Those motors weren’t designed to be environmentally friendly, a rainbow behind them is normal especially when idling in one spot for a little bit. The crankcase has a built in puddle valve that empties excess fuel buildup out the exhaust. If an obnoxiously large rainbow is forming then look for obvious leaks, I don’t believe a crank seal sees that much fuel mix to be an issue, especially on a running motor. A very common issue I’ve seen with these motors is fuel pouring back out of the carb for various reasons but especially when starting and becoming flooded from over choking the motor. Check all the fuel line connections and fuel line connector for leaks as well. From what you describe it sounds like normal operation or a leak somewhere but pictures would tell the tale.
