Seal it. As long as you have sufficient air flow to the cooling head, plenty of smoke and it still overheats, it's probably sucking air. Read the tuning tips and tricks at the top of the nitro forum. It explains sealing. Make sure the tank cap, header and coupler are sealed as well. Check where the head bolts to the cylinder too. If it's leaking there, most times flipping the shim over and re-tightening evenly solves it.