The carburettor mixes fuel and air in the ratio the engine needs, over the whole range from idle to full throttle. It has no electronics and few moving parts, which makes it easy to understand and easy to blame. Before you change jets, rule out the simple causes.
How it works
- The float bowl and float. Fuel enters the bowl through a needle valve, and the float closes the valve when the level is right. A wrong level gives a rich or lean mixture.
- The pilot (idle) jet. Supplies fuel at idle and just above. The idle mixture screw adjusts it.
- The slide and the needle. The throttle lifts the slide, which opens the air passage, and the tapered needle in the main jet regulates the fuel at medium throttle.
- The main jet. Controls the mixture at full throttle.
- The choke or tickler. Enriches the mixture for a cold start. A tickler floods the bowl.
Common makes on classics are Amal, Mikuni, Keihin and Dell'Orto. They differ in detail, but the principles are the same. Use the manual for your carburettor.
The usual causes of trouble
- Dirt and varnish. Old fuel leaves deposits that block the small passages. The idle jet is the usual victim.
- Air leaks. A cracked inlet rubber, a worn throttle shaft or a loose manifold lets in air and leans the mixture.
- A wrong float level or a worn needle valve. Causes flooding or starvation.
- A worn slide or body. Common on old carburettors. The slide wobbles and leaks air.
- Ignition and valve clearances. Often mistaken for carburettor faults.
Check the ignition, the compression and the valve clearances first. See electrics and ignition.
Cleaning a carburettor
- Photograph the carburettor and note the positions of the screws and the needle clip.
- Remove the bowl, the jets, the needle and the float.
- Soak the metal parts in a carburettor cleaner. An ultrasonic bath gives the best result.
- Blow the passages through with compressed air. Do not use a wire or a drill to clear a jet, since that enlarges it and changes the mixture.
- Replace the O-rings, gaskets and the needle valve. Use ethanol-resistant parts: see fuel, ethanol and the fuel system.
- Reassemble, set the float level as the manual says, and set the mixture screw at the standard number of turns.
Finding an air leak
With the engine at idle, spray a little carburettor cleaner or water mist around the inlet joints and the manifold. If the speed changes, the joint is leaking. Be careful: carburettor cleaner is flammable, and a hot engine is an ignition source. Use a very small amount or a smoke tester, and keep a fire extinguisher at hand.
Balancing twin carburettors
On bikes with two or more carburettors, the throttle opening has to match. Set the cables so that the slides lift together, then use a vacuum gauge set (a synchroniser) or the manual's method to balance them at idle. An unbalanced pair makes the bike vibrate and run roughly at low speed.
Reading the spark plugs
After a run at steady speed, stop the engine without idling and look at the plug insulator. Light brown means a good mixture, black and sooty means rich, and white or blistered means lean. Use the plug colour to confirm what the bike tells you, but do not tune only by plug colour.
Changes in weather and altitude
Air density changes with temperature and altitude. A bike set up in summer may run rich in winter and the other way around. If the bike runs well and starts easily, leave it alone. Change one thing at a time, keep a note, and change it back if it is worse.
