Although I had some trouble keeping the fuel to the carb in the beginning, and plugging the exhaust helped, ultimately it turned out that the exhaust nipple was getting plugged. Once that was cleaned, as long as the engine was turning, the fuel stayed at the carb. I think the one I had just needed an idle bar glow plug to idle down to 2000. As far as the back pressure goes, I would look at the stock muffler, and then try to duplicate the opening size on the new muffler to be somewhat close to that, And after doing some thinking, I've decided that with all the adjustment we have via the needles, I would want to duplicate the exhaust opening for fuel economy, and from my experience with that italian ST G20, whatever it was. it really sucked down the fuel.