89 D50 running badly--need help

We just purchased this d50 out in loomis. stopped and put gas in it and drove it up the hill to camino. started having issuses with cutting out, black soot coming out of exhaust. at an idle it runs like a top. took it to a mechanic, he couldn't find anything..., thought maybe water in the fuel. Put in fuel treatment and filled the tank. seems to be somewhat better...but still having problems off and on...
seems to happen sometimes at higher rpm and if you shift up it helps. although I did come up the grade out of placerville no problems at all until I got off the freeway.

Seems to be an off and on issue. sometimes it runs great other times not.

Any ideas?

Would appreciate any help.

The very first thing you need to do is make sure the choke is opening up all the way. Pull the air cleaner, get it hot and look at the choke. Is it open all the way.

What type of carb does it have on it. Stock, aftermarket, Webber? Also get us a picture of whats under the air cleaner. Black smoke is over fueling. One trick is to snap rev it, then take a rag in your hand and cover the intake. This will cause fuel to suck from all the holes in the carb. Don't let go of the rag. When it slows down remove the rag and get it to idle and clear out the excessive fuel. Do that a couple of times and test drive it. You might have a chunk of rust in the needle and seat. This can sometimes blow it out of there.

OK...so we did camoit's snap rev trick...(by the way it is a mikuni carb.) husband was playing around and we unpluged the vacuum hose on the air control valve on the air cleaner intake. I don't know if it was the snap rev trick or removeing that vacum hose but it is now runing about 95% better. only does the stuttering at the upper end rpms. cruised out to get the boy at the bus stop and it drove fine as long as I didn't rev it up to high.
Now the question...the hose we disconected off of the air control valve goes to another valve? inside the air filter housing and another line goes from there to another unit with a bunch of vaccum lines on it that is down under the carb on the intake maifold. What is that piece called?

Can you get us some pictures to see just what you are talking about. Air cleaners normal have the air flow control damper on the neck. That directs air from the exhaust manifold into the air cleaner. Then there is a thematic control valve. That controls how far the heat damper valve is open. From there it will go to a manifold vacuum or some other thematic control valve to shut off vacuum to the air cleaner system when the engine is warm enough. It should rev all the way up once you get it fixed. Sounds like there is more to go.

My 89 d50 4x4 does the same thing, has no power on hills, sputters when its floored, so I got to looking,whith the engine off I twisted the throttle all the way open and only one bowl is getting gas?