Sounds to me like the gas cap isn't vented correctly either the holes are
plugged or you replaced the cap with a non vented cap. What happens is
everything is fine for a few minutes then the engine quits you wait five
minutes (the vacuum in the tank has equalized by then) and then it will run
fine for a few minutes then repeat. Try unscrewing the cap a little so that
it doesn't pull a vacuum on the tank to eliminate this as a possible cause.

Good luck, Rich


"JohnR66" wrote in message
...
I have a 3.5 hp B&S "classic" engine. While mowing, it started sputtering
and surging like it was about to run out of gas but it has plenty. It does
this in any throttle position. I took off the carb and cleaned the
strainer.
It had some junk, but wasn't very dirty. After I put the carb back on, it
ran fine for 3 minutes then started surging again. One thing I noticed is
that it will run smooth seconds before it runs out of gas. Sounds like a
carb problem, but I'm not sure.

Thanks for any help.
John