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Backfiring flames not starting

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I've got a 2000 drz400. The bike sat for 3-4 years before I bought it. The jets were clogged, so I rebuilt the carb, cleaned the gas tank, changed all the rubber, and the bike ran great. I used the bike for commuting to work daily, never had a problem. Then, I was heading home after an 8 hour shift and the bike lurched forward a few times, lost all acceleration, and backfired severely. There were flames out of the exhaust. Since then, I have re cleaned the carb, but the bike won't run. It will whine the starter, but never catch. If I spray carb cleaner into the intake it stumbles for a second, then dies. I checked the carb, cam chain timing, and compression. i also replaced the spark plug. Any thoughts?? I feel like its a fuel problem. I pulled the plug today and it barely smells like gas.

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It has the vacuum pulse petcock. I took apart the petcock and cleaned it well. Gas flows out of the main, but not the smaller.

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Yes. The smaller pipe coming off the petcock. I'm not sure if the petcock is working properly.

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2 things spring to mind...

 

You have checked the compression - how?

Have you done a leak down test or a proper compression test or does it just feel like there is compression there?

 

Have you checked the valve clearances to see if one is now more open, or stuck open.

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It's got spark. And, I have done a proper compression test. I have not checked the valves. I have the feeling it's a fuel problem. The plug looks like its running very lean.

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