A carburettor is set for the air that reaches it and the exhaust that leaves. Change the exhaust, the air filter or the carburettor itself, and the mixture can move out of range. The engine may stumble at one throttle opening, run hot, pink or foul plugs. A change of one jet is not always enough, because the jets overlap in their ranges.
Is it safe to ride?
Ride with care and gently. A lean mixture overheats the engine and can burn a piston. If the engine pinks, stalls or runs hot, stop.
Quick checks first
- Check that the new parts are fitted correctly and that there are no air leaks at the exhaust joint or the intake boots.
- Check the air filter is clean, and that nothing is loose.
- Return to the standard parts if the problem began at once, to see if it clears.
Diagnose it step by step
Each part of the throttle range is mainly controlled by a different part of the carburettor. The idle circuit runs from closed to about one quarter throttle. The slide cut-out has most effect up to about one third. The needle jet and jet needle control about one eighth to four fifths. The main jet controls from about two thirds to full throttle. The ranges overlap, so a change in one affects the next.
1Does the trouble show at idle and just off idle?
- Yes: see Find and set the idle mixture screw. The idle jet is rarely changed.
- No: go to step 2.
2Does it hesitate or stumble in the middle of the throttle range?
- Yes: see Move the jet needle clip.
- No: go to step 3.
3Does it feel weak, run hot or pink at full throttle?
- Yes: see Change the main jet and read the plug.
- No: read the plug. See Read a spark plug.
To test a throttle range: run the engine at that opening for about a mile on a safe road, then read the plug. Decide whether it is lean or rich, then change the part that controls that range.
Carburettor makers publish tables of jet and needle sizes. A test on a dynamometer can help, but is not the same as the road. A friend's success with a jet change does not mean the same result on your bike.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Air leak after the change | Idle speed climbs and falls, see the intake leak test | Reseat and tighten the boot or gasket |
| Main jet too small | Plug white, engine hot at full throttle | Next jet size up |
| Needle in wrong groove or wrong needle | Stumble in mid-range | Move the clip |
| Idle mixture off | Rough idle, poor pickup off idle | Reset the idle screw |
| Poor quality copy jets | Odd results with new jets | Use jets from the carburettor maker |