18 HP OHC - turns, won't start

I have a Poulan Pro with a B&S 1 cylinder OHC 18hp B&S model 31D707 0458E1 that ran fine, until it started surging mid-summer. It always was a "rough start" - meaning, I'd turn the key, it would pause, then crank, then pause, then crank and then eventually start. I replaced the battery and replaced the carb because it was so nasty it couldn't be adjusted, replaced the air filter and after I bolted it all on, it started and ran like a champ. Although - it still wouldn't turn over very...

18 HP OHC - turns, won't start