I have a 16 hp Briggs mower in my lawn tractor. I was mowing last Saturday, and the engine gave out a "poof" sound with a puff of smoke. Someone at my office thought it might be that the valve seat came loose. When I tried to restart it, after checking for gas, it spun really easy with no resistance. It is a two cylinder.

I had the same problem with my 5.5 Briggs on my push mower half hour later. Tried to pull start it, and it was real easy with no resistance. Same type of thing happened, just a pop and it quite running. :mad:

Any suggestions are greatly wanted and appreciated!

Side valve flat heads? (l head) take the heads off to double check the seat possibility...

Also what color was the smoke?

The tractor gave a whiteish puff of smoke, while I am not sure what the push mower did if anything. The push mower is a side valve (plug is horizontal) while the tractor is top (vertical). I think the tractor is a Vangard motor?

Naa, what Meant was in what valve config. They had... side vale flat head (valves operate beside the piston) or over head valve (valves operated via pushrods and rocker arms over the head/piston)
Puff of white sounds bad (blue to white means oil) does the oil smell like gas? Is it to the full mark? Make sure the pistons are still attached to the connecting rods... insert a screwdriver into the plug holes and turn the engine over, the driver should when held in, move in and out if your touching the piston.