A worn (flat lobe) on a cam will cause a rhythmic (as opposed to a random) backfire through the carb, less at idle, worse the more the throttle is open.
But, you say it's been neglected, so good chance the valve lash has not been properly set. That needs to be checked if it hasn't already. While the cover is off, inspect the valve train carefully for broken springs, bent pushrods, and you can visually see that each valve is opening equally. If heavily carboned, clean it up, but be careful not to dislodge carbon and let it fall into the crankcase. Scrape and vacuum it out, don't try to blow it out. If the gas tank has to come off, dump and rinse it out with some E85 gas.
Once the valves are set, run a compression test. Look for differences between cylinders, 10% is a good number to shoot for.
I would then revisit the ignition system, especially if it ever backfires out the exhaust (symptom of raw gas getting in the exhaust). All ignition components are now aftermarket and questionable quality. Very common to get bad or mis boxed parts new out of the box. Condensers are especially troublesome, if you have an old one, try it, it may be better than a new one. Check the distributor shaft for side play, any wear in the shaft bushings and the points will not stay set.
Take a fuel sample from the carb drain plug. If contaminated, so is the inside of the carb and the tank (unless you already cleaned it). Usually, the carb can be eased apart and cleaned without having to buy gaskets or parts. While apart check the float level, and check for fuel inside the pontoons, shake and listen.
For best results, do these things one at a time. You don't want to multiply problems or not know what fixed it. A shop manual would be a valuable investment!
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Today's Featured Article - Old Time Threshing - by Anthony West. A lovely harvest evening late September 1947, I was a school boy, like all school boys I loved harvest time. The golden corn ripens well and early, the stoking, stacking,.... the drawing in with the tractors and trailers and a few buck rakes thrown in, and possibly a heavy horse. It would be a great day for the collies and the terrier dogs, rats and mice would be at the bottom of the stacks so the dogs, would have a busy time hunting and killing, all the corn was gathered and ricked in what we c
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