17-18 TC/SX Run on at Idle (jetting issues)

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12/13/2017 12:55 PM
Edited Date/Time: 12/13/2017 1:31 PM


Been reading various jetting threads with great interest. Seems a lot of SX/TC250 owners are rich at idle or just off idle. This seems to be the reason why some are going to a richer slide (my opinion, just a guess).

I and it seems several others are having the opposite issue. Running on at idle (or words to that effect) which indicates a lean condition at idle or close to. I have seen several people post with this problem (I have it too) with no remedy, or no answer at all. I was kinda waiting for someone to come up with the answer but I have not seen a cure.

In full disclosure I have a 38 PWK short body on pre order from JD jetting. I also have a STIC Metering Block on order from Harris Performance all in an effort to quelch my jetting issues.

I think I have found a potential cure for the stock TMX. I say potential because it is winter time here in West Virginia and I will not be able to test. This is a cure on paper (I hope)!

I know all about the main tuning features of a carb, although I am far from an expert. I usually jet on the rich side for safety. I have messed with the pilot, main, needles, needle jet, air screw etc. I just came upon a couple of references online about the 38 TMX short body where they reference a "starter jet". I have not heard of this critter before. I always thought that was a non tune able portion of a carb. Well low & behold this TMX has a tune able starting system.

In the video below at the 1:48 mark, you will see where this little critter lives.

This is a blown up pic of what it looks like.


I do believe (guessing) that if we go richer on this jet this MAY solve our run on at idle issues. Plus the first time I tried to start my new bike, it was a bear to start & needed a little throttle to start. I am damn near to the point of convincing myself that this is the answer. What say you?

There is VERY little info about this starter jet on the interwebs. I really had to dig & scratch around to find these three. In this link , where I got the pic from above, they talk about the starter jet (Controls enrichener circuit on many Mikuni carburetors) available in 20 to 100 sizes. In this link which is a PDF file, on the bottom of the first page is a reference to this jet in 20 thru 120 sizes.

Also found this link over at Thumpertalk where they briefly talk about this jet.

The stock size on a 18 TC250 is 80. So far I have not been able to find which way to go for a richer jet. Best guess is a larger number. Although that is just a guess at this point.

That is all I have at this point. More to come as I learn more about this elusive critter.

A big thanks goes out to Donnie at Morgantown Powersports and RJ at Beaver Creek Cycles

12/14/2017 5:23 AM

I'm not sure what your results will be, but I'm not optimistic. Mainly because I've ridden SX's with completely different carbs and they all have issues. My opinion is that it's not a carb issue. I believe it to be a cylinder head and ignition issue. The TMX isn't a great carb, but it would work acceptably given a well designed engine to work with. Go to KTMtalk, and search a guy called powercdi. He has built an ignition with 3 dimensional mapping that works with a powerjet Keihin carb and it's claimed by him that it cures the issue. Expensive fix though, I'm waiting impatiently for KTM and Husky to fix the issue before I buy mine. Until then, my YZ runs perfectly every time out.


12/15/2017 7:03 AM

From what I have been able to find, this jet is for the choke system only. Bummer...


A big thanks goes out to Donnie at Morgantown Powersports and RJ at Beaver Creek Cycles