Carburetor Balancing Using Vacuum Gauges

Mikuni BS 36 SS Carburetor from a Suzuki GN400

Wusel007/Wikimedia Commons/CC BY-SA 3.0


Carburetor balancing on multi-carb, multi-cylinder engines is very important. Each carb must supply the same amount of mixture (fuel and air mixed) for the engine to run smoothly, develop good power, and maintain fuel economy.

A typical application of this design can be found on many Japanese four-cylinder engines manufactured from the 70s onwards, such as GS Suzuki's, Honda CB's, and Kawasaki Z series machines.

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Balancing Carburation Systems

Diagram of Balancing Carburation Systems
A = adjuster between carbs one and two. B = adjuster between the banks (one and two and three and four). C = adjuster between carbs three and four.

John H Glimmerveen

The most accurate method of balancing these types of carburation systems is by using vacuum gauges. When attached to the inlet systems, the vacuum gauges measure the amount of vacuum drawn on each gauge as the engine is running. The effectiveness of this system is evident as the carbs are adjusted: Small adjustments can be seen on the gauges as the carbs are adjusted.

Capable of Greater RPM

For example, as the carbs are brought back into adjustment (assuming they were out in the first place) the engine idle rpm (revs per minute) will increase. Effectively, this indicates that for a given throttle position, the engine was capable of pulling greater rpm.

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Using Vacuum Gauges

vacuum balance tube on motorcycle
The vacuum balance tube (arrowed) is molded into the inlet manifold on this Kawasaki Z900.

John H Glimmerveen

To balance multi-cylinder multi-carb type systems, it is necessary to warm the engine first. However, if the mechanic has access to a large cooling fan, this should be placed in front of the machine during any subsequent running to maintain constant engine temperature.

The vacuum balancing gauges should be fitted to each inlet tract (many Japanese machines have either a removable screw or a capped tube on each inlet) and the engine re-started. Reference to a shop a manual will list the correct rpm to set the idle to when vacuum balancing (typically around 1800 rpm).

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RPM Increase

The first adjustment should be made to the link between carbs one and two. As the adjuster position is altered, the gauges will synchronize as the vacuums drawn are matched. It should be noted that as the carbs are brought back into balance, the rpm will increase. The idle should be adjusted down to the same setting as used at the start; for example, 1800 rpm.

Next, the mechanic should follow the same procedure for carbs three and four; again re-setting the rpm as required.

The final adjustment is between carbs two and three. This adjustment will bring the two banks of carbs (one and two, three and four) into balance.

When the carbs are in balance, the idle setting should be returned to normal; typically 1100 rpm.


  • Yamahas fitted with YICS (Yamaha Induction Control System), are an unusual case and need a special tool to balance them.
  • Some Japanese four-cylinder machines are not set with equal amounts of vacuum on each cylinder. This setup is designed to compensate for the slightly different running temperatures between cylinders exposed to air flow (one and four) and those on the inside (two and three). Mechanics must check their shop manuals before carb balancing.
  • After rebuilding a set of multi-cylinder carburetors, the slide (or butterfly valve) opening balance can be set statically on the bench by using the lollipop stick method.