While this is the O.S. support forum, we do service the Tower engines. Likely, the carb's not quite set correctly. Start out with a super-rich idle mixture setting and a very rich high-speed needle setting. Start the engine at part throttle and open to full. Get the high-speed needle set correctly, first. Then retard the throttle and adjust the low-speed mixture. You should be able to work the mixture and throttle opening down to a point where you can get a good, reliable idle at 2,00 RPM or lower, and still have good acceleration.

There are no additional extensions for the muffler, but they are easy enough to make from bar-stock aluminum.