| Jetting is NOT brain surgery or voodoo... |
| Date: Wednesday,
May 2, 2001 From: Andy Sickle The object of "piping and jetting" is to increase the airflow and fuel supply in to the engine, presumably as much as possible so long as they (air and fuel) maintain the correct ratio. Torque is directly proportionalto how filled the cylinder is (as well as many other factors). Because of intake and exhaust resonances (as well as port characteristics), cylinder filling, and thusly torque, varies throughout the RPM range. The "pipe" is actually a pretty complex resonator that it is trying to 'pull' as much air out of the engine as possible during the exhaust stroke. This can only achieve so much if there wasn't already a proportionately sized 'gulp' of exhaust to 'pull'. This 'gulp' had to have been delivered in to the engine during the intake stroke. That is why a high-flow intake (K&N, BMC, individual filters, etc.) is necessary to gain the most performance from a pipe. So we buy a good air filter to permit more air to the intake stroke. The airbox is a resonator just like the pipe, and it is designed to 'pump' air in to the engine at a chosen rpm range(s). Okay. So now we have as much airflow as we can get 'pumping' in to the engine and being 'pulled' out. This is where jetting comes in. I started out by mentioning a "correct ratio" for air and fuel - for any given quantity of air inside the combustion chamber there is an ideal quantity of vaporized fuel (ideally about 12 or 13 to 1 for maximum power under acceleration). Engine manufacturers typically shoot for a 14.8:1 mixture (or slightly leaner) to minimize emissions and maximize fuel economy. This means that even if we stuck with the stock pipe and filter, we could probably get a modest gain from larger jets alone. If we install an exhaust and filter without rejetting we will definitely be running too high of an air-to-fuel ratio (too lean). If we rejet too big, we get too low of an air-to-fuel ratio (too rich). If we rejet just right we get maximum torque. In reality, any jetting will probably be a little off at some parts of the rpm range due to the aforementioned resonances. So how do we know if we've got the right jetting? The real answer is EGA (Exhaust Gas Analyzer) and Dyno testing. A properly equipped performance shop with a knowledgeable tuner can get your jetting to its best within a few tries. This can be expensive; not all performance shops a properly equipped; and most tuners are nowhere near as knowledgeable as they would have us believe. The next option is do-it-yourself. Here's how: 1) Start with the biggest jets in the kit and the recommended needle
settings. Set your mixture screws according to jet kit specs. 2) Check your sparkplugs for condition and color, or better yet, but in brand new plugs. Run the bike at idle, cruising, and hard acceleration. If your engine revs well but without a lot of power and it runs hotter than usual, STOP! You are too lean. This can cause serious damage to your engine and may even cause piston seizure (I know this from experience, that is why I have since studied so much about mixture). 3) Barring any symptoms of overly lean mixture, check the plugs after fifty miles or so. They should have a tan-like color and no white powdery edges. If they are whitish, you are lean. If they are sooty, oily, or they show dark (blackish) finish, you are rich. 4) A rich mixture:
A lean mixture:
5) Mixture screw is only relevant when throttle is almost completely closed (at idle and during closed throttle deceleration). An occasional backfire when engine braking is OK, this indicates a slightly rich mixture setting. 6) Pilot jets (sometimes called idle jets) should not be strongly affected by intake and exhaust changes. However, if your bike heats up much more rapidly during idle you might want to close your mixture screws a little (try a half turn or less at a time). Another solution is simply to not let your bike idle for long periods of time. As a last resort, upping your pilot jets by one size should do the trick. 7) Needle settings only affect partial throttle acceleration and constant speed cruising at low to mid rpm's. Raising the needle (clip moves toward pointy end) richens partial throttle operation and lowering the needle (clip moves toward squared end) leans partial throttle operation. 8) Altering the airbox IN ANY WAY changes the resonance of the intake charge. This will shift torque surges and dips to higher or lower rpm's. It is possible to shift the airbox's surges to fill the exhaust's dips - resulting in a nearly flat torque curve. 9) Finally, you can just stick to a known formula: results: 72.5 bhp at 9550 rpm P.S. I'm sure that there is infinitely more useful information from many people's experiences and that my information is disputable. I have tried to keep it to what I know for fact, and even then, I reserve the right to be wrong... Andy Sickle
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