Getting Started with Jetting

Engine condition

In order to get good jetting results, the engine must be in good working order. This includes: good reeds, clean air filter, good packing in the silencer, no leaking crank seals or other air leaks, and good top end condition.

Rich and Lean

Rich and lean in the context of jetting refers to the fuel:air ratio. More fuel relative to the amount of air is richer. Less fuel to the amount of air is leaner.

Jetting rich and lean is not the same as fuel ratio (the ratio of gas:oil) rich and lean. In fact, in the extreme, more oil actually means leaner jetting because there is less gas in the fuel mix. At typical fuel ratios (60:1 to 32:1), you can pretty much ignore fuel ratio variations in your jetting.

Lean in a range makes the engine very peppy and fast revving under light/no load, but lacks power.
Rich in a range makes the engine dull and slow to rev.
Rich on a low throttle circuit, then lean as the throttle opens farther, produces a strong power surge or hit.

More power is produced when you are slightly rich as opposed to slightly lean. If you set your jetting for perfectly clean operation with absolutely no stuttering under very light load (such as very slowly rolling on the throttle on flat smooth conditions) you are probably giving up a bit of power or grunt, especially at the low end.

Pilot Circuit

Lean causes the engine to surge at low RPM’s, bog or cut-out when the throttle is opened quickly and rpm will hang and slowly settle down.
Rich causes hard starting, plug fouling at low RPM and sputtering as the throttle is cracked opened. Can cause randomly timed pipe bang when unburnt fuel burns in the expansion chamber.

Main Circuit

Lean causes a hesitation, then resuming when the throttle is rolled on. Will cause engine to run hot and melt the piston if very lean.
Rich causes sputtering and a dull sound and slow acceleration at WOT. Will increase fuel consumption.

If the engine surges when the throttle is closed, the low speed circuits are lean.
If the engine hangs at a certain rpm, then finally drops to normal idle, the low speed circuits are lean.
If the engine stalls or comes close, then returns to idle, the low speed circuits are rich.

The Needle

The needle is the main influence for most of the throttle range. The needle taper, starting diameter, and clip position are the major contributors to jetting from about 1/8 to 7/8 throttle.
That annoying stutter at 1/4 throttle is often corrected with the needle taper and clip position.

Throttle chop method of setting air screw to adjust pilot circuit

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High Speed test to set main jet

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