starters, check for spark. hold plug against head and turn engine over briskly.
an idea to get a real plug. NGK b6hs (5 or 7, whatever, read around a bit and the numbers become clear)
no spark... either the magneto is fried (if it has a white wire, thats more than likely) you have a bad earth or connection somewhere, or the cdi unit itself is cooked. thats unusual.
spark present? and still no go? ok. pull the carb off the manifold.
pour a capful of petrol down the manifold, now try starting the engine.
it should fire up, then (if its unloaded) will rev out, and die. thats a good sign that the fault lies in the carby. strip, dismantle, clean, paying attention to the JET and its hole. should now be fixed
if it still doesnt start... forget the carb or fuel issues. we have another problem, which is most likely the key on the magneto is broken or missing.
sometimes they are easy to remove, sometimes they arent. laminated magnet. one plate shifts and it locks itself onto the shaft. 50-50 whether it will happen or not. thats only annoying, makes it hard to remove in one piece, but isnt an issue regards running.
a broken key is an issue, as it means the spark could be happening literally anytime instead of at approx 10 degrees before top dead centre...
therell be spark, but its useless if its timed wrong!
last but not least...is the engine actually turning over, or is it just slipping somewhere? the main pinion gear has a key, pretty common for it to be missing from the factory. they are hard to get in! (handy hint...try glue when installing). that will slip, and so will the clutch if its out of adjustment, etc.
some ebay motors are simply lemons and what you get is a matter of luck... i know zbox or mtbimports is more expensive, but ive never had a bad engine from either. and they very happily send replacements when you find, say, a crack in the main pinion.
