I also have a 18 hp vanguard on a troybilt rider. After it runs for a while and I drop the throttle down to idle and let it idle for a minute or two and then try to throttle up again it sputters and dies. It will restart right away. Carb is nice and clean. New plugs and filters. Compression is very good. It does not seem to have the power it once did.

Hi,
Try the listed site and the OHV models:
http://www.smallenginesuppliers.com/html/engine-specs/briggs/common_specs_english_all_engines.pdf
Peace,
Clarke