I have a lawn tractor that's not running right.  It starts okay but quits after about a minute.

It has a 12HP Briggs & Stratton engine.

I changed out the plug & air filter...no change.

What do you think?  Carb problem?  Sticky float?

Thanks!

Dirt in the carb. Did you leave it full of gas over the winter?
Maybe dirt in the gas line - the carb bowl fills slowly when the engine's off, but when you start it, it can't fill fast enough to keep up.

Or maybe the choke?  Does it have a manual choke?

No manual choke.  I ran it out of gas last summer when I put it away but it sat around for severl months.  I think I remember seeing a little in-line fuel filter on it...perhaps that is it???

 Make the sure the vent on the gas cap is free of restrictions. loosen it , start the mower and see if it runs.

If you have an in-line fuel filter you're lucky.... mine didn't come with one and it clogged up the carb.  

So yes, change the filter.  You can buy one in most  auto parts stores.

If it has a float and needle setup then it's sticking. You'll need to take off the 1/2 screw at the bottom of the float bowl, slide out the float pin and remove the float and needle, clean off all the varnish with carb cleaner and get a new needle [some come with a seat also] hose every thing out  that has a "hole" with carb cleaner, install new parts. Should work. If you have a jet in the shaft in the middle [you'll have to look] that has a screwdriver slot, CAREFULLY unscrew it and clean it out well. reinstall and snug down but do not overtighten. fill with clean new fuel, go mow.

Thanks for the replies...I'll report back when it's up and running!

We have a Forest Gump style Snapper that is able to be stored tilted up on its ass.  Part of the enineering that makes this possible is a vented gas cap (close before tilting it back).  If you fail to open the vent and run it, it will indeed start to crap out after a while.  The tank vacuum that results from fuel usage eventually becomes more powerful than the intake vacuum.  It's kind of funny to look back at the plastic tank and see it puckered in like it gets...