And dont try to play all innocent like........
???
Let's see... Seized one chainsaw solid, burned the piston on a bike, done my share of work on these things... Oh, I have been learning what I can about these engines. Here goes nothing.
1) supposedly, the port-timing (in degrees) is biased toward low-end, with a power peak of ~ 5000 rpm. (I wish I had the URL where I read that, but did not save it. Duh...)
2) there have been a fair number of comments on the board about 'wrist pin bushings' and overall engine fragility.
2-B - pictures, and actually seeing one of the engines myself. They didn't spend more time or money on these things than they had to.
3) Based on what I've seen and read, a too-large carb tends to make for poor low-speed running - and with one gear, that's a source of trouble.
Is 20mm 'too big'???
Now, some questions for *you*:
What's your jetting look like? (Main, pilot, needle position, etc.) A larger carb means less velocity at all engine speeds, which usually means larger jets. (and poorer fuel atomization at low throttle openings... Poor idle, loss of low-speed manners, perhaps a tendency to foul the plug? Oh, and worse fuel consumption.)
Your plug - its color?
You spoke of '35-40 miles an hour' as being an irresistable lure. That would seem to indicate top end performance is what you want - and yes, a larger carburetor *will* help there - as will cleaning up the ports, a freer-flowing exhaust (or, better, a tuned pipe), or potentially adding duration to the exhaust and intake ports.
What I meant was 'you pays your money, and you takes your choice' regarding these engines. Your later post more or less summed up what else I was thinking, with this possible addition: brief bursts of full throttle. Long wide-open stretches at 'max revs' tend to cause trouble for engines in general.