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recently purchased used vn800a carb off ebay. put it on bike and idles well but backfires through carb under heavy throttle. i have no clue what i am doing.. help
 

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First thing I'd do is take it off, tear it down, clean it and restore stock needles and jets. Does your bike have stock airbox and pipes?
 

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Sight unseen...keyboard diagnosis difficult..can only offer possibilities or best guess...but here goes......

prior to installing, did you inspect the cleanliness of the emulsion tube and main jet? And the float valve?
 

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back-firing out thru the carb throat usually means a lean condition (too much air, not enough fuel)
 

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back-firing out thru the carb throat usually means a lean condition (too much air, not enough fuel)
Back-firing thru the carb can mean also that the intake valves are not closing ?
Or the symptom, in the case of valves not closing, is different ?
 

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why did you buy a carb for your bike? what was wrong with original '97 carb?
 

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Open the carb up, inspect the diaphragm for cracks/tears, check the jet sizing, give the thing a really good clean, blow out the passages with compressed air. Once you're sure the thing is clean, clean it again. Carbs can be a bit finicky. You can check for vacuum leaks after the carb and airbox are back on by spraying some carb cleaner around the carb connections/intake manifold. If the RPMs change it means you have a vacuum leak, and the bike is running lean.
 
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