A carburettor bike that ran well in one place now runs badly after being taken high into the hills or down to the coast, or when the season changes from cold to hot. High up the air is thin, so the mixture goes rich: black smoke, a sooty plug, poor power and high fuel use. Lower down, or in cold weather, the air is denser and the mixture goes weak: hot running, spitting back and pinking. The jets are fixed, so they no longer suit the air. Fuel-injected bikes correct for this themselves.
Is it safe to ride?
A rich engine is safe to ride gently to a workshop. A weak engine can overheat and damage the piston: ride gently and fix it soon, most of all on a two-stroke.
Quick checks first
- Is the air filter clean? A dirty filter adds to a rich mixture.
- Is the choke fully off?
- Did the problem start only after the change of place or season, with nothing else changed?
- Does the engine run normally again when you return to the old place?
Diagnose it step by step
1Did the trouble start with the move, and does it go away when you return?
- Yes: the jetting suits the old altitude. Go to step 2.
- No: look for another cause: Engine runs rich, Engine runs lean.
2Read the spark plug after a short ride. Is it black and sooty, or white and clean?
- Black and sooty: rich. Check the air filter first: Service the air filter. If it is clean, go to step 3.
- White or very pale: weak. Go to step 3, and ride gently until it is fixed.
- Help reading it: Read a spark plug.
3Will the bike be ridden at this new altitude most of the time?
- Yes: change the main jet using the maker's chart, then retune the needle and idle: Adjust jetting for altitude and temperature.
- No, a short trip: a small rich condition is safe. Ride gently and refit the old jet on return.
4After the jet change, does it still run badly at part throttle?
- Yes: the needle or pilot needs setting too: Tune a slide carburettor in the right order.
- No: done. Write down the jet sizes used for each place.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Main jet too big for high altitude or hot air | Black plug, smoke, better when you return lower | Jetting for altitude |
| Main jet too small for low altitude or cold air | White plug, hot running, pinking | Jetting for altitude |
| Dirty air filter adding to richness | Filter dark or oily | Air filter service |
| Needle or pilot not matched to the new main jet | Fault only at part throttle | Tune in order |