I have been chasing my tail trying to figure out this intermittent almost random backfire at idle in what seems to be the carbs. I have recently installed 78 mm pistons and rebuilt top end. Everything I am about to describe I have checked multiple times. I have checked cam tension, TDC, valve adjustment, verified timing, float level, carb sync, and for leaks around carbs/exhaust. I swapped out for new and correctly gapped spark plugs (0.030 inch NGK BP7ES). I have checked my ignition coil and spark plug wires. I have checked carbs and plug color to verified it isn’t a problem with tuning being to lean or rich. When I have used the color tune; I‘ll hold for laughter, it would indicate rich only when it popped at idle. When it isn’t making this random once in a while pop everything is normal. My battery is good and no issues with charging. I have also checked brushes on alternator, rotor ohms, magnetism, and seating was correct. I have found no issues with wiring and have checked pretty much any that is in the system. I have checked the slide diagrams for damage and function. I should also say I didn’t have any issues with carbs before I took this bad boy apart. I have done heat cycles and break in procedures to ensure rings were sealed and have rode around 40 miles afterward. I have completed a compression test on both cylinders. The engine has been re-torqued to specs. Also even though I am not 100% sure the whole tank has premium I put that in my last two fill ups so its at least 2/3rd 93 octane. CHECK OUT THE VIDEO AND THE POP HAPPENS AROUND 0:16 WHEN IM FOCUSING ON THE TACHOMETER.
My next thought is to maybe open the gap in the spark plugs?
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