The carb is gummed up with varnish from having old gas in it for a year. Remove the carb and clean it (preferably by soaking in carb cleaner) and make sure all passages on the main jet are cleaned out. You might try first removing the main jet and cleaning it with a spray type carb cleaner and running a tiny wire through the passages, but if that doesn't work, it's soakin' time.

Remove the air cleaner. It's probably saturated with oil. Try starting. If it "clears up" and runs smooth, you should be good to go after putting on a new AC. If not, report back. in fact, report back if everything's OK. We like to hear good stories once in awhile!:-)

Hey, I know its an old thread but just wanted to say I eventually realized that using motorcycle oil in the mower was not a good idea although as far as I can tell it did no harm. Now using Castrol Edge 10W-30 Synthetic in both 4-cycle mowers. The thing about motorcycle oil is does not have friction modifiers because it may be used for a wet clutch. Regular auto oil does have these which inhibit wear and help the engine run cooler. Actually, and I haven't thought of why yet, the mower with Tecumseh engine has always had 30W oil and it always blew just a puff of oil smoke on starting. With the lighter 10W-30 that you would think would be more likely to get past the rings into the combustion chamber while the mower sits, the reverse has been true - it has not blown a puff of oil since I changed to the Syn 10W-30. Whatever, I really like the way the mowers run with the Edge so its now my mower oil.

Ok so my mower was fine this morning. I stopped for about an hour and a half to run some errands then came back and it wouldn't start. Gas looked low so I refilled it but nothing. I have had the mower some 7+ years and done nothing to it so I ran to Home Depot and got a new air filter and spark plug. Came home, changed both (oiled the threads of the plug before inserting), crossed my fingers and nothing. I don't get it. I am not mechanically inclined so I think I have used all my knowledge and need some help. Help also means if you say "check the carburetor" I would need some help figuring out how to access it. lol Oh - if it means anything - when I pull the cord, I can see some smoke on the right side of the mower but it doesn't even seem to be almost starting, i get pretty much nothing.