Idle drops when touching clutch

[TheProtoSmith]

Hey folks so I just picked up a killer barnfind 1984 720 4x4 king cab. That had less rust than my 5 year old daily. So I've been wrenching to get it a 100 percent but I am stumped on this one. When you so much as look at the clutch pedal. Touch it to preload (but not depressed) the engine will loose about 700rpm and start spewing an over rich black exhaust. I have no idea why. I think I still have a vac leak somewhere, but this problem only happens when using clutch. Break booster is working fine ( maybe a little stiff but she old, so I don't think it's that). Is there a vacuum like that goes to thrust bearing or transmission? Looking for any new rabbit hole to jump down..... Thanks folks.

[datzenmike]

A vacuum leak would not cause black smoke, indicative of an over rich condition.

 

The'84 has a fuel cut off circuit that turns the idle cut solenoid off shutting off fuel to the idle circuit when decelerating only at highway speeds. This fuel is not needed and is usually just sucked out the exhaust and wasted. There is a switch on the clutch and another for neutral in the transmission that turns the idle cut back on (so engine doesn't stall)  if down shifting. 

 

It shouldn't be doing what you describe.

 

 

This 700 rpm loss is sitting still at idle???? or driving??