I did pinch the exhaust nozzles closed to gain muffler pressure. Not closed, but restricted to match what the stock muffler has for backpressure. What I would like to know is what is the test for achieving the optimum back pressure? What I would do is adjust the back pressure to the point that I could still get the high needle to go full rich at full throttle. Hopefully, it would still draw well at lower rpms. Openning up the exhaust would yield more power. More power is always nice to have.