What is carb balancing?
Most bikes with more than one cylinder have separate carburettors delivering fuel mixture to each combustion chamber. These are linked so that when the throttle cable tugs one slide open, they all open. For smooth, efficient running it is critical that the action of the carbs is synchronised so the same amount of fuel/air mixture is delievered to each cylinder.
If the carbs open at different times, the cylinders will make varying amounts of power. This causes vibration, most noticebly at tickover. Throttle respnse and fuel consumption are also adversely affected by unsynchronised carbs.
The difficulty of checking your carbs varies from bike-to-bike, depending on access. Twins can often be checked with the tankin place, but on four-cylinder bikes it'll have to come off to allow access from the above. Either way, one thing you'll definitely need is a set of vacuum gauges.
The gauges measure the air pressure in the carbs' intakes simultaneously (there is no vacuum, but a low pressure area is caused by the suction of the downward moving piston accelerating the air) showing up any differences. Gauges can have dial-type indicators or tubes of mercury (or a mercury subtitute - much like a bank of thermometers. For the home mechanic, the tube-type gauge is the best bet as these are relatively cheap and stay calibrated.
If youhave to remove the tank, block the smaller of the pipes leading to the fuel tap. Unless you make up a dummy tank to deliver fuel, you'll have only the time offered by a carb's-worth of fuel in which to do the check. A washing-up liquid bottle can be used to refuel, but using bike's tank - with the fuel tap in teh RES position - and an extra-long pipe, is a better and safer option.
Each inlet rubber (engine side of carb) has a screw in it, or a short pipe capped by a rubber stopper (though one often carries the vacuum pipe back to the ruel tap) and these must be removed. Then, with the help of adaptors supplied with the gauges, eash inlet is connected to the gauges by a rubber pipe. The engine must be at operating temperture for the checking. If the readings are the same for all cylinders, great. If not, they'll need a tweak.
The aim is to have a 'datum' carb that is not adjusted, then balance the other carbs to it. Using the screws on the throttle linkages, starting nearest the datum carb, adjust the others until they are all the same. The correct-sized, long-shafted screwdriver is essential as the screws are easily chewed and forcing one will open the throttle and mess up the readings.
When you're happy that the carbs are in balance, blip the throttle, re-check them and re-set tickover, before putting everything back togather. Be very sure to replacethe screws/caps on the inlet stubs or it'll run like a bag of the smelly stuff.
Is it really worth doing?
Typically, carbs need balancing every 8000 miles (12,800 KM), however, with a manual, the instructions for the gauges and some patience, most folk could accomplish the task and it's super-satisfying to get right. On a twin cylinder bike, with just the one balancing screw, it's a much easier job than on a four.
Extracts from Bike March 03