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I have a KFX 400 that will start and idle, but when you give it gas it starts back firing badly. I have put a big bore kit in. Put new valves and shims in the head. Replaced the carburetor. Triple checked the timing. I replaced the generator, voltage regulator, and a new AGM battery. I am unsure where to go from here?
 

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If everything is the same it has to be your are leaning out. If you increased the bore size then the swept volume has increased. Keep in mind stock carb tuning is usually on slightly lean side. So when mods are applied that demand more fuel you will get popping and backfiring. There should have been carb mods suggested with the kit information. Usually bigger jets. Stock ignition timing and curve should be OK since it tends to be on the mild side.
 
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