My yzf 450 sat a few months in garage I cleaned

If your atv has been sitting this long, the carb is probably stopped up internally... take the carb to a qualified professional and have it cleaned and set...(take a few pictures so you know where all the hoses re-attach)

ALL it takes for an engine to run is:
1. A timed spark
2. Adequate compression
3. Proper fuel/air ratio delivered

Did you the drain the old fuel from the carb? You need to clean out the old gas in the carb. by draining them.

1, Clean the carb 2, check battery 3,clean the airfilter 4, clean spark plug 5, check spark

Clean the carbs

the ignition cant fail from sitting, perhaps the kill switch has corrroded, always put a spark plug in and ground it to the head when checking for spark , also check with a new spark plug

YUASA IS THE BEST ALL OTHER PAIL IN COMPARISON YOU MAY HAVE GOT A BUM BATTERY

check the compression

Sound like a fuel problem.drain gas from gas tank and carb.you may have to take off the carb and clean it .If it is fuel injection they will have to be clean to.This will happon when a bike set that long