Rejetting-Lean Or Rich

A lean condition is the end result of too little fuel and too much air. Slightly lean conditions create drivability problems. Lean mixture burns hotter. Worst-case scenario: Lean conditions can and do destroy engines. Holes in the pistons, burnt valves and trashed main bearings are the direct result of lean mixtures.

Rich mixtures are the end result of too much fuel and too little air. Rich mixtures waste fuel, contribute to carbon buildup and pollute the air.

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's 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

TIP. To determine if the bike is lean at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich, remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich.

You could also use the choke to troubleshoot these potential conditions!

In tracking down the correct mj on a '99 Yam R6 I would set the choke on after warm up and apply full on throttle on the track.

Up until finding the correct mj's(main jets) I had a lean spot at 12,5k RPM +/-; if I was still to lean on the mj's using the choke would let me pull through the lean spot by reducing the amount of air that was entering the carburetors, but not affecting the fuel delivery as I was full on throttle, and allowing the motor to spin up to the the rev limiter at 15,5k RPM.

Eventually I found the correct mj's and could spin up without a lean condition throughout the power band, then of course you have to hang on for dear life while the little beast wants you off!

I know that it is not a great thing in the entire scheme of life, there are far better things to accomplish, but tuning for performance has the potential to deliver allot of satisfaction; especially when you eventually end up yarding (cannot stay in your draft) naysayers on bigger bikes who have doubted your tuning commitment/understanding!

Wondering how your engine is doing? Look at your spark plugs