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Hey guys I have a 2002 crf450 r and I was riding it yesterday and it started to lose power like it was having a fuel or spark problem. I cleaned the carby drained the fuel and changed the spark plug and took it for another spin.. It still lost power under full throttle and started backfiring and then stalled.. I did this 3 or for times and it even locked up on stalling a couple of times.. I thought I may have seized it.. "Oil was fine" but it would still kick over.. I took it back to the shed and changed the coil pack thinking it must be electrical but when kicking it it felt like the compression suddenly built up to take a real effort to kick it over. Anyway symptoms are bogging at full throttle with extreme backfiring and what appears through the kickstart to be raised compression. Any ideas on the cause would be great guys cheers:)

Welcome to CRF's Only. Start by checking valve clearance and check the auto decompression system. Verify auto decomp is functional, spring is good and gap is set correctly.

Is the gas cap vent plugged?
That will cause a fuel starvation issue.

LD is right...we've seen that before especially if you have an aftermarket vent cap. If it is faulty or clogged it will prevent fuel from flowing. It's always a good idea to make sure you have fuel in the float bowl when having these kinds of issues.

Ken

Hey guys it's getting plenty of fuel it's almost as if it's choking on it at full throttle. I'm beginning to think it might be a timing issue or a workshop job:(

check the float height and function. it could electrical as well and you'll need the manual to troubleshoot the electronics unless you have known good ones to swap in/out.