The engine pulls at low revs and at high revs, but in one band in the middle (for example from a quarter to half throttle) it goes flat, stumbles or stops pulling, then picks up again. Carburettors meter fuel in overlapping ranges: the pilot circuit near closed throttle, the needle and needle jet from about a quarter to three quarters, and the main jet near full throttle. A flat spot in one band points to the part that controls that band. On injected bikes the cause is usually a sensor signal or an air leak.
Is it safe to ride?
Yes. Avoid relying on that throttle range for overtaking until it is fixed.
Quick checks first
- Note the throttle position (not just the speed) where it goes flat, on a road test.
- Is the air filter clean and the airbox closed properly?
- Has anyone changed the jets, needle clip or exhaust?
- Fuel-injected: is the warning light on?
Diagnose it step by step
1Is the air filter clean, correctly fitted, and the airbox and intake rubber sealed?
- No: clean or replace the air filter and refit the airbox. A missing or oily filter upsets mid-range fuelling.
- Yes: go to step 2.
2Has the exhaust been changed, holed, or has a baffle been removed?
- Yes: a changed exhaust changes the airflow and often causes mid-range flat spots. Refit the correct exhaust. See Exhaust has become much louder.
- No: go to step 3.
3Does the idle change when you spray water on the intake joints?
- Yes: Find an intake air leak.
- No: carburettor go to step 4; fuel-injected go to step 6.
4Carburettor: has the needle clip, needle or needle jet been changed or is the carburettor old and dirty?
- Changed: refit the parts specified for your bike, with the clip in the standard groove. The needle and needle jet wear oval with time and can also cause a rich flat spot; replace them as a pair.
- Old or dirty: Clean the carburettor, and on a CV (vacuum) carburettor check the diaphragm for holes.
5Carburettor: is the float level correct?
- A low float level gives a lean flat spot as the jets draw harder. Check the float.
6Fuel-injected: is a code stored, or does the throttle position reading jump in live data?
- Read engine fault codes and check the engine sensors. A worn throttle position sensor can have a dead spot in its track.
- No code: clean the throttle body.
7. Still flat? Check ignition: service the plug. Then see Hesitates when opening the throttle.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Dirty, oily or badly fitted air filter | Visual check | Air filter |
| Changed or damaged exhaust | Louder, baffle missing | Refit correct exhaust |
| Intake air leak | Water spray test | Intake leak test |
| Dirty or worn needle and needle jet, wrong clip | Parts history, inspection | Clean carburettor |
| Torn CV diaphragm | Pinholes against light | Clean carburettor |
| TPS dead spot (injection) | Live data jumps | Sensor check |