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02 crf450/490 big bore backfiring

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I have a buddies bike that I have been working on that has me completely lost any help will be appreciated. It's an 02 crf 450 with Athena big bore 490 kit, the bike ran perfect before a fuel screw fell out while riding now all it does while running is backfire.🔥💨 

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11 minutes ago, Str8mobbin said:

I have a buddies bike that I have been working on that has me completely lost any help will be appreciated. It's an 02 crf 450 with Athena big bore 490 kit, the bike ran perfect before a fuel screw fell out while riding now all it does while running is backfire.🔥💨 

You probably have an old oring stuck up in the fuel screw passage....assuming you have it assembled screw/spring/washer/oring, it couldn't be anything else....directly related.

Dental pick it out

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I checked no old o-ring I had this issue on my bike so I made sure that was the first thing I looked for when it wasn't running right and I have it set up exactly like that screw,spring,washer,o-ring cause that's how it was before 

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I

21 hours ago, Str8mobbin said:

When reving the bike up with choke  off backfires, with choke on idle is pretty high 

I'd go right to the pilot jet.

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