I have a early 1994 Cub Cadet 2165 with a Briggs Vanguard V-Twin 16 engine. This summer I've had a problem that I can't seem to nail down. It starts great, runs great for about 20 minutes at full load and then begins cutting out. Letting it sit and cool will allow me to start and run another 20 minutes. See, we've already narrowed it down to about a dozen things I've done all the basics, new plugs, fuel filter, air filter, oil change. I suspect it's developing a lean condition since when it cuts out applying the choke helps me get it back to the barn. One consideration was vapor lock since it seems more pronounced on hot days. I've tried running the carb a half turn richer but the same symptom occurs. I've checked the flow rate out of the fuel pump and it's giving a constant stream about 1/8 inch in diameter, does this seem sufficient? Something that has troubled me from day one is that this engine will start with an almost full fuel filter and after 3 to 5 minutes at full throttle the fuel will slowly drop to the bottom and appear to bubble out of the supply hose, is this normal ? I've run the engine with no gas cap to rule out a plugged tank vent. I'm thinking it may be time to look at the carb but suspect there's probably a bunch of experts out there that know the quirks of this engine. Any ideas ? Thanks in advance for your help.