I have always thought that the Fox 35 does not like muffler pressure; that they like standard vent tanks. That idea may be changing........I had Lew Woolard fix up an old 40th Anniversary engine with his normal rework and added one of those blue heads with the offset plug. I haven't really flown it, but I put it on a profile Smoothie, and have run it in the backyard. I'm using a GRW 4oz. uniflow with muffler pressure. The first run was sort of wierd because I hadn't hooked the pressure line back on the muffler, and hadn't blocked the overflow -- duh! The second run, with the plumbing hooked up right, was great. I set the needle for a kind of fast 4, with the engine trying to break into 2 every couple of seconds. When the nose went up, it kicked into a nice 2, but smoothly, not harshly. It maintained the 2 until the nose came level and then smoothly went to 4. Seems to run the same way inverted. I may have to readjust after flying it, when centrifugal force comes into play. We'll see. Anyhoo....this particular Foxy 35 seems to like pressure...........Glen