Some kits offer features with...
Some kits offer features with greater adjustability.
Top end; 3/4 to wide-open
At this stage of the game, the majority of the fuel flow is controlled by the main jet (remember some fuel is still flowing through the pilot jet). The time-honored method of determining main-jet size is via a plug chop. If you've never done one, a plug chop is pretty simple. Start with new spark plugs. Run the bike flat out (full throttle), in as high a gear as possible, for eight to 10 seconds (don't hold the bike at the rev limiter, as it'll give you a false reading). Without cutting the throttle or slowing down, simultaneously pull in the clutch and hit the kill switch. Coast the bike, with the clutch still in, onto the shoulder and remove the plug(s). The plug insulator should be tan to grayish-white. A white plug indicates a lean mixture; a dark gray or black one means it's rich. You then select the main-jet size accordingly.
Unfortunately, plug chops present a few problems. Some bikes require an inordinate amount of work to remove the plugs--at least more than you want to do on the side of the road with a smoking-hot bike. And in most parts of the country, the law takes a very dim view of banzai runs down the main drag followed by an impromptu tuning session on the side of the road.
The alternative, and the method I personally prefer, is to use a dyno. It's certainly a lot easier and arguably more accurate, but it may not be convenient.
One of the reasons I'm so stoked about jet kits is because they remove 95 percent of the guesswork. Be that as it may, even the best kits require some fine-tuning. Essentially a jetting problem can always be reduced to one of two choices: Is it lean or is it rich?
To determine if the bike is lean (to little fuel for the amount of air reaching the engine through the carburetor) 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 (too much fuel in the air-fuel mixture)), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich.
If your bike is equipped with a manual fuel shut-off (or you're clever enough to disconnect and plug the fuel lines), take the bike for a ride with the fuel turned off. If it runs worse as the float bowl drains, it's lean. If it gets better, it's rich.
A lean condition is the end result of too little fuel and too much air. Slightly lean conditions create drivability problems. 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.
Main jets (bottom) may be...
Main jets (bottom) may be exchanged for larger or smaller size to make the mixture richer (a greater amount of fuel mixed with the incoming air)) or leaner (less fuel) at full throttle.
- 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.
- 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.
- 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.