234wheeler

Remove the carb top cover and make sure the set screw that clamps to the throttle shaft is tight. It's a t9 torx set screw. They have been known to come loose. A little loctite helps if from backing out!

Yeah, I tried 127.5 and even a 120 main and the problem got worse. The only issue it had once I put the 132.5 main back in was a stumble after backing off full throttle. This bike runs fine at lower elevation, which tells me it just needs more air than the intake system can provide. The 1990 DR250 came with a 31mm carb, the 350 with a 33mm. I am wondering if this could also be an issue, but I doubt it.

I finally decided to spend some time on my DR350 and get it running right at high altitude (7000 ft.) After spending the better part of the weekend trying every combination, I ended up back with the Thumper Racing jetting suggestions for their 346cc Big Bore Kit (1990 DR250 dirt model). 4th notch down on the needle, 132.5 main. The bike accelerated fine but had a weird stumble when I backed off the throttle. Here's what fixed it. I removed the boot between the carb and air box and the problem disappeared. I rode it on the street to reduce the chance of sucking dirt. The answer...the air intake system on this bike is too restrictive. I opened up the top of the box as big as it would go, cleaned the foam air filter and lightly oiled it, I even left the side access cover off and the problem returned. The solution....If you ride at high altitude, you need an aftermarket air box. I would be willing to bet the problem is even worse on the bigger engines (440 kits etc.) Hope this helps