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