I have 4 saito 150s and 2, 180s. I've used both med. and large tubing on the 150s and couldn't tell the difference. The 180's definitely get more fuel and run better with large fuel tubing. You have to drill out the clunk and use larger brass tubing into the tank as well.

I don't really have an answer to your question though. I've always used the standard muffler that came with the motor and I've never had a problem with any of them starving for fuel. You might make sure the muffler pressure line isn't clogged any where. Also stick a pin or small allen wrench up the nipple inside the muffler and make sure that's not plugged. It sounds like you might not be getting enough muffler pressure to pressure the tank. Jim