Too rich ?

I have a 2000 WR400, starts great, runs good close and 3/8th to 1/2 throttle after that it starts to belch and after fire (out of the pipe). It has all the mods done and I am running a WB R pipe with spark arrestor. I think I am running to rich and need to move the clip down on the needle ? I am I right ?

I had the exact same problem on my 426. Well actually I caused it, I thought I was running lean so put a richer needle in (JD blue, was running red). The bike started and ran fine right up to about 1/2 throttle when it started popping and surging. Put the red needle back in and all's fine again. So yes you're right, you're running rich, try dropping the needle a clip or maybe try a slightly leaner needle.

Now I am confused. The needle was sitting at the 2nd position so I moved it to the 1st. and it still did the same thing. So I went the otherway dropped it to the 3rd and it ran a little better but not much. Went to the 4th and still the same problem. So looked at the BK mod and did that. getting better but I still think I am rich someway in the sequence. Man what an ordeal.

Try a completely different needle, possibly the one for a 2000 YZ400.

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