well i can relate i am having the EXACT same problem with my 03 grizzly 660 and i'm 98% sure it is the woodruff key i have no idea how it got broke but when the piston is at TDC the timing marks aren't close to lining on the crank sprocket gunna go at it tomorrow and see what i find

 

wow ! aright heres how you can eliminate the guessing game shut off the fuil to the carb remove the spark plug squirt about a table spoon of gas into the cylinder replace the spark plug start the engine if it starts and runs for a couple seconds then the electrical is fine next repeat the same test only into the carb throat if it runs the valve timing is fine and it truly is the carb that is the problem if it fails check the fly wheel key since you'll have it off replace it then repeat the afformention test if it still fails the look at your valves a side note just don't over doit on the gas or it'll flood remember isolate and eliminate