Small Engines (Lawn Mowers, etc.): Craftsman 7 hp missing, back firing, blown head gasket, briggs and stratton

Question
Hi Ben, I have a 7 hp Briggs and Stratton model number 128602 type 0259-e1
Its on a Craftsman self propelled mower. When I start it, it will back fire and sputter through the carburetor. It will run if you push the handle down, tilting the mower front wheels off the ground, it will spit and sputter for a couple minutes, then straightens up and runs great. I have cleaned the carb out, (not rebuilt), replaced fuel line and replaced gas tank, replaced a blown head gasket, adjusted the valves, intake on .005, exhaust on .009. I'm at my wits end on this thing, its a good mower, powerful, when it runs right. I would like to get it running right again if possible. Any help would be greatly appreciated.

Thank you,
Sam

Answer
Sam
A lot of possibilities are already mentioned here. Dirty carburetor, fouled plug, etc. Check all the common sense things. I just have one additional possibility to try. Remove the muffler, and see if that helps. Sometimes (especially after burning a lot of oil), soot can clog the muffler, and cause these problems. A motor has to breath both ways.

Disconnect spark plug, remove and clean  gapped portion of spark plug. Wipe off with rag, reconnect spark plug. It sounds like you have too much oil in the crankcase. Did you replace the air filter? if you did not do so and that should solve your problem. the air cleaner gets oil soaked and not enough air can get through and it will do just what you said sputtering and backfiring or not run at all