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Old 01-05-2011
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Join Date: Nov 2007
Location: Alexandria, MN
Posts: 3,617
Default Adjusting the jet needle on Mikuni TM 38 rack carbs.

The pictures I'm using are from the carbs on a Polaris 600 VES motor. The carbs are the flatslide TM 38 racks. I'm pretty sure that this writeup will apply to any size TM rack carbs used on a wide variety of sleds from multiple manufacturers.

The jet needle effects your jetting from right above idle to about 3/4 throttle. From idle, the needle's effect rises until its' peak at about 1/4 throttle. From there, the needle's effect falls until about 3/4 throttle where it is no longer a factor.

On my particular application, the 2000 600 VES motor from Polaris, the stock setting of the needle clip is in the third slot from the top. Since factory jetting is on the rich side for this sled, it is safe to raise the clip one notch which will lower the needle and lean out the midrange. Leaning out the midrange will help you if your sled is loading up when you crack the throttle.

Well, lets get some pictures posted and tear into this simple little project. Lift your hood and address your carbs... Remove the two screws on the top cover:

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Here's what you'll see inside:

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That tiny allen head screw holding the j-shaped retaining plate is your first order of business once you're inside. Take a 2.5 mm allen wrench and loosen the screw.

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You don't need to remove it. Just loosen it enough to turn the retaining plate out of the way. It should look like this when you're ready to remove the needle.

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Once you've got things looking like the above photo, you're ready to remove the needle. Take a needle nose pliers and carefully pull the needle out. Be cautious because the needle is fragile aluminum.

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Here's your needle once you've got it out. Notice that there is a 1mm plastic washer under the E clip. Don't lose it or forget to reinstall it.

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Here's a better picture of the needle so you can more clearly see the five grooves in which the E clip can fit. Observe that my clip was in the second groove from the top. That means I was already one setting leaner than stock.

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My goal in this project was to raise my clip one groove so that I'd be in the top setting. This means that the needle would be as low as possible, thereby impeding the flow of fuel in the midrange, leaning out the idle to 1/2 throttle area. The only reason I was putting my clip in the top ring is that one groove above stock was still too rich. At the time, I was riding high elevation with warm spring temperatures. I don't recommend the top groove for any normal riding conditions.

Once you've carefully removed the E clip and put it in the desired position, simply replace the needle making sure the 1mm plastic spacer is still on there. Pivot the j-shaped retaining plate back over the top of the needle and tighten the allen screw. Replace the top plate on the carburetor, and repeat all the steps on the other side.

Performing this five or ten minute adjustment should improve your throttle response quite a bit from 1/4 to 1/2 throttle.

Last edited by Rubi : 01-05-2011 at 07:11 PM.
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