went through that with my 871 gas not long ago. You can try 437's but I suspect that the fouling cause is still not identified.
Drain the tank and put fresh gas in. Check your float, and blow out the carb and jets and make sure float is not sticking.
I suspect you have a timing problem more than a carb issue.
I ended up replacing my timing gears because they were so worn that there was slapping on the worn gear teeth and I could not keep it timed and the plugs kept getting black soot. Wether I used 437's or 216's. That fixed it and quited down the knock to a non-existant knock.
Try gapping the 437's at .030. and the 216's if used at .025.