Have you had this engine apart for any reason? Just asking!
Have you checked the valve clearances? If the exhaust stroke is forcing the fuel back into the tank the intake vlave isn't closing soon enough or the exhaust is openiing to soon. Maybe!
As far as keeping the intake maniflod tight go with the teflon tape as hobbsy suggested or some high temp silicone will hold both the intake and exhaust nuts. I've used that on some of my other four strokes with success.
[&:]