'86 420 20hp Onan puffing black smoke...surging

You need the usual fuel system cleaning. If all fuel lines haven't been changed do it. Get new pickups in the tank and clean out the tank. At the very least spray seafoam into the carb while its running. Also put some in the gas tank and run it.
If the lines haven't been changed since 86' they need to be. Also clean the tank!
If thats too much work for you, I'll take it off your hands!

Chad,

You should also get the manuals for your tractor from Technical Publications.

Terry

Had the same problem just last week. Turned out to be a vacuum leak from under the carb. Replaced the gaskets (and I say gaskets cause I have a 316 onan and exhaust and intake had to come off to get to the bolts for the carb) and it runs like a top now

Most likely teh slow/idle jet is clogged.
Can be cleared in 10 min.
Take off air cleaner assy and take carb top off.
You will see on drivers side of carb (as in car side) a brass screw towards teh front. use a good fitting screw driver and take it out and clean it. You should be able to see through all the holes and through it long ways.

I have done this many times on my 420 and finally did a whole fuel system cleanup this past winter.

Thank You everyone for the info...going to get the seafoam and checkout the slow/idle jet...(nice one Glenn with that statement I'm guessing my problem isn't terminal) I'll update the outcome!