Make sure you have a good seal where the mufflers meet the headers, if you have a leak there that could cause popping. That being said, a correctly jetted engine with low restriction exhaust should pop a little. You can try reading your plugs but it is harder to do with ethanol treated gas. If the intake is stock you may get away with a bigger pilot and maybe a shim on the needle but a dyno run is the best way to see where you are now making it easier to judge what needs to be done. A dangerously lean engine will run great right up until you burn a valve.

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