Tom I havent tried any different props yet but Ive had several different mufflers on mine doing some testing; mainly for sound reduction. All these are with the same 20X10. The stock muffler runs at 5800 rpm. Several different mufflers Ive tried vary between 6300 and 6600 and with a straight header it turns 6800/6900.

Your reading with the 18X8 is right there with anybodys Poulan Ive read about. I dont know why the MA would drop so much?? That prop may be a power hog.

Adam