atv will not stay running?

Make sure you are getting a steady supply of fuel to the carb and that your gas tank is venting properly. If it runs good for 30-45 seconds I don't see how it could be the carb, spark, or compression. If the float in the carb was stuck shut, it wouldn't be getting any gas. My vote goes for plugged tank venting.

it could be the magnets around your stator sometimes they come loose and move from their positions causing sudden stall after a few seconds I have read about this problem and some guys are unlucky they can slide off the magneto and destroy your stator. if I were you I would take the sideplate off and check them. could be a quick simple fix and stop a worse one from happening

Make sure your gas tank is venting properly and not starving your carb for fuel.

Possible electrical problem.

Check if the spark plug fires when it won't run.

check your fuel lines and carburetor. make sure your petcock, if it has one, is working properly. it sounds like its just running out of fuel and takes some time for fuel to run back through the lines.