Bell Mouth Watermain Fittings

Carburetor Tuning and theory

Carburetor tuning has the greatest effect on engine performance. When a motorcycle manufacturer builds a bike, they usually install jets in the carb that are too rich. The manufacturers sell the same model worldwide, so they couldn’t afford to install different jets in the carb to suit all the different climates and types of fuel. In addition to the climate and fuel, the manufacturer would also have to consider many other factors, such as the terrain and type of riding. And then there is the most important jetting consideration, the rider.

When I worked as a mechanic, I was in charge of jetting the bike over the course of the day. During morning practice sessions, the track was usually muddy and the air temperature was at its lowest point. I had to jet the bike rich for practice because the air density was

greater and the mud put more of a load on the engine. Then I had to watch the rider and the bike perform on different sections of the track. I would go to the obstacle on the track that presented the greatest load on the bike, typically an uphill straight section. I’d listen to my engine and watch the rider. I’d listen for pinging or knocking noises or excessive smoke from the pipe. I would watch to see if the rider had to fan the clutch a lot and how my bike pulled in comparison to others. Getting feedback from the rider is difficult because they are concentrating on riding not the bike’s performance. At a pro national there is one practice session, followed by a series of qualifiers and eventually two race motos. The time spacing of the riding sessions over the course of the day was such that I had to compensate the jetting two or three times. Otherwise, the bike would either seize from being too lean in the morning or run too rich for the second moto.

Race mechanics have different techniques for carb jetting. These techniques range from asking other mechanics what jets they are running to using precise measuring gauges to monitor the engine performance. In motocross races, where most of the riders are of equal skill levels, a holeshot in the start can mean the difference between a place on the podium and 30 minutes of roost in your face! The difference in horsepower between the bike that gets the holeshot and the bike that brings up the back of the pack may only be a few ponies! The race mechanic can give his rider an awesome advantage if he carefully monitors the carb jetting.

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