I have a 1995 Craftsman lawn tractor with the two-cylinder 18.5 horsepower
engine. Last weekend, it started without a problem but after about 15
minutes or so of mowing it began to sputter and backfire through the
muffler. Fortunately, I was just finishing up at the time but I planned to
check the engine before the next mowing.

Well, I didn't and the same thing happened yesterday; ran fine for a while
then started the sputtering. This morning I adjusted the carb screws per the
manual but when I turned it on, it ran very fast and wouldn't slow to a
reasonable idle speed. I turned down the fast fuel mixture screw a bit,
which helped. I did a bit more mowing but this time it began sputtering very
quickly.

I had pulled the plugs before starting and they looked good, no noticeable
difference between the two. The intake manifold seems tight, as do the bolts
holding the carb to the manifold. Any ideas on why it starts acting up at
full operating temperature?

Paul