I haven't had a single issue as far as "lack of fuel tank pressure" on my Cherokee. I am using a Magnum 46 motor and it runs like a top with this muffler. I have probably about 30 LONG flights on this plane and had only one dead-stick. That was because is was 110 deg F out and I didn't adjusted the needle setting since that previous time out when it was 80deg...I wouldn't worry about clamping off one of the pipes unless you are SURE you are having back pressure issues.