445 Engine missing

A new tractor, new problems. I just picked up a JD 445 it starts right up and appears to run well, but when fully warmed up it's stumbling & smells rich. Apparently I didn't fully warm it up during the test drive before I bought it. Doe! I was happy to see it cold start so well. I'm going to dive in shortly to see what I can figure out, but what are the usual suspects for this engine?

thanks!