At part throttle the bike runs well, but when you open the throttle fully the engine gets slower, chokes, or will not rev out. Often it pulls better if you close the throttle a little. Full throttle needs the most air and the most fuel. If either is restricted, or the exhaust cannot get the burnt gas out, the engine chokes.
Is it safe to ride?
Yes, with care. You may not have the power you expect for overtaking or climbing.
Quick checks first
- Is the choke fully off?
- Is the air filter clean (not soaked in oil or water)?
- Does it improve if you open the fuel cap?
- Is there black smoke at full throttle?
- Two-stroke: when was the exhaust last decarbonised?
Diagnose it step by step
1Does it pull better at full throttle with the choke pushed fully off and the air filter clean?
- Yes: the engine was rich (too much fuel for the air). Clean or replace the air filter and check the choke.
- No: go to step 2.
2Is there black smoke and a fuel smell at full throttle?
- Yes: still rich. On a carburettor, a high float level or a worn needle jet can cause it: check the float. See Engine runs rich.
- No smoke, engine just runs out of fuel: go to step 3.
3Does it improve with the fuel cap open, or is it worse on a low tank?
- Cap helps: Check the fuel tank vent.
- No change: go to step 4.
4Carburettor: is the fuel flow from the tap strong, with a clean filter?
- No: Check the tap and lines and replace the filter.
- Yes: a partly blocked main jet or low float level. Clean the carburettor.
5Fuel-injected: is a code stored, and is the pump strong?
- Read engine fault codes and check the fuel pump. A workshop pressure test finds a weak pump or blocked filter.
6Is the exhaust restricted?
- Two-stroke: heavy carbon in the exhaust and port is common. See Exhaust blocked with carbon and Decarbonise a two-stroke exhaust.
- Four-stroke with a catalytic converter: the K-Light 125 manual lists a catalyst blocked by burning oil or internal damage as a cause of power loss. A workshop checks back-pressure. Something stuffed in the silencer end (a rag, a mud wasp nest) does the same.
7. Still bogging? Confirm with a road test and see Will not reach normal top speed.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Dirty or oily air filter | Visual check, black smoke | Air filter |
| Choke partly on | Choke check | Choke check |
| Fuel restriction (vent, tap, filter) | Better with cap open; weak flow | Tap and lines |
| Blocked main jet or low float (carburettor) | Good flow but starves | Clean carburettor |
| Weak pump (injection) | Low pressure at workshop | Fuel pump |
| Blocked exhaust or catalyst | Carbon, rattle in catalyst, back-pressure | Two-stroke decoke |