When on start up can not get on it and ride have to let it warm up for about 20 minutes

check pulleys-mandrels -and bearings for failure

Check the tank vent. Try loosening the fuel cap (not totally removed) see if this stops the problem. If so, try a new fuel cap.

Check for leak's on manifold rubber between cylinders!
Try to squirt a Carbu cleaner inside the manifold when running,if the idle going fast or dropped, then you have a leak,.

May be a bad coil. It will only fail when hot.

DO not turn the engine over. Drain and replace all fluids, including the fuel. Replace air filter(s). Open almost everything and drain water. You can use air to help. Unplug all multi-plugs and completely dry with blown air.
Pull plugs and turn engine over. This will expel excess fluid. Put plugs back in. Start the unit. Run until quite warm, but do not ride it hard. Work out any misfiring.
Drain and replace all fluids, while still warm. Then run the unit again for longer than 20 minutes after fully warm.
Make note not to ride in water again.
Doc

make sure the intake is not cracked or leaking spray carb cleaner arround gasket and carb sounds like you have a hot condition vacume leak

I would say your ignition coil is bad. They get hot and quit working until they cool off then work again.

if it has a carb, clean it.

It is most likely a thermostat. As your car gets older the thermostat begins to lose its spring like properties and stays open all the time causing your car to never warm up.

If you can't tackle right away, start your car in the morning and let it run for about 15 minutes to warm up. Be sure to turn the heat on full blast and recirculate the air.

my mtd lawn mower wont start . when I turn the key there is a buzzing soung where the battery conectors are near the shifter but the starter is hot and getting juice