318 is hard restarting after running 20 minutes or more.

Replace the pluge.Old Plugs tend to get week, If that don't do the trick Take the air cleaner off. Remove the Air filter Housing 3 #8 screws 2 #10 screws. Remove the chock &throttle cable, Remove the top of the carb. Lift it Strieght up So the float pin will not fall out, Lay it on your 12 pac of BUD , then you'll find it again;;Now take a Couple paper towels and remove all the gas and sediament that has accumalated in the bottom of the carb, Now carefully remove the jet that is streaght down near the front side of the carb, You will problely find a littli piece on dirt or carbon in it I At this time carefully blow it all out including the Jet; With care Put it back togather , Check your air cleaner, It can never be to clean, However dirty Makes the engine run ritch and may wash the cylinders and lead to premature engine Falure. As I have stated in prior posts I service John Deeres and it is not uncommon to see 3500 hours on that engine, And still runs like new, I have also seen some with 450 hours that were ready for the junk Yard, Also I do NOT use Carb-out , Gum-out, Sea-foam, If It's dirty take it apart and Clean it; Good luck. Taken care of the 318/onan will last many years.. OH some times the coil gets weak when warm. you never stated what year you have.