headgasketitsnot.....
Check your head gasket. I broke my gasket for my mitsu 49cc and it made that sound, and had near 0 power. It could be a loss of compression - i didnt read that you tested for it?
Thanks mate, one should consider all possibilities....one of the good things about the gx 31 etc. is that it is 'forced air cooled' and doesn't depend on the air flowing past it trough the 'forward speed' like so many other motor bikes, cause that means even if you are going slow, like going uphill in 'summer', or stop/start in town (in summer).... it still gets enough cooling....
my old (1970) bmw (motorbike) use to get so hot at times that i had to put an oil thermometer on it to make sure it wasn't getting too hot at times....
I (gx31) go from sealevel to 400meters twice weekly and to over 800meters once every 2 weeks (with a 3 speed gearbox/hub) and i was worried the first time i did it in middle of summer in the middle of the day...., but the 'baby' just does it, again and again.
i bet many normal 'aircooled' small motors would have given up long time ago....
i have a compression tester and after 10000km wanted to know anyway what the compression is like and how much the motor is worn because the BIG QUESTION for me is still out there to be answered: how long does a 4stroke 30ccm motor last in a 'motored bike', especially when it gets the workout i give it.
it needs to last at least 20000km otherwise i (and others) would have to replace it every year (no good). sofarsogood: no significant oil usage, no 'metal' ever in old oil, no obvious wear/pitting on visible valvegear, even the valve clearance is still within its tolerance etc. (having used only fully syntetic oil).
BUT, you just never know...., even without a compression tester one could check a (cold) motor/headgasket with soapy water around the headgasket and look for bubbles forming....
But, "you just never know" (did i say that before)....ones in middle of winter (long long time ago) my bmw crapped out, wouln't start and my mates and me spend the next hour or two (three) pulling the bike's electrics to pieces....and you know what it was....i (or somebody else) leaning on the bike had moved the (then usual) handlebar mounted (small and hardly used) 'killswitch' accidentally onto its off position....
or the time dirt got IN BETWEEN the 'points' (no electronic ignition) and you look for everything else but THAT easy (but not likely) one....
e.g.: a good way to look for (some) 'spark problems' (especially if it happens in rain) is to look at the coil/cable/magneto/sparkplugcap etc. in the DARK, then, any 'shortening' will be clearly visible....
Hence asking MY MATES on this excellent forum...., because no matter how much you know or how many miles you have done on two wheels, THEY can give you just THE HELP you need, when you need it....
(and i hope to be able to return it one day....)
Thanks a lot everybody.... Have Fun.... AL.