This one's just about got me stumped, let's see if we can figure it out as a group.

'85, stock but strong 12A, RB 'street port' exhaust, no rats nest, direct-fire HEI ignitors on leading spark, regular distributor/igniter on trailing, Nikki carb. Zero issues starting hot or cold, idles somewhere between 800 and 900 RPM (can get it down more I suspect, no signs of vacuum leaks). The accelerator pump is adjusted so the rear of the arm is parallel to the cover at idle as it should be from reading threads on here. Floats are set right (just below the middle of the sight glass). Manually working the throttle looking down the carb, the pump shots are strong and clear, without any perceptible 'slack' between the throttle arm and the flow beginning.

Here's my issue - once fully hot, every now and then as I get on the gas from closed-throttle (whether it be between gears or (more often or at least more noticeably) from a stop) the engine briefly cuts out. If from idle, revs will drop concerningly low and recover (though today, for the first time, it completely stalled on my as I hit the gas to pull away from a stop sign, long before the clutch pedal came off the floor). When between gears at speed it will lug hard when this happens (again this is as soon as you get on the gas after the throttle has been closed, unrelated to driving style / clutch-work - can be several seconds after being in-gear ) It used to do this, then didn't for a long time, and it has recently come back.

I have adjusted the idle mixture up/down a quarter turn to see if it makes a difference, and it doesn't. I adjusted the accelerator pump linkage as mentioned above, didn't solve it. It's such an intermittent problem it's hard to judge if something makes it 'better' or not.

I think my next experiment will be adjusting the idle down a hair and see if that does it. If not, maybe I'll move it up a bit too. Do these carbs have an idle transition slot that maybe I'm into with the 8-900rpm idle setting? I'll readily accept any further suggestions of things to try!

 

Your accel pump diaphragm is probably old. Or maybe the little check ball down inside the carb is sitting at a less than round section when it settles in the machined cup. I have some carbs with an intermittent weak accel pump shot and I suspect that is what it is. Maybe the machined cup is just a bit worn out. I've gotten them to about 90% accel pump shot frequency by over-adjusting the 7mm nut to put some preload on the diaphragm. This works but it runs out of travel about 10% too soon (mine are set to squirt all the way through to 100% throttle opening but on these troublesome carbs it reaches the end of its travel with still 10% throttle opening left, if that makes sense).

But if your car has a lot of stock stuff like rats nest and shudder valves with potential vacuum leaks, maybe it's one of them causing an intermittent problem.

 

This is a timely thread for me as well. At DGRR I had some fuel pump issues caused by a poor ground and I fixed that with no issues and completed my DGRR fun. I did notice that I was experiencing something like what you have described. So I did the following:

1. I replaced my current fuel pump setup (old dual facets) and put my Carter 4070 and regulator back in to provide a constant 2.5 psi at the carb.

After this I still experienced the same issues.

2. I checked my timing which seemed a bit on the advanced side so I went a but more conservative to see if it had any affect.

It may have but if it did it was minor. I did this because I had fixed an issue prior to DGRR with the hold down nut and bolt for the dizzy which caused me to restab the dizzy after fixing it up.

3. I looked for vacuum leaks and found the left front bolt holding the top cover on completely loose. Turns out it was too short and needed to be swapped for a longer one from the other side. Also the throttle body to the carb base nuts were loose on 3 out of the 4 there. So I cranked them down securely. The carb base plate bolts all seemed tight. The front of the carb is covered in a fine greasy black coating as if something had been leaking. I suspect fuel/oil was maybe leaking out or something from that loose top.

This seemed to help but once it got good and warmed up the idle was still a bit more lumpy than before I did the prep work for DGRR.

4. I rechecked the timing and also messed with the mixture screw as well. Took it all the way in and backed it out 1.5 turns. This seemed to give me a better, more stable idle.

The symptoms are better but not gone completely. Its also hard to start cold and at DGRR it was blowing by oil a lot more than it ever did. I hope I have to build a new engine!

Anyway thats my story so far. I'm pretty sure I had some vacuum leaks and some timing/tuning issues from mucking around with the dizzy. I'm also sure the last DGRR was hard on the car as I drove very hard most of the time <grin>. I don't know if this helps or not but its always good to hear about others experiences.

 

I discovered that hitting a bump or compressing the left front suspension will cause my car to immediately stop running if at idle. I was able to reproduce this by letting the car idle and then standing on the front bumper and giving it a good bounce, car shuts down like an off switch was hit. If I bounce the passenger side, no issue.

Soooo, I have a short in my loom or somewhere something is flopping and causing a short or break on a critical circuit. It just happens that all my wiring is on the drivers side for the HEI ignition and and fans and such.

I'm sure this will be fun to find.

 

I once had an issue similar in the GLC. I probed the wires and did all kinds of back flips when all it was was a front turn signal bulb (1157 or 1156) that had loosened out of its metal base and the wires were shorting. It would blow fuses and burned out a blinker unit, blowing smoke. Good luck.

 

So its actually not an electrical issues <whew>. It seems that at idle my air mix, throttle speed adjustment and fuel pressure all have to be just right so that when the carb gets bounced around I don't loose idle. I suspect the nikki work I did created the issue because I didn't do the modifications of any of the air or fuel bleeds at the top of the carb. I just adjusted my air mix at idle until the car would die when I stood on the bumper and jump up and down on it. Kind of redneck but it worked.

At some point I need to visit the whole nikki process and do some of the mods Jeff has determined that make things smoother.

Sorry to hijack like this, now back to your program

 

Air bleeds are the most important aspect of these carbs. They make or break it. Everything else is secondary, even the secondaries!

 

While looking into another issue I had the top plate off the carb, and noticed fuel would come out of the screw indicated below when I worked the accelerator pump. Tightened that up. Should be back on the road shortly (barring any new major problems) and I'll update this thread accordingly.

 

I once came across a carb with a similar stumble that had a loose screw there as well. It is rather uncommon so it is not one of the first places I usually look. Glad you found it.

Fuel can still come up past the threads regardless of how tight you make this screw, and I'd caution you against over tightening it as that will not stop the leak. The threads are straight and loose fitting as all the other threads in the top of the carb. They do not seat or seal against anything because the actual seat is further down the screw to seal the chamber which contains a steel check ball and a thin brass weigh. This chamber is one of two valves in the accel pump circuit that allows fuel to go one way only. The head of the screw is stamped 30 which tells us there is a tiny .3mm drilled hole allowing for a "siphon break" which prevents fuel from literally getting sucked out of the accel pump nozzle under heavy acceleration. There are two holes in the float bowl which connect to the accel pump. The lower hole draws fuel in. The upper hole is related to the anti-siphon chamber above the seat. I hope I explained this well enough.

One thing you can do is wrap the threads with teflon tape. This will reduce the leak down to practically nothing. For NA, seems to be fine. If boosting, you can add a layer of RTV on top of the screw head then scrape off the excess with a business card, leaving a disc of material, flush with the top of the carb main body.

 

I'm happy to report that after tightening down that screw (tight but not excessive) and shooting some carb cleaner into various orifices (might as well while it's open...) I've not encountered this problem again. Went for a 70 mile partially-spirited drive this evening and had no major stumbles or bogs. There was the occasional very slight-hiccup but that's pretty much to be expected. I'll continue to monitor the issue, but I believe tightening the screw above (which was completely loose) solved the issue.