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Old 01-23-2011, 06:44 AM   #115
artbone's Avatar
Joined: Dec 2010
Location: San Miguel de Allende, Mexico
Oddometer: 73
Originally Posted by JollyMeanGiant View Post

Here's my experience with jetting the XT250 for higher elevations. I got the main jet part number from a parts fiche and then I tried variations of that number at to find the other sizes. These jets were used by a bunch of Yamaha thumpers in the 1980s and 1990s.

Yamaha part number - main jet size

288-14343-70-00 is #140
288-14343-68-00 is #135 standard main jet for 2008 XT250
288-14343-65-00 is #130
288-14343-63-00 is #125
288-14355-61-00 is #122
288-14343-60-00 is #120
288-14343-59-00 is #118

I ride from 5000ft up to 12000ft. On long hills and full throttle in 4th and 5th gear, my XT (with #135 main jet) had a bad rich stumble. I drilled some holes in the airbox, but it wasn't enough, so I got the smaller jets. I finally ended up with #122 main jet. I did not make any changes to the needle. The bike runs much better and I'm happy.
I found the jet kit on Ebay and it's from 6sigmajetkit. I think you can put in 6sigma jet kit and go to their online store. When I got the kit I was impressed that they included a sheet that address my bike specifically. I had sent them an email that the bike was rich and that I lived at 6500 ft and they sent me 2 smaller jets (#125 and #130) and several small shims, etc.

As far as the needles are concerned, on my bike the needle only has one groove. There is a thick plastic shim under it. They sent enough shims that you could adjust it up or down but, after changing the jet, I didn't feel that I needed to do that.

The kit included a drill bit and wood screw to pull out the alum. plug that covers the low speed adjustment screw, which I did as per their instructions and that is definitely something that anyone that has one of these bikes needs to do. My bike starts so much better with just that change. The stock position of the air screw on mine was about 1 1/4 out and I changed it to 2 1/4 out and it really made a difference.

The last thing I did was take out the spark arrester and saw off about 6 inches and reinstall it. With these mods the bike will now touch 80 MPH on downhills and hold about 75 on level ground. More importantly, it starts easily from cold after sitting a few days and doesn't have any weird lurches and stumbles when I roll off the throttle or on transitions from full throttle to part and back to full. It runs like it's supposed to.

The kit includes a small drill to drill a hole in the metal part of the vacuum diaphragm which is supposed to improve the throttle response but I elected not to do this. I ride in the dirt and am more interested in smooth, predictable response in low traction situations than in peak power.

The kit cost about $40, the shipping was about $9, then I had to pay another $10 customs so it wasn't cheap but it included everything I needed and I didn't have to take the carb off, figure out what I needed, order it, and wait 2 or 3 weeks to finish the job. Whole thing took me about 8 hours from taking the tank off to completing the test ride.

If I had it to do over again I would just order the jets from for about $8 each. I've already got all the drill bits, wood screws, etc.

The hard part of this job is getting the carb off the bike without screwing something up. I'm thinking of taking a lot of the unnecessary EPA crap off. Has anyone taken the carb heater off? It never gets to freezing where I live so what do I need with a carb heater? Also, the little plastic box that mounts above the intake boot between the air box and carb, and the sort of pump or valve that mounts to the frame on the left of the carb and attaches to the airbox with hoses and vacuum lines. I'm thinking of taking all that stuff off and plugging up the holes. That would really make the bike much simpler to work on and I don't think any of those things has an effect on performance.

Any thoughts on this?

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