No, no cowling. I'd like to get the fiberglass cowling, but I doubt the engine would fit very well, so I'll just leave it as is. Anyway, I'll include a picture of how the carb sits. The vent holes (it has two if them) are both facing forward. I would think if they were the cause, it would happen all the time, not after being in the air for some time.

It's got a Master Airscrew Classic Series 18x6 prop. I don't have a tach, or access to one, and don't know the max RPMs. I also don't think it's heat, once it leans out, I can let the engine coll off completely and start it back up, and it's still lean. I open the needle to get it running right, take off, and it starts 4-stroking like it's incredibly rich, land, lean it out, and fly with it running great for 10 minutes or so, until it leans out again. It's almost as though the needle valve is moving, but I've checked it and made sure it's right where I had it when I took off.

I'm going to start with the kit, it surely can't hurt anyway...

For what it's worth, I ran this engine a number of times over the past 3 or 4 years on an airboat, and it never gave me an ounce of trouble.