I have Mac trimmer power head that I picked up to tinker with - I believe it's a 28 cc (Roadrunner). I've got it running, but at full throttle it makes this loud knocking sound that I'm having trouble figuring out - despite taking it down a couple of times. It will occasionally backfire too. The only thing I haven't done is go into the crankcase - but everything feels tight by hand.This engine does have one thing unusual about it - someone stripped out the upper spark plug threads and when I got it, it had some type of adapter that I've never seen before. It screws down into the lower threads and then you screw the plug into the top of it. It creates its own little chamber and has about a 1/4 " opening at the bottom. Could this be the cause of the knocking? How do I correct it? Is there a longer plug that could do away with this adapter? It currently uses a CJ7Y. Thanks...
