I have a Yamaha moto 4 80 cc it has spark is getting gas and has compression but won’t start what is causing this

 

Change the plug, it could just be that, even though you see a spark. That engine is notorious for the valve clearances closing up, so are you sure you have good compression? Check the clearances anyway. Engines need just the right amount of fuel, too little or too much and they won't start, so clean the carb out and check the inlet manifold for cracks, if the above "fixes" haven't worked.

 

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