Tuning the Jet NeedleMaybe a definition is in order before we go any further. The jet needle is that straight, needle-looking thing that you see in the throat of the carburetor when the slide rises in the carburetor bore. The jet needle controls fuel flow from just above idle to about three-quarter throttle. Essentially, the jet needle rises and lowers in the needle jet portion of the emulsion tube, which is a predetermined diameter, and the taper on the needle regulates the fuel flow during partial throttle operation.
It is unlikely that you need to change the jet needle from the one supplied. In the unlikely case that you do, you should be aware of how it works and how to tell if the one you have is too large (leaner) or too small (richer) for your particular engine setup. The initial straight portion of the jet needle affects the mixture from idle to approximately quarter throttle, at which point the needle's tapered portion takes over.
If the jet needle is too lean, part-throttle acceleration will be flat. There may also be some detonation during part-throttle acceleration, although this can also be caused bu other factors. A lean jet needle will also result in a slow warm-up. Raising the needle in the slide by shimming will effectively richen up the mixture by presenting a thinner portion of the needle in the emulsion tube at a given throttle setting.
If part-throttle acceleration is flat, install a jet needle one size smaller (or shim the existing one) and compare the performance. If acceleration is improved, leave the smaller jet needle (or shim) in and take a fairly long ride at a steady speed to give the spark plugs time to color evenly.
Rich ConditionWhile a black, sooty spark plug is a sure sign of richness, there are other indicators that are a bit subtler and better ways of dialing in the jetting.
If your engine responds crispy at low throttle when it is cold, chances are the main jet is one size larger than it needs to be. Assuming, of course, that the idle circuit is correctly tuned.
Poor fuel economics is another sign of richness and because of the way most of us ride our motorcycles, that richness is usually the result of a needle that is too small.
The color of the end of the exhaust pipe is a sign of mixture strength. Dark gray with some black is normal with today's lead-free fuels. If the exhaust color is black, chances are you can drop the size of the main jet.
It may be that you prefer a main jet that is slightly on the rich side of the correct range. A slightly over-rich condition lets most motorcycles accelerate better at very low rpm and very low throttle settings. Be aware that you will lose some fuel economics if you choose to do this.
If you decided to also do the spark plug check, take a spark plug wrench with you and, after a few kilometers at steady speeds, stop and remove the spark plug for inspection. Be careful the spark plug are extremely hot... The body of a spark can be from light gray to brown to dark gray. If the body is black and has a sooty appearance the main jet is probably too rich and a smaller one will need to be fitted.
Lean ConditionA lean condition can quickly become a problem as the engine begins to self-destruct with detonation. Holes in pistons, burned valves, and power loss top the list of reasons to make sure the jetting is on the money. Pinging and knocking, poor acceleration, surging, and excessive popping during deceleration are all good signs-of too lean a mixture. Also, if the motorcycle seems to take a very long time to warm up or needs the choke longer than other motorcycles, chances are some tweaking of the jetting may be in order.
Interesting enough, however, there's a fine line between maximum power and the maximum repair bill. An engine running leaner than ideal street-ability will make the most power. If you are after those last elusive fractions of horsepower, expect your motorcycle to run a little rougher than one that's tuned for ridability. That is to say, you'll have more popping out the exhaust, longer warm-up, tougher starting, all those tell-tale signs that you're running lean.