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 ATV Jetting basics...

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Bootlegger
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Joined : 2008-01-15
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Location : Crossville,TN

PostSubject: ATV Jetting basics...   Mon Apr 20, 2009 10:20 pm

This was wrote by Scribner and copied to post here.

Quote :
Air/Fuel Mixture
If you are having idling issues or poor throttle response, the first thing to check is your air/fuel mixture. The air/fuel mixture controls from idle to 1/4 throttle. The best way I have found to check it is to let the bike warm up to operating temperature. Let the bike idle in neutral and give it wide open throttle. If the bike has a hesitation your mixture is off. If the bike bogs down and then revs up fast it will be lean. An up and down or "rolling" idle is another symptom of a lean mixture. If the bike idles rough and kind of low, it will be rich. The best place to start with your settings is about 2 turns out. Tighten the screw lightly until it seats and then back out and count slowly.

Pilot Jet
If you get more than 3 full turns out on your air/fuel mixture, you will need to step up on your pilot jet size. If you get under 1 full turn then you will need to step down one size on your pilot jet.

Next Is Your Needle
Your needle will control 1/4 through 3/4 throttle positions. If your bike is "popping" or "missing" at a constant speed, your needle will need to be adjusted. If you give your bike full throttle from a constant speed and experience a hesitation, you are lean on your needle. You will need to lower the clip on the needle (this raises the needle), allowing for more fuel flow through your needle tube (dump tube). If the bike is trying to "load up" at a constant throttle position, your needle setting is to rich. Raise the clip on your needle (this lowers the needle), which will restrict the amount of fuel coming through your needle tube.

Needle Tube
Your main jet screws into a brass tube in your carburetor called the needle tube (dump tube). If you run out of adjustment on your needle then you will need to replace it, although very few times have I seen that this needed to be replaced. The only time that I have seen it need to be replaced is when running methanol.

Main Jet
Your main jet controls 3/4 to full throttle. If you are at wide open throttle and the bike is revving up but not really pulling, you will be lean. If the bike is slow to rev, then you are rich. The best way to check your main jet settings is to do a spark plug reading. Run the bike at full throttle until top speed is reached, then shut it off using the kill switch. Keep the throttle wide open until you come to a full stop. Pull the plug and look at the porcelain down in the plug. The all around best color is a light chocolate color on the plug. If you run the bike at full throttle all the time you might want to run one step bigger on the main jet, to keep the motor from burning up. If you run short distances at full throttle (ie. racing) you can run one step smaller on the main jet for maximum power, but keep the full throttle distances short.

Some More Jetting info....

lets start with elevation,
higher altitude means less air density which means smaller jets

usually you would need to go down one main jet size for every 1700 to 2000 feet of elevation you ascend.

The example would be, say you are at sea level and you are running a 160 main jet, you'd need a 140 jet for 4000 feet.

in colder conditions you need a larger jet as cold air is more dense so the air temperature plays a role in it as well.


you'll also lose horsepower the higher you go up in elevation.
usually this means You can figure on losing about 3 percent of your power for every 1000 feet you go up.
even with rejetting your ride.


Pilot jet:
pilot jets control the low speed and idle of your mix.
if adjusting the mixture screw won't improve the low-end running speed it's usually time to get a different pilot jet.


Needle:
the needle controls the fuel mixture throughout the midrange. The shape or taper of the needle determines how much fuel goes into the engine at a given throttle position,

usually your needle is for fine tuning once you have the right main and pilot jets figured out.

If the machine stutters or hesitates before it brings on the power, that part of the needle's taper is usually too small and sometimes the only way to fix it is to get a needle with a different taper.
Finding the right needle can be a real pain so try moving the clip first, in minor cases this will fix the problem, but with engines that have been severely bored and have a higher performance exhaust, you are most likely going to need a new needle and a new pilot and main jet.

Main jet:
main jet comes into play at three-quarters open to full throttle
we all know a bigger jet means more fuel to the engine!!!

but it also can mean a cooler running engine,
while a one size smaller jet can mean more power for a while, it can also mean burning your engine out.
a slightly richer main jet can help your engine last longer by helping it run cooler and still make good power, so be sure that your main jet is big enough.

Afterall you can't ride your machine if the rings are shot or the piston skirt is broken due to overheating.

A quick tip is Don't change more than one jet at a time.


Some more info from a different source,,,,



Let's start by talking about the effective range of each carb circuit. Remember that the each adjustment effects a range of throttle positions, NOT engine RPMS... Always try to think "throttle position" -not- RPM.

The three main carb circuits are;

* The main jet - 3/4 to full throttle
* The needle + needle jet - 1/8 to 3/4 throttle
* The pilot jet + pilot screw - idle to 1/8 throttle


The Main Jet
The main jet primarily controls fuel flow between 3/4 and WOT (wide open throttle). Once the throttle is open past about 3/4 the needle is pulled high enough out of the needle jet that the size of the main jet begins to control fuel flow. Main jets are identified by a number. The larger the number, the larger the hole in the jet. A larger hole will allow more fuel to flow, giving a richer mixture. So basically, the higher the main jet number, the richer the fuel mixture will be between 3/4 and WOT.

The Needle and Needle Jet
The needle and needle jet are the componets that primarily regulate fuel flow between 1/8 and 3/4 throttle. The needle jet is seldom changed in everyday tuning, but it's still worth mentioning the fact that it's there. The needle itself is a basically a tapered rod connected to the throttle slide. As the slide opens the needle is pulled upwards. The needle is tapered so that as it's pulled up, it takes up less space in the needle jet. This allows the fuel flow to be gradually increased as the throttle is opened. The needle has a clip that allows it to be lowered or raised in relation to the carb slide, which gives an overall richer or leaner setting. Raising the clip up a notch drops the needle down farther, causing a leaner mixture. Lowering the clip raises the needle up, causing a richer mixture. You can also get different diameters, and tapers of needles. If the clip is lowered all the way, and the mixture is still lean, you need the next size smaller needle. If the clip is raised all the way, and the mixture is still rich, you need the next size larger needle.

The Pilot Jet, and Pilot Screw
The pilot jet, and pilot screw control fuel delivery from idle to approximately 1/8 throttle. The pilot jet is similar in design to the main jet, basically a small screw with a calibrated hole in it. As with main jets they are identified by number, a larger number pilot jet is richer, and a smaller number pilot jet is leaner. The pilot screw on an ATV regulates fuel flow.. Turning the screw in makes the mixture leaner, and turning it out makes it richer. If the pilot screw winds up being turned almost all the way in, you need the next size smaller pilot jet. If the pilot screw is more than about 2 1/2 turns out, you need the next size larger pilot jet. Basically the pilot screw is an adjustemnt that allows you to fine tune the pilot circuit.

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