Help needed for intermittent poor running

[bnam]

My friend in his 71 kombi and I in my 71 vert went for a 70km drive this morning. Both cars drove well. When we got back to my place he had me look at the slightly uneven idle on his bus. I pulled out#3 wire on the distributor cap and the idle actually improved. That surprised me. I put it back and the bus returned to the uneven idle. I pulled out #4 with the same result. I confessed to being baffled.

He wanted me to help clean out the idle jet which we did. We also removed the carb to see if the choke coil was ok inside. We the put back the carb and tried starting the car. It started and idled ok (better but not perfectly). But it would not rev up. When throttle was opened it would sputter and die. Once it backfired. And would emit black smoke occasionally.

Prior to restarting, we had also swapped plug wires with new ones since the old ones had rat bites on 1 & 2 wire. Plug 1 was checked and it was a good tan color. After trying to restart, plugs were checked - they were dry but heavily sooty. We checked for air leaks.

This poor running had happened a few times before, but each time seemingly resolved by unrelated fixes - first time by removing and plugging booster hose, the. By replacing a burnt fuse and another time by having an electrician repair the harness as it had been rat bitten.

My friend has left the car with me as we could not get it to run beyond idling. I’ll post a sound track tomorrow of the engine.

Any thoughts on what could cause this and where I should start? We swapped in a spare Bocar 34pict 3 with same result. We cleaned out all jets and passages with compressed air. Put a soft wire into transition orifices to make sure they were open

Freshly rebuilt 1600dp with bocar 34pict3 and a dvda rebuilt by Bill. Idle mixture and timing was rechecked and reset a week ago. Bus ran well since then.
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1971 1302LS Convertible (RHD) owned since '74
Click to view image
1965 Karmann Ghia Coupe - under restoration
1966 Fiat 1500 Cabrio (with 1600 Twin cam)
1952 Citroen TA 11BL

[Tram]

Vacuum leak on the 3/4 side would be my first SWAG. What do the 3/4 plugs look like?
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[Angus II]

Hello,
Not idle related but, your friend needs to put out some DECON rat killer were he stores his bus...
Recheck correct firing order of plugs wires, bead blast and re-gap plugs(or get new ones),clean all electrical connections in the ignition system, check condensor wire for nibbles(voltage leaks). Just go back over the basic. And change the fuel filter, could be a plan of action.

[TomWesty]

Keep it away from rats.
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[SGKent]

check the dual port boots. One may have a small crack. Also check the timing and dwell. If it has an electronic ignition then swap it with another known good one to see if that affects it. When you pull the wires out one at a time, look for an RPM drop. I believe your hearing is not the best way to judge an even idle.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[bnam]

Unfortunately, we only checked plug 1 before refitting the carb. After we refitted it and experiencing the problem we checked 2/3/4. All were csrbon fouled but dry. Not sure if this was from beforenor fromnour attempts to getnit started and running.

I had checked timing and dwell the prev week. Timing was correct and dwell was at 48deg. Will check again. Rechecked Firing order umpteen times. But will pull valve cover make sure of #1 tdc and check again.

Hadn’t looked for bittten or cracked manifold boots. Will check that.
_________________
1971 1302LS Convertible (RHD) owned since '74
Click to view image
1965 Karmann Ghia Coupe - under restoration
1966 Fiat 1500 Cabrio (with 1600 Twin cam)
1952 Citroen TA 11BL

[Tram]

Well, it sounds like you are exhibiting the same plug condition on all 4, so it's time to go through each system with a fine tooth comb. Ignition first.
If points ignition- sounds like possibly a bad condenser, or maybe burned points. Does the coil get really hot? If so it could be failing. What brand and part number of spark plugs?

How is the charging system output? Ground straps and grounding points?

Fuel pump pressure?
Hard to play armchair diagnostician but it doesn't sound like a vacuum leak. Carbon fouled plugs usually indicate a rich condition.
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