Jetting in a nut shell

[racin450r#66]

What is Rejetting? Rejetting is a repetitive process in which jets are swapped in the carb until the desired A/F (air to fuel) ratio is obtained.

What is the Air to Fuel Ratio? The air to fuel ratio (A/F) is the amount of air in proportion to the amount of fuel. The stock engine from the factory has been jetted to provide the correct A/F ratio.

Why do I need to rejet? To maintain the correct A/F ratio to avoid damaging the engine.

What kind of things would cause rejetting to be needed?1)If performance parts are added to the engine, 2) if the temperature changes, 3) if the elevation you normally ride at changes, 4) even the barometric pressure effects the A/F. For example an aftermarket exhaust increases the flow of air thru the engine and that leans the A/F ratio out. Rejetting is the process of correcting the A/F ratio for any performance modifications. When the A/F is leaned out larger fuel jets are required to correct the A/F ratio.

What jets do I need? Due to variations in each engine from the manufacturer, different intake modifications, different exhaust modifications, elevation, temperature, humidity, barometric pressure and other differences, it is nearly impossible to to determine with any accuracy the exact jets needed.




Does temperature effect the jetting? Yes it does.

Does elevation effect jetting? Yes it does.

Does barometric pressure effect jetting? Yes it does.
Will a pilot or slow jet be needed? Pilots or slow jets usually help if the engine backfires when you let off the throttle. Even if no backfiring is present the pilot (slow) jet may be too lean. The pilot (slow) jet is one of the most commonly overlooked jet in the carburetor. If the engine needs the choke for a long time in cold weather a larger pilot or slow jet will help. If unable to obtain a smooth idle with the mixture screw going up a size or two in pilot (slow) jet will help. I put on an air filter and exhaust and my engine seems to run fine with the stock jets, do I need to rejet? The engine may appear to run fine but must be checked make sure the engine is not running lean. Most intake and exhaust modifications require rejetting.

Will the engine be damaged if I run it lean and don't rejet? The fuel in the air and fuel mixture helps cool the cylinder. When the air flow thru the engine is increased with performance parts the cooling effect of the fuel decreases. The engine now runs hotter than it was designed to and may be damaged with prolonged high temperature use.

What symptoms indicate the engine is too lean?
a. Acceleration poor, surging.
b. Performance improves slightly as engine warms.
c. Better performance when choke/starter is used.
d. No color, overheated spark plug electrode (see reading plugs FAQ).
e. Removal of air cleaner makes condition worse.
f. Popping back thru carburetor.
g. Engine pings or knocks, excessive engine heat

The order which provides the best performance is correct the: 1) idle, 2) slow or pilot jet, 3) re-adjust the idle with the new slow or pilot jet, and finally 4) the main jet. As the temperature fluctuates thru the year rejetting will be needed to compensate for the change in the A/F. Since the A/F changes it is good to have a few extra sizes of slow or pilot jets and several extra main jets.


Idle screw: Let engine warm up and adjust for smooth idle.

Pilot screw: (sometimes called a mixture screw) Turn screw in GENTLY until it bottoms. Turn out 1/4 turn at a time until smooth idle is obtained. Normally do not turn the pilot screw out more than 2.5 turns. If you must turn it out more than 2.5 turns go up to the next larger pilot screw.

If the above adjustments did not get the engine running properly, the jets will need to be changed.

Pilot jet (Slow jet): If unable to obtain a smooth idle with pilot screw, increase the pilot/slow jet until a smooth idle can be obtained. The pilot/slow jet will eliminate or reduce popping that occurs when letting off wide open throttle. The pilot/slow jet will also improve cold weather starts. Go up 1 pilot/slow jet at a time, don't skip sizes. After installing a larger pilot/slow jet, re-adjust the pilot screw.

Main jet: Start with the biggest numbered main jet and run at wide open throttle, the engine should stumble at wide open throttle. Install the next smaller size until the stumble is gone. Once the main jet is correct re-adjust the pilot screw first and the pilot(or slow) jet if needed until proper idle is obtained.

And one last thing one of the biggest mistakes people make is not adjusting the valves, out of adjusted valves will leed to hard starts and make it seem like you need to adjust your jetting.

more to come soon. i am really tired from typing right now.

[Bootlegger]

Not to steal your thread....but here is a easy way to look at it and tune your carbs....

1/8-1/4 is your pilot jet
1/4-3/4 is your needle
3/4-WOT wide open throttle is your main jet.

I tune carbs alot. An example...if your running great at 3/4-WOT but your midrange is popping or just falling on its face you know to move your needle.


Good right up thought...

[wildwillie]

i put a slip on muffler on it and it was to loud so i put the stock one on and now it runs bad and no power do i need to re set the comp.on it and how do i do that?