One of those Chinese YB110 mini quads came to me as the local engine guru 
to "have a look can you as it wont rev....was OK last time it was used".....which turned out to be some 6 years ago!!!
the plug was thick black and sooty and sure enough it wouldnt rev past a very lumpy and un-happy idle
first thoughts were carb so after cleaning it I ended up replacing it and the spark plug...well, they are cheap enough....but it was no better
I had just changed the oil (of which there was hardly any in it) and checking again after maybe 30 seconds of idling it was very black, stank of combustion fumes and there was a lot of air coming out of the filler plug hole and oil breather pipe
I then looked a bit deeper and found this horror story....oh dear
new block & piston set duly ordered
what might have caused this....it did used to run OK as I had it in for an oil change when it was last used
to "have a look can you as it wont rev....was OK last time it was used".....which turned out to be some 6 years ago!!!the plug was thick black and sooty and sure enough it wouldnt rev past a very lumpy and un-happy idle
first thoughts were carb so after cleaning it I ended up replacing it and the spark plug...well, they are cheap enough....but it was no better
I had just changed the oil (of which there was hardly any in it) and checking again after maybe 30 seconds of idling it was very black, stank of combustion fumes and there was a lot of air coming out of the filler plug hole and oil breather pipe
I then looked a bit deeper and found this horror story....oh dear
new block & piston set duly ordered
what might have caused this....it did used to run OK as I had it in for an oil change when it was last used
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