Remove the shrouds then you can do it in two ways .If the coil is already set right you can pull the fly wheel and sand or wire brush the face of the coil .You can remove the coil and leave the fly wheel in place.You will have to pull the spark wire off the spark plug ,remove the bolts holding the coil in place (move the magnet so it is away from in front of the coil first) ,lift coil remove wire off spade connector .Then you can clean the coil with a wire brush or sand paper (you need to clean the face and the fly wheel face and make sure you wipe so no dirt is left) the you reverse the removal and reset the coil if you have a manual it will give you proper coil gap. If not you can use a thick peace of paper to gap it.
Could be plugged fuel filter, sticking float:Change fuel filter, clean carb.
Clean out the carburetor with gas, the reason it wont keep running is its not getting enough gas through the carburetor.
Check fuel filter or sticking float: replace filter clean carb. hope this helps
the seat on the exhaust valve is dirty so the valve does not seal it.... not sure how to clean it
Because your carburetor is not getting fuel to the engine - could be a plug in the fuel line or it could be dirt in the screen in the pick up tube in the carburetor. I would suggest you clean the carbutator.
if it starts and runs for just a few seconds then dies it is generally a carburetor issue. clean or replace the carb and that should solve your issue.
If the valves are new you grind the end of the valve stem to the proper height . Otherwise you just clean the valves up real good and put them back in , there is no other adjustment .
check your coil out
Clean the carburetor with special attention to the high speed jet and fuel passage.
You could try cleaning the spark plug. That worked for us. Take the spark plug out with a wrench and use sandpaper to clean it.
if it's a foam filter , clean with dish soap, when it dries put a little motor oil on it and sqeeze out excess, you want a very light coating of oil throughout filter it helps trap dirt. if it's a paper filter you can't really clean them, they are available at most home stores if the square type, about 5 dollars
I would start with float sticking or maybe fuel filter plugged filter and run some good carb cleaner threw a tank of fuel if that does not fix your problem you may have to take the carb apart and clean it.