I have the same good results with my OS 120AX. I have the engine in a Venus II. Originally the tank was just behind the firewall and with a MAC muffler, the preassure was just fine. I got sick of re-trimming the airplane as fuel was used and I kept forgetting to re-trim for the next flight. Now I had to use lots of up elevator until I could get my hand away from the stick to trim it. I moved the tank back to the CG and installed a Perry pump. All is great now. I use a 17X6 APC pattern prop and I am very pleased with that combo. I also had to lean my low end at least 1/2 turn initially and another 1/2 turn after I installed the pump.