On older tractors, its fairly common for them to spew some white oil smoke after they've been sitting. This more likely when you have just topped off the oil. My old Tecumseh motor had could belch smoke when you first started it if the oil was fully filled and I had left it sitting. Check your oil level. If you have room to, top it off, start it and run it for a few, and see what happens.

Based on experience, even if you are leaking oil, it doesn't smoke much due to the fact the engine block does not get hot enough (especially on startup) for large amounts of oil to burn off at once. In the unlikely case it's winding up on the exhaust, it would often catch fire before too long. Also, if it was leaking, it would find your garage/shed floor, as a mower frame has plenty of holes for it to escape through.

I assume this is out of the ordinary.

My first guess would be that you have the belt routed wrong and it's rubbing somewhere. Examine the belt. There should be an obvious dark spot where it was rubbing, if that's the case.

Did you have the tractor tilted at a severe angle when you changed the belt?

You changed the belt and it started to smoke. I think you have the belt installed wrong.

hmmmm.. oil over-filled?

When working under Craftsman and other tractors, a usually tilt the entire machine up on the two right side wheels, and support it with a jack stand under the rear fender. There is a drive belt routing diagram on the bottom of the Craftsman tractors. A simple mistake is to route the belt around a guide, which will heat the belt making it smoke. Tilting an engine will oftentimes produce smoke as the mufler can will fill with oil. A good 20 minute run outside will burn this out.

IMO Depends on the model you have, beleive it or not some models are fairly easy. usually the first time they are hard cause you are learning. if you positions the engine anyway up, down, or sideways it will usually smoke on start up. tiny amount of oil makes it pass the piston rings and creates alot of smoke in the comb chamber (usually BLUE smoke not white) I equate white smoke when an engine burns water.

IMO Usually when you install the belt wrong it makes a heck of a squealing sound, does nothing meaning the tractor won't move or the deck won't turn, or lugs the engine down. I would double check the belt installation as recommended above, verify the oil level, remove the plugs spray some carb cleaner down the spark plug holes motor the engine over for 30 sec. with the plugs out to wash out any oil that may have or suspect got pass the piston rings. clean and reinstall the plugs and fire it up and run it long enough to see if the smoke cleans, usually take 3 or 4 min. depending on how much oil is in the comb chamber and muffler. if enough was in there to get pushed into the muffler then it will take longer to clear.

Thanks all of you for your answers.
After taking the belt off five times and having the same thing happen,I bit the bullet and called the mechanic.He said a pully keeper was bent and the belt was rubbing on a bolt on the wheel. I insatlled another new belt, bent the keeper a little, WD40 on everything and fired it up. The smoke stopped after about 5 min. and I finally got the lawn mowed after three weeks. My neighbors are thrilled! So am I. Thanks again.

There you go.