I have a Baja 90 I bought used and ran fine. Now starts and idles fine. Runs good at low rpms. At high rpms, past 3/4 throttle, engine sputters, misses, and backfires. No fuel issues, I changed spark plug and checked rotor and pickup for loose brackets thinking timing changed. About 3 months age cdi box worked itself loose and laid against engine and started to melt, but still ran fine. I'm assuming this is the problem but looking for any suggestions. Thanks.

 

It very well could be electrical but it sure sounds like a carburator issue.

 

nhahn

I was out today with my 250 wilderness Baja and went through some deep mud holes and after that it ran really rough, same as you described, starts and idles fine. Runs good at low rpms. At high rpms, over 1/2 throttle, engine sputters, misses, and backfires. I thought the electrical system was wet,as it started to do it right after I got out of a deep water hole but it never went away, at least until I found a loose wire on the coil, the small wire not the coil wire. Once I put it on tight it ran fine.
If it's not the wire see if you can check the coil itself, may have a bad coil.
Hope this helps
sunchaser81

 

I have a Baja 150 atv that had the same problem you describe. It would start fine and run good up to just about 2/3 throttle. I tried just about everything I could to get it to perform better on the top end, both by replacing electrical components and with the carburetor. The first time I removed the carburetor, I just sprayed it down with throttle body cleaner. Then after talking with an ex-Baja technician, he told me to completely disassemble the carb, feed a small wire though all the jets and clean it with PJ1 Professional Carburetor and Choke cleaner. I did as he suggested and now it runs like a top! Better than it did the day we bought it, leading me to believe there was something stuck in the carb from day one.