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Backfiring, especially if it's happening through the intake, can also be a symptom of a too-lean condition. Too lean makes temperatures go way up and the fuel/air mixture sometimes ignites as soon as it enters the cylinder, before the spark plug fires and before the intake valve closes (hence the popping back through the carb). And it can become a vicious circle because the resulting too-early ignition can increase temperature even more. The end result is holed pistons. I've never worked mine hard enough at night to see if the exhaust manifold glows, but it wouldn't surprise me to see it glow just a bit. There really is a lot of heat there. However, it may be indicating that it's *too* hot. The next time you're running it hard at night, and it starts glowing and backfiring, I'd leave the throttle open and adjust the main mixture screw out and see if the manifold glows less and the backfiring stops. The richer mixture has a cooling effect and slows down ignition just a bit. In fact, the old Kawasaki air-cooled 3-cylinder two-stroke motorcycles (most of my engine knowledge is based on bikes) ran a larger main jet in the middle carb just for this reason. To help cool down that middle cylinder with its limited ability to dissipate heat. Another symptom that should occur if yours is running too lean would be a cold-blooded nature. The machine wanting choke or partial choke for a pretty long time when first started. I would also be suspicious of other things that could make the engine too hot, which could cause it to be seemingly lean when it's really just too darned warm. Make sure there's enough coolant and it's able to flow. Make sure there are no exhaust obstructions. And you might also want to check your points gap. If the gap is too small, the points can open a tad too soon, which has the effect of advancing the timing, which can contribute to overheating. All that said, if the backfiring is happening through the exhaust, it's possible that you've got an electrical problem (I think Dell mentioned the coil). What happens if the ignition starts working intermittently is that raw fuel ends up in the exhaust because of an ignition stroke that didn't ignite, and the next time exhaust gas gets pushed out, it ignites the raw fuel in the exhaust, causing a backfire through the muffler.
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