25 HP Briggs After Running Awhile Starts Back Firing

A friend asked me to put this out. He has a JD lawn tractor with a 25 hp Briggs & Stratton engine. The engine will run just fine then it will start back firing and running crappy. Leave it for awhile and will run fine again. The fuel system was gone through as will the ignition system. He had it at a small engine shop and they can not figurer the problem out. Any suggestions?

Valve adjustment could be off or carbon build up on the valves. Have him try 1 quart of ATF in 3gal of gas and see if it clears up. That will clean the carbon up

When you say leave it for a while. Are you shutting it off? Or are you letting it run and it eventually clears up on its own?

If you are shutting it off, Then it is cooling off and the problem only occurrs when it is hot. I would check valve adjustment. You probably have a valve with almost no clearance and when it is hot it hangs open a bit

Another thing: check the oil. if it has gas in the oil it can foam and go through the valve into the carburetor, where it makes the fuel mix too rich and they really do run crappy when that happens.

If you are letting it run and it clears up on its' own, You probably have a small amount of water in the gas. Ethanol in gas does that sometimes, it happens most when there is a lot of moisture in the air. Since Ethanol is water based, it likes water and will absorb water. Eventually there is too much water to absorb any more and then you end up with water in the gas.

The third idea is to check the spark from both cylinders. Sometimes when a bad coil is hot it makes erratic sparks.

I also recall a Toro once that the delay module on the chassis was giving a partial ground to the ignition when it was hot. I am thinking it had a Kawasaki engine, though.