I have a Ryobi 31cc engine mounted on my GP Giant Stik. When I advance the throttle forward the engine will hesitate and then throttle up but only at about 50% power with throttle full open. I have a pitts style muffler mounted on the engine, when muffler is left off I have no problem with the throttle going to full power. No hesitation and l00% full power with no muffler.

Can anyone out there help me solve this problem?