Yamaha YZ 80 Not Starting

Check the reeds pull the carb and take the 4 bolts from the intake and look at the reed cage and make sure they are closed

If you have a good fat spark and timing is stock, with good compression, your problem is likely fuel/air mixture. Assuming you have good quality fresh fuel/oil. Set air mixture screw one and a half turns out. What is the condition of the reeds and reed holders. Is there a build-up of fuel/oil residue on the piston port skirts or the crankshaft? Check for absolute seal between carby and reed valve manifold. Check for obstructions through airbox inlet, filter and inlet to carby. Ensure the governor is not fouling. The site link below has exploded view diagrams for all components of the bike to see how it all works. Good luck, cheers.

http://www.cmsnl.com/yamaha-yz250g1-1995_model10278/partslist/

if you mean not to start they could. a reed valve is simply a one way valve that captures the air fuel mixture in a two stroke motor forcing to go into the combustion chamber rather than back out the carb

Look at ?

crank seal (drawing air)

ring wear

main jet (blocked jet's)

fuel starved carb