Balancing Multiple Carburetors
Words and Photos: Randy Bolig
If you are riding a bike with an inline engine—be it a two or four cylinder, you are also probably running multiple carburetors. Inline motorcycle engines are often equipped with one carburetor per cylinder. Since each cylinder is fed through its own carburetor, what you essentially have are independent engines joined by some shared hardware. For that reason, it's important to keep them all working in sync. One way to accomplish that is by balancing (Syncing) the carburetors. By balancing the carburetors, you are ensuring that each cylinder is working at peak efficiency, and that they are always at the same rpm and fuel ratio.
If the carburetors do get out of sync with each other, each cylinder will be receiving a slightly different air/fuel mixture. If and when this happens, the engine will be constantly trying to speed up or slow down, because again, each cylinder is receiving a different fuel signal. As small as these mixture differences may be, they can cause the engine to vibrate, make the idle fluctuate, and cause off-idle hesitations and/or surging at steady speeds. Look at it this way; consider a twin cylinder, twin-carbureted engine with carburetors that are out of sync. When you twist the throttle, one carburetor is supplying enough mixture for the engine to run at 3,500 rpm, but the other is supplying enough mixture for the engine to run at 3,600 rpm.
Usually, each carburetor is controlled by a single cable that comes from the handlebar, and they are connected to each other via linkage. When this linkage wears, small amounts of play develop, and the carburetors can get out of “sync” with each other. Since most times, it is simple wear that creates the problem, the problems tend to manifest themselves slowly. Many times, this wear doesn’t even result in required adjustment. We just mention it so you can check for it.
We should mention, that if you are trying to balance a set of carburetors on an engine that is tired (i.e. down on compression, needing a tune-up, etc.), you can spend days trying to balance the carburetors to never get a satisfactory result. Before any carburetor tuning can begin, you must make sure that the engine is in proper working order.
If you feel that you need to sync the carburetors, start by at least checking that the linkages are in sync. To do this, have someone operate the throttle while you watch each carburetors’ butterfly. If one moves before the other, that carburetor’s linkage or cable needs adjusted.
If you have access to a set of vacuum gauges, you can be more precise about your adjustments. Vacuum gauges attach to the carburetors and measure the amount of vacuum that is drawn into the engine as it is running. The effectiveness of using vacuum gauges is evident as the carburetors are adjusted; small adjustments make the needle move on the gauge.
To balance multi-carburetor systems, you will need to get the engine to normal operating temperature first. The vacuum balancing gauges should be attached to each carburetor. Begin adjusting the mixture screws on the carburetors. As the mixture screws are moved, the gauges will synchronize as the vacuum intakes are matched. Keep in mind, that as the carburetors are brought back into balance—if they were out, engine rpm will increase. The idle can be adjusted as you go. Continue to follow this procedure for all of the carburetors if there are more than two. If there are four carburetors, start by pairing the two outside carburetors first, and then make your final adjustments between the two inside carburetors. This will bring the two pairs of cylinders (one and two, three and four) into balance.
Some motorcycles have adjustable cables that will allow the adjustment of the carburetors travel or linkage.
Multiple carburetors are typically joined by a shaft that has adjustability built in. This adjustability allows each carburetor's "movement" to be adjusted to match with the others.
Proper tuning requires at least a set of vacuum gauges. With a gauge connected to each carburetor, they can be individually tuned and synced. All that is required is to adjust the mixture screw on each carburetor until you get the highest possible readings.