In the first place, try flying the plane with the cowl off (recheck the balance first). That will validate any overheating problem.

I've heard all sorts of formulas for inlet/outlet ratios. The most valid one comes from Andy Lennon who recommends 1:1.4 inlet to outlet. To keep his engines cool, he baffles the inside of the cowl so that all the incoming air cools the engine and doesn't become trapped in 'dead-air' pockets. All the airflow is used. The baffling is done with lightweight stuff so as not to weigh down the front of the plane.

Just my $.02

Bob