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Adjusting the jet needle on Mikuni TM 38 rack carbs.

Snowmobile How-To's

 
 
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Old 01-05-2011
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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 06:11 PM.
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  #2  
Old 01-05-2011
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It's a damn shame you didn't send a resume to Polaris when I brought it up a while back.
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Old 01-05-2011
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Nice write up Rubi, when I was tuning my RMK after the sap to TM40's I always carried that allen wrench, phillips screwdriver and some needle nose pliers. But pone thing I will add is performance shops sell E-clip washers in a couple different thicknesses to really fine tune your carbs.
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Old 01-06-2011
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Take this next tidbit from sumone who knowz: buy some of those e-clips and plastic washers to keep in a small pill-bottle inside the over-the-brake tool container of your sled. Cuz when you lose one on the trail...they'll come in handy.
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Old 01-06-2011
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So if your running rich what symptoms would you see? and do I understand that you move the E-clip up on the needle?
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Old 01-06-2011
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Rock by moving the clip up you drop the needle down leaning the sled down.
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Old 01-06-2011
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E clip up leans you out in the midrange, and E clip down richens the midrange.

I don't really know how to describe the symptoms of being rich in the needle too well. Taking off from an idle, you will be boggy, smoky, and rich. The rich condition will disappear once you get above 3/4 throttle. If you run it at 1/4 to 1/2 throttle for awhile and hit the kill switch, you should see a black or rich reading on your plugs. I'd say you'd probably foul plugs on a warm day if you were rich on the needle.
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Old 01-11-2011
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Rubi, this is excellent. Thanks for the detail and pictures.
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Old 02-10-2011
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Quote:
Originally Posted by TheKuskokid View Post
Take this next tidbit from sumone who knowz: buy some of those e-clips and plastic washers to keep in a small pill-bottle inside the over-the-brake tool container of your sled. Cuz when you lose one on the trail...they'll come in handy.
Bryan recommends also carrying an extra allen head screw in that emergency kit. That would have saved Powersledder from making a trip from Houghton to Rockland last week.
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