You may have heard the term "jetting" before and wondered what it meant. Jetting is the act of adjusting the separate fuel circuits in a carburetor to optimize the fuel/air mixture going into the engine. To the untrained person jetting can come across as being very complicated and some people fear the process. Once you understand which circuits control what part of the throttle cycle, you will learn that jetting isn't at all that difficult. The following will describe what each circuit does and how to adjust that circuit for optimum performance. This assumes that you have a Keihin FCR type carburetor (the information translates to other carburetors as well) which is found on almost every modern high performance four stroke ATV engine.

Fuel screw: Adjusts the "hang up" of the engine when you go from wide open to idle quickly. If your engine doesn't smoothly idle back down or "hangs up" when you let off the throttle you need to adjust the fuel screw out (richer) to clean this up. If your engine idles down and then dies you need to turn the screw in (leaner) to allow less gas into the engine. Play around with the screw until your engine idles down smoothly.

Needle jet: Controls 1/4 to 3/4 throttle. You can tell the most when this is out of tune when you do holeshots. If your engine is bogging when you dump the clutch on a holeshot or you are riding at a constant 1/2 throttle and you engine is bogging then you need to move the c-clip up one notch (drops the needle making it leaner). If the engine is cutting out in these situations you need to move the c-clip down one notch (raises the needle making it richer). Setting the c-clip in the middle slot is a good place to start.

Main jet: Controls 3/4 to full throttle and also affects all other jets slightly. This circuit is tested when you hold the throttle wide open. If your engine is bogging the you need to make the jet leaner (lower number) and if it is pinging or cutting out you need to make it richer (higher number).

Jetting can be very touchy and tedious. At any rate your engine should rev at a constant RPM at any throttle position and should rev up and idle down with no hesitations. Once you start getting your hands dirty and listening to your engine you will become an expert at jetting. Hope this helped and good luck!

Rob Shoemaker is an experienced ATV racer and mechanic. He invites you to visit [] where he has published numerous articles on ATV safety, maintenance, riding, racing and much more. You will also find a video section with high performance ATV racing and riding featuring some of the top pro's of this era. To gain more knowledge about ATV riding and racing go to: []

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had some people ask me some carb questions so I came up with some answers as to how they work and some basic principles to follow for jetting
ATV Front Drive Check
On behalf of Expert Village, my name is Tom Roland here at Cottonwood Motor Sports and I am here to tell you about doing service on a Polaris Quad and this is a fairly representative of macho, many of them. There are a couple of U-joints on the Polaris for the front drive and they don't have brackets on them any more but they have a pin that locks them on. You need to make sure that they are not all rusty and that there is no damage and that the U-joints don't look like there is no play back and forth in them to where they are going bad. This is a new unit so we don't see a lot of problems with the U-joints on the Polaris but it is all stuff that has to be looked at because the prevention and maintenance cures a lot. It is a lot cheaper to fix it now than it is to fix it after it breaks and it's gone away. This clear tube right here is the vent tube for the front differential, front gear case and this allows expansion and contraction of the air due to temperature changes and altitude changes so that it equalizes outside. It goes up high so that it keeps water from getting in the gear case. But people sometimes go through real deep water on these and it is possible for water to get in there. The other thing that you want to do is make sure the line isn't damaged and that it is still attached to the gear case because if it's not, that just makes it a lot easier for dirt and water to get inside that gear case.
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ATV Carburetor
High Performance ATV Carburetor Jetting Explained
By Rob Shoemaker
Pilot jet: Controls 0 to 1/4 throttle. This jet requires the least amount of adjusting over the rest. If your engine cuts out when you rev it up then you need to make this richer (higher number) if it bogs when you rev it up you need to make this leaner (lower number).