Craftsman II, 12.5hp, 38" deck. New belt, engagement arm functional. Blades spin after engagement, but there's a stop and start theme. Makes an awful racket, too. Engine strong, drive belt seems fine, too. Just can't keep the blades spinning. Any advice?

Woodruff Key or shear pin?

I have no idea! I just drive it, I don't fix it.

We thought you wanted ideas on how to fix the issue. If you just drive and don't fix then call Sears or take it to a shop nearby. Do not continue to engage the blades. http://www.searshomeservices.com/lawn-mower/repair?sid=HSRx20090527x00058

I do want to fix it. I'm a beginner.

OK lets start with who installed the belt, you? Did you follow the belt diagram on the Sears deck? sears tractor belt diagram - Google Search (http://www.google.com/images?hl=en&q=sears+tractor+belt+diagram&um=1&ie=UTF-8&source=univ&ei=XW75S_uFFcL6lwfI98XqCg&sa=X&oi=image_result_group&ct=title&resnum=1&ved=0CCgQsAQwAA)