2-stroke exhaust tuning?

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Last year I posted that I couldn"t get my Brad Smith upright maytag running. I figured out that my rings were not making 100% contact with the sleeve, so I made a small lap and honed the OD of the rings until they were shiny --and lo and behold it runs like a champ. BUT- if I install the muffler it quickly dies, yet I can take a 6 inch machinist rule and cover the 1/4 inch pipe exhaust port by approx. 80% and it doesn't bother the engine one bit- it seems to be a length issue because I can role up a tube 3/4 in diameter about 6 inches long down to 1 inch long and the engine still dies. What is wrong and what is the fix, port timing- sucking too much ex gas back in or what. Thanks for anyone's help. Kelly