As a general rule (even with full scale aircraft) you should have at least 1-1/2 times the outlet area as you do inlet area. Having more outlet area than inlet creates a vaccuum that will actually suck more air into the cowling and past the engine thereby cooling it better. Having more than this amount of outlet area won't necessarily create more vaccuum depending on the design of the cowl. Inline cowlings like say a P-51 Mustang cowl will not cool any better by having two or three times the outlet area whereas a radial cowl like on a Corsair tend to work better with as much outlet area as they can get. Most scale models of inline or tubine engined aircraft need to have more cooling inlet area than what would be scale because the full sized birds have radiators and intercoolers to keep them cool whereas most scale models have air cooled engines that need larger inlet area to promote better airflow and cooling. Try enlarging the inlet area and the outlet area and I would be willing to bet money that it will help to solve your problem if not correct it completely!

Also check for air bubbles in your fuel line and make sure that the clunk in your tank isn't too close to the back of the fuel tank. I Always test run my engines without the cowling and make sure that everything checks out before installing the cowl. That way you will know if the engine doesn't run properly that there is a problem with the way the cowling restricts cooling or fuel flow to the engine.

Good luck!