I have a 8 year old (hours unknown) toro 44" with the briggs 18hp vanguard engine. I bought it used about 3 years ago and it has run strong until recently. It is now putting out black smoke when running and loses power when you turn on the pto. I have cleaned the fuel lines and carb but no change. The engine races some when running but generally it is just weak. The initial trouble started when mowing a tough tall lawn a few months back. The mower stalled then, and barely ran when restarted, but when I tried it the next day it ran decent, and I admit I used it on a few lawns while it was smoking, out of necessity during a busy time. But now it is really just unusable. The mower is old enough I'm not interested in a new engine or really spending much on repairs. But if it's something simple I'd hate to sell it for parts. Any ideas from the engine experts on what problem this is? Just worn out?