Hi crazzeddie, love your commercials from the 80's by the way.

Sounds fuel-related. Had a similar problem a week ago. Mownie, here on the forum, advised me that it might be a clogged gas cap, which after 20 or 30 mins does not allow fuel to exit the gas tank very well. Might try loosening the gas cap a bit and give that a try. New gas cap works after that for a permanent fix, if that were the cause.

Otherwise, check for blockage in/at the governor linkage - grass, a stick, etc.

If none of the above, also check your gas line - might have a bit of roughage in it from the winter. You would have to remove the carb and clean out the bowl w/air compressor, where the needle underneath seats with the carb. I believe its easier on the B&S 18 twin to remove the intake manifold as well, to clean. Be careful not to lose the needle, accompanying spring and chrome hinge pole thing. Be careful to note the original set-up of the spring before ripping it apart.

Good Luck!

Did you remove the flywheel shroud and check in under it, for a build-up of grass, acorns, nuts, and a mouse toilet and nest? Sounds like the engine is not getting cooling air flowing around the cylinder head, etc.
Mousies just love to live under that shroud! And, they store nuts, acorns, grass clippings, and POO!

Have blown out the gas cap and areas around cylinder head and under sheet metal shroud. No debris came out and flashlight inspection confirmed clean. Will try operation with gas cap replaced by clean filter rag held by rubber band when it quits raining here. Also intend to monitor fuel filter (I purged all the air out). No fuel pump on this model. Will also try removing Fuel solenoid on carb. Recognize that this engine runs great for the first 30 minutes so requires time and operating load to cause problem. Thanks for the suggestions. Will let you know how it turns out.

You know, it might help if we knew just what 18 hp B&S we are talking about since there are at least 4 that I can think of and some drastically different. MODEL NUMBER.

Walt Conner