Craftsman 20 hp B&S 42 cut 5 yr old Auto trans Last yr started to backfire and lost power for a few min after 2 or 3 hrs of running I plug wet 1 plug dry installed new magneto runs ok but sometimes wont start at all Then out of no where starts right up Ant ideas It seems like an electrical prob.When it wont start it just floods seems like no spark had this prob now 1 year.I thuoght maybe get second magneto and new wiring harness

sounds like you have a stuck valve or out of adjustment

sounds like one of the jets in the carb is plugged up

loose piston wrist pin will seize an engine eventually

Carb is running a tad rich and the fuel collects in the muffler. My 20 twin has done it since I bought the thing and that was almost 8 years ago.
Cut the engine idle down before you cut the key off and minimize the bang.
Someone else may give a better explanation of the why but it is just annoying and personally after rebuilding the carb and checking the fuel pump on mine it still does it on occasion. Hasn't hurt a thing on in all these years and I actually thing it blows the soot out of the muffler when it does.

Try cleaning your gas cap vent. It sounds like it may be partially plugged. If it is plugged, the engine will eventually stall.
Regards!, --W/D--

Check your spark first,if you do not have spark your problem is the coils-if you do have spark the problem probaly lies withone of your valves getting stuck after getting hot and soonas it cools down a little it closes and then you can restart it till it gets hot again.Hope this helps

When you let it sit did you remove all the fuel or add a additive? If you did not then I would recommend that you remove all the fuel and add brand new fuel to your system. Gasoline only last for about 3 to 4 months before it will start to go bad and this sounds like your problem

Have you checked your fuel filter? it may be clogged or contain water, which will limit flow of the fuel. This would be simplest to relace. Otherwise I believe it to be a carb problem.

check to make sure you are getting fire

You have a bad antibackfire valve. It is located on carburator below the fuel bowl. You can replace it, or as I advise others, install an inline shutoff valve. Turn the fuel valve on to start it, then turn it off when you shut the engine down.