What size venturi? What size/weight/amount of drag is your plane? How rich/lean is the mixture that you prefer? The cold air/hot air effect on my friend's plane is the way I described. I always install at least 4.5 ounces with FP40s/FP35s. Come to think of it my last profile, a flea marked purchased Prowler ($10) needed nearly 5 ounces for the pattern. A thick airfoil porker weighing 44 ounces, powered by a Tower 40, underwent a very interesting tuning process. I wound up going to a large venturi .283, a very free flowing tongue muffler (holes on both sides of the tongue), taking all the head gaskets out of the engine. This in order to get a break. After some stubborn resistance on my part (taking out all the head gaskets was Mike Palko's and Dan Banjok's recommendation) that was a tune that worked. Nice even run with break on 10% 50/50 Powermaster. I just realized that I forgot to mention taking into consideration the amount of nitro in the fuel, when predicting engine run. On 10% fuel, mileage for pattern was inadequate with a 4.5 ounce tank. On 5% there was no problem. But on %5 engine was not happy. Danny suggested mixing fuel, half 5% and half 10%. That worked. I really liked that plane. Best profile I every flew. Tracked. Cornered. At our contest, plane went in. A hinge near the flap horn (far as I could figure) pulled out enough to give me no down (which meant up) when doing the inverted laps. Kaboom. Broke fues along old break. Not bad.
The Magician weighed 33 ounces. Magician airfoil much thinner. No problem with the 4.5 ounce tank. I'd pull out a little under 1/2 ounce when flying an official. That engine (same engine) was tuned with two extra head gaskets and a .272 (mid size) venturi when flying the Magician. On the Prowler, running %10, with engine tuned very differently, 4.5 ounce tank was too small.