Sounds to me like it's running lean. Could be getting too much air and not enough fuel, thus making it run lean. They will also backfire shutting off when lean. Also run ~~HOT~~ as well. Check carburetor mount bolts, make sure they're tight, but don't overtighten them. Could enrich the mixture screw some. Sometimes the intake manifold gasket will have a crack, or suck air making them run lean. I've seen some get so HOT the muffler was red.
