Learn To Change The Jetting On Your Bike

Tip #281 from the pages of The Total Motorcycling Manual.

motorcycle jetting, total motorcycling manual

Carburetors mix fuel and air in a precise ratio (by weight, about 14 parts air to one part gasoline). The problem comes because there’s less air (by volume) at higher elevations, and because some engine modifications (new exhaust pipes, new air cleaners) change the amount of air flowing through your engine.

Jets are small brass nozzles inside your carburetor that spray liquid gasoline into the moving 
air, mixing them together. These small, removeable fixtures have 
a hole in the center that’s precisely sized. When you change how much air flows through your engine (or if high-altitude operation changes it for you), you'll often have to change your jets to match, fitting new nozzles with larger or smaller holes as needed.

It’s not tough. Most engines have two jets, one for low-speed operation (the pilot or idle jet) and one for high-engine-speeds at large throttle openings (the main jet). A long, tapered needle controls the fuel/air mix at throttle openings between ¼ and ¾.

If your engine is too lean (too much air), it will often pop and backfire on you. If it’s too rich (too much fuel), it stumbles and burbles. The best way to judge is to read your spark plugs or run the engine on a dyno.

Changing the jets is a trial-and-error adjustment method that simply involves screwing in larger or smaller nozzles (jets) or moving the needle higher or lower (richer or leaner) in the throttle slide.