience, about how to synchronize carburetors on twin (or more) cyclinder engines with carburetors for each cylinder.
Purpose:The reason to do this, is that each cylinder of the engine needs to have the same amount of airflow and fuel running into it so that the cylinders are firing with the same power. Otherwise, one cylinder will be "fighting" the other.Process:
The way to balance or synchronize the carburetors is to measure the vacuum between the carburetor and the engine on each carburetor and compare it to the other. Then adjust the throttle and mixture (for the idle mixture) so that the vacuum on ach side is the same.
A differential manometer works very nicely for this as it compares the vacuum of two different sources, and can be easily built for just a few dollars.BMW
R75/5 SynchThe BMW R75/5 carburetors by Bing have a port that easy to access, and the idle screw and idle mixture screw is easy to access as well.
When I first put my manometer on my BMW, the vacuum on the right side was far stronger than the left. I turned the idle speed screw up on the right side and brought the vacuum closer to being in balance, but to get it perfectly balanced, was going to cause the idle speed to be about 2000 rpm. Something was definitely wrong. The left side just would not generate the vacuum the left was. So, I suppose the valves are out of adjustment. I'll be checking that next.
Later: I checked the compression, and its fine on both cylinders. While checking it, I discovered that the choke cable had come loose on the left side carb, and so it was running rich. I corrected that, rebalanced the carbs and now they balanced very nicely and it runs great.
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