The engine coughs or pops back through the carburettor, often with a puff of fuel mist from the intake or air box. It happens most when you open the throttle from idle. The Amal tuning guide in the course lists spitting back as a sign of a weak mixture: not enough fuel for the air. A weak mixture burns slowly, so the flame is still alight when the inlet opens for the next charge. On a two-stroke, a broken reed lets the flame out the same way.
Is it safe to ride?
Ride gently and only to get it fixed. A weak engine runs hot and can damage the piston, most of all on a two-stroke. A spit back can also set fire to an oily air filter.
Quick checks first
- Is the choke fully off, and does the engine spit only when cold? A cold engine often spits until warm.
- Look for a split or loose intake rubber between carburettor and engine.
- Is there enough fuel in the tank, and does fuel flow freely from the tap?
- Was the air filter, exhaust or jetting changed recently?
Diagnose it step by step
1Does the spit back happen only when the engine is cold and stop once it is warm?
- Yes: this is normal on some bikes. Use the choke longer and warm up properly: Cold start drill.
- No: go to step 2.
2Does the engine run better with the choke partly on at the throttle position where it spits?
- Yes: the mixture is weak there. Go to step 3.
- No: go to step 5.
3Is there an air leak at the intake rubber, carburettor flange or manifold?
- Yes: fix the leak first: Find an intake air leak.
- No: go to step 4.
4Is the pilot jet or a passage partly blocked?
- Clean it: Clear the carburettor jets. If it is clean, set the pilot screw and check the slide cutaway: Tune a slide carburettor in the right order.
5Two-stroke: are the reed petals cracked, chipped or not closing flat?
- Yes: Check the reed valve.
- No, or four-stroke: go to step 6.
6Four-stroke: is the inlet valve clearance too tight, or the valve not sealing?
- Check: Check and adjust valve clearance. A burnt or bent inlet valve needs a workshop.
- If the valves are fine, check ignition timing. Late or early spark also causes popping: Check ignition timing.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Air leak at the intake | Idle changes when you spray soapy water on the joint | Intake leak test |
| Pilot jet partly blocked | Spits just off idle, better on choke | Clear the jets |
| Weak jetting or wrong cutaway | Spits at one throttle position after a change of filter or exhaust | Tune in order |
| Broken reed petal (two-stroke) | Petal chipped or open against the light | Reed valve check |
| Tight or leaking inlet valve (four-stroke) | No clearance at the rocker, low compression | Valve clearance |
| Ignition timing wrong | Timing marks do not line up | Timing check |