Take the tube off of the exhaust & take the tube off of the carb. Plug 1 and blow into the other (Without any fuel in the lines or tank). Push some breath in there and then listen for air escaping and feel if the pressure remains the same or if it diminishes.

If it is not cracked, chances are that the tank is fine. Have you sealed the engine? I run the ribbed pipe and my SH runs a little lean when the tank is less than half full.

When starting hot, I turn my throttle adjustment on the receiver to open the carb a little more. This usually works fine. With 290 degrees and no air leak, are you sure that your tuning is not lean? I have not been over 265 degrees even running full throttle for 15 minutes on grass with the body posts raised high.

If your tuning is real erratic, or your temps don't go down, try sealing the engine.