ORIGINAL: Pelle Gris
Mine did 8200rpm open exhaust, and 7900 with the DA 50 can on a 8,66in header, all on the APC 18x10.
I don´t know the type of ''muffler'' that´s attached, but it does not look like it does much in the way of silencing the engine, so I would guess that you might as well run it without. It could be that this is holding the engine back.
I read in another thread that you had changed your carb to a WT610, did you ever get that to work for you? One problem I am having is that the power and idle needles on mine (HDA48) are missing the washer/o-rings which I think would lead to some issues, unless this carb just doesnt need them (its actually marked 21-245). I can get a 610 fairly cheep, but if I could just find the o-rings that would be even cheaper but no one local has any clue if the o-rings are interchangeable, etc. Very frustrating, but the carb is rebuilt so I would kind of like to see it through, plus if the 610 is going to cause issues I would be no better off. It seems par for the course when you get a "good deal" on a motor!