[wyattdunc]

My 2008 crf450r is having problems starting and everything I have tried has not fixed it. I was riding it when it bogged down and cut off and I haven't been able to get it started since. I have rebuilt the cab, put new ignition harness in it/spark plug, and new airfilter. I have tried choking it and not choking it, different positions on the pilot, giving it 3 cranks on the throttle, holding the throttle and the only thing I can get it to do is back fire at most. It has good compression also. Anything to try would be helpful. thank you

[Leardriver]

Since you were riding it when it quit, all of the usual gummed-up carb issues are not my first thought. This is going to require a little more methodical approach.
Are you getting spark? Next, is your valve timing correct, using the left side inspection mark and correct cam marks? Something could have slipped.
If the first two items are OK, then remove the air filter and use a blast of starting fluid into the carb bell, and kick it to see if you can get it to fire for a second. Starting fluid is not my favorite stuff, but for a diagnostic aid to see if carburetion is the problem, brief usage is ok.