Hi, I was wondering if anyone could help. I recently had a 2.1 L put in my 87’ VW Vanagon. The old motor had a knock so I bought a used motor from a National supplier and had it put in at the local shop by a guy who owns 3 vanagons. The motor came with a 30 day warrnty. It is about to expire. The motor runs fine untill I go up to the mountians on dirt roads or climb a big hill on the highway. It starts to chug like it is flooded or power is cutting out or missing and then it will kick back in.I recenly noticed that there is black carbon smoke that comes out the exhust when this happens and the tachometer drops down and then kicks up again. This is a sparatic problem I live in montana so this happens alot. I have tuned it up and put in a new fule and air filter. The oxs sensor is not hooked up because the leeds are broke. It was not hooked up on the old motor and i did not have this problem. I can go 75mph down the hiway with no problems untill it hit a big hill then it will chug. Sometimes after that it will go away and sometimes it won’t! I spent all my money on the motor and swap so i dont have funds to throw at try this or try that witch is what the VW guy is telling me. The injectors are the ones that came with the used motor and the distributor is as well. It has a new rotor cap plug and wires. The motor runs greaat and has about 85,000 miles on it. Can someone please help so if it is the motor i can at least not be down 2 motors and no money. There is no point in owning a camper van if you cant safely or unstressfully camp. And my wife wants to kill me and the VW guy. Thanks Larry
sounds like the ECU is adjusting the mixture too rich when the engine is under a load. Of course it could just be that it doesn’t have much power anyway and it can’t make it up a hill at 75 MPH 
All bets are off until you get the oxygen sensor hooked up. The ECU is running in open loop mode because of the lack of input from that sensor, and that means a rich mixture.