Typical Lean & Rich Conditons; How They Affect Performance
A lean condition is the result of too little fuel and too much air. Slightly lean conditions can create idle and low speed drivability problems. In a worst-case scenario, extremely lean conditions can and do destroy engines. Holes in the pistons, burnt valves and trashed main bearings are the direct result of this kind of lean mixture.
Rich mixtures are the result of too much fuel and too little air. Rich mixtures waste fuel, contributes to carbon buildup, pollutes the air and may cause small performance issues.
A properly set-up carb will require the choke / enrichener on a cold start up, be able to pull away cleanly on half-choke within a couple of minutes and run well without the choke within about a half mile or so.
With a properly tuned carburetor, the engine should idle and run smoothly, and it should be easy to maintain a constant speed without varying the throttle position.
Typical Lean Conditions:
- Poor acceleration; the engine feels flat.
- The engine won't respond when the throttle is snapped open, but it picks up speed as the throttle is closed. (A too-large main jet also mimics this symptom.)
- The engine runs hot, knocks, pings and overheats.
- The engine surges or hunts when cruising at part-throttle.
- Popping or spitting through the carb occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle.
- The engine runs better in warm weather, worse in cool.
- Performance gets worse when the air filter is removed.
Typical Rich Conditions
- Engine acceleration is flat and uneven and loses that "crisp" feel.
- The engine "eight-strokes" as it loads up and skips combustion cycles.
- The engine idle is rough or lumpy, and the engine won't return to idle without "blipping" the throttle.
- The throttle needs to be open continuously to maintain acceleration.
- Black, sooty plugs, a sooty exhaust pipe and black smoke from the tailpipe that stinks of unburned fuel.
- Poor fuel economy.
- The engine works better when cold. Performance falls off as it warms up or the ambient temperature rises.
- Engine performance improves when the air cleaner is removed.
Thanks to MotorcycleCruiser.com