94 Dodge B250 conv van jerk when decelerating

I recently brought a used conv van, the only problem was the catalytic converter. The van ran fine except for the noise the converter was making. I ordered the converter and a maintenance guy installed it, but he messed up the muffler too, so I ordered the tail pipe and muffler. He installed it.

Now heavy black smoke comes out the tail pipe, which i thought it was ok since the converter was not working properly and now all that gump is coming out.

But when driving at 40 mph or up to 70 mph, the van starts to jerk when I attempt to decelerate (when I take my foot off the gas pedal) . I have to pressed the gas pedal again for the jerking motion to stop. It also happens when I slow down at a stop light, it would jerk again.

Friends of mine said that the maintenance guy did something to van beside working on the exhaust system, is that true?

Could the van be sort of like in shock now that it can breath again due to the replacement of the exhaust system (tail pipe, muffler and converter)? Is there something else I was suppose to do when replacing the whole exhaust system?

Could you help determine what causing the black smoke and the jerking motion?

Thanks in advance...

First of all, if the maintenance guy didn't offer to pay for the new muffler and exhaust(and labor to install), find a new place to take your van!

The black smoke could be from running rich. Check the inside of your tailpipe; if it's black and wet the van is likely running rich. I'd check or have replaced the O2(oxygen) sensor.

The transmission may not be releasing the lockup feature, which might be why it's jerking when you slow down. How many miles on the van? I'd think that a fluid and filter change for the transmission should be in order.