I have an atv that won't start.

Try and check your timing?

Check that you have a spark .
The gap may be lost on the flywheel

Turn off the fuel tap the spark plug should be on the motor has a thick wire going to it, remove the plug and move the wire away from the hole to avoid a fire then crank engine over to push all the fuel out of the cyl, then fit a new plug .connect wire and see if it starts then turn fuel back on.

can you tell me what year and make of atv sounds like you might have a timing problem were you do have spark but at the wrong time need more information to know for sure and also is it 2 stroke or 4 stroke

check the compression, REPLACE the spark plug with a new one, clean or have the carb professionally cleaned. compression should be at least or more than 100-110 lbs.

if this has been sitting,,even if it hasn't, #1clean or replace the air filter, #2have the carb professionally cleaned and set, and #3 replace the spark plug.

I would check the spark plug after cranking to see if is getting gas-clean and re-install-also if your battery is weak you may have a dead cell that may be effecting the quality of the spark thus not firing -cheers Denny

then the carburetor jets are cloged up. take it apart and do a good cleaning on it.

The battery voltage on a Suzuki I had would drop low enough during cranking with the electric starter motor not allowing the spark to be strong enough to run, the voltage regulator also didn't function correctly once and as the battery voltage dropped during operation engine would start to bog down and cut out due to low voltage for the ignition system.

The spark plug is on the left side (shifter) of the head,,..?/ this is August.. if the atv has been sitting the carb may need a qualified professional to clean it again.. Replace the spark plug before you squirt anything in the engine...