Ive got 77 pickup. 20r with weber 32 36. Its began to have a strange issue, after ive been driving for a bit, engine is nice and warm. I'll come to a stop sign and it will be idling super rough and slow. Like its about to die. As soon as i give it gas it cruises and drives fine. Its just at idle and is super random. It will happen at one light and not the next. Also if shut it off and back on it will sometimes come back to a normal idle. When it is happening, ive pulled over and messed with the idle mixture screw and if I richen it will go to a normal idle. But will later be rough again wanting me to lean it back out.

To me it obviously sounds like a vacume leak. Yesterday I took the carb and adapter plates off, I took the gaskets out and used silicone between all the plates. Ive had a leak there in the past so I assumed it was there. But it still happened today, and it seemed maybe worse. It almost felt like it wasnt getting fuel, but it reved up just fine, if it was a fuel issue it would have died as I press the gas. Where else do these engines commonly have a vac leak. Ive sprayed carb cleaner around and found nothing. Its got the weber so theres only the vac line to the timing advance. Not many places it could be besides the intake manifold gasket. Could be a dirty carb? This carb is only 6 months old, its never been apart so I doubt theres a leak on the carb.

Probably unrelated, but the positive of my battery was arcing out on my hood for a little while, not sure when but it looks like someone tried to weld on the battery terminal. Voltage is still good.

Also if anyone has any info or recomondations on jetting this weber for 7500ft elevation that would be real helpful. I called redline, they can't help at all, said its all trial and error. Right now I'm at 2000 ft and I dont think that should have an effect on the idle, it started 3 weeks after I got here.

 

Today when it got warm, it wouldn't idle smooth (cold and warm) until I backed out the screw to about 4 1/2 turns. But then ran like a sewing machine.
Next time I drove it ran rough again, so rough. I turned the screw in to 2ish turns like it should be and it smoothed out nicely again. Could it be a clogged pilot jet? If it were an air leak it would have to be so big to make it idle that rough, and if there is a huge air leak why is it sometimes not leaking?

 

Can we get a pic of your engine bay?

 

It's probably not your carb, but I can tell you some of the things it most likely is. I have a '78 that has had the same issues. Turns out it's the throttle linkages have wear in certain spots. I will explain.

The '75 to 78 pickups all use a throttle rod system, from the gas pedal to the carb. From '79 onwards, Toyota switched to a throttle cable system. My '78 was doing the exact same things as you describe, with a fresh pro rebuilt carb on it & no vacuum leaks found anywhere (truck is also desmogged, so that eliminated a lot of possible vac leaks).
Random changes to the idle speed & quality - one time low & chugging like you say, other times too fast, other times right on. Everytime I got off the gas pedal I never knew what it was going to do. Also made it impossible to tune the carb & setup a good idle. It almost drove me nuts trying to find the cause.

Once I started looking into this, I have been taking it in steps, and it has slowly improved. The two main points to look at are the rod & bracket behind the gas pedal, and the hinge on the firewall that the carb throttle rod connects to. In both cases, the rod or the rod in the hinge that acts as a pivot, pass through holes in a stamped sheet metal bracket.
Over time & use, dirt, wear and a lack of lubrication, the rods can get grooves worn in them from the bracket. This can result in the rod not always being in the same spot in the bracket, moving every time you step on the gas pedal. It can bind (too fast idle), drop into the groove (low chugging idle) or land on a spot on the rod that isn't grooved (normal idle).
That changes the rods position, which changes what force is being exerted on the carb linkage, changing the position of the butterfly plates in the carb.
This sometimes causes the plates to open a bit too much, throwing off the idle fuel/air mixture, hence the bad and random idle qualities.

First thing I did was check out the rod behind the gas pedal & take it apart. Sure enough it had some grooves in the rod from the bracket. I welded up the grooves then ground & sanded the welds & rod back down to a perfectly round shape. Reinstalled, put a few drops of oil on the pivot points and it works fine. It solved about half of the random idle issues, with them happening less often, but they did still keep happening. That's when I took a look at the throttle rod firewall hinge. Grabbing the arm & moving it up & down, it definitely has some play in it, again likely from grooves worn into the pivot rod. Bad thing about this part is it's not going to be easy to get apart to fix, as the picot rod is held in the bracket by peening over the metal of the bracket. (Sorry I don't have pics of it).
From looking at it, I'm assuming I'm going to have to hacksaw the peened ends off the bracket to get the old rod out. Then I'll have to find a new rod of the right diameter, and come up with some sort of fastening system to hold it in the bracket.

In the meantime, I have tried both oiling & greasing the pivot points on the hinge, so that the hinge part doesn't get hung up on the grooves on the rod. Both worked to reduce how often the random idle quality happened, with the grease method being better. It only happens every so often now but when it does, I just slap the gas pedal & it usually straightens right out to a smooth, good idle.

Choices? Many say the best solution is to retro-fit a throttle cable system like the '79 & up & be done with it. I've never done one so I can't offer any advice on how to get that done. I'm either going to try to fix the hinge I have, or get a low mileage one from an RV model that likely doesn't have so much wear. Yoda Jims should have some. http://yodajims.com/

 

Makes sense how it could be the throttle linkage especially for a high idle, but how could it cause a low idle? The idle speed screw is a stopper that wouldn't allow the butterflies to close anyway. I'll look into it though.

Months ago I was in the process of removing the old fuel pump in the tank, in the meantime I was driving around with a gascan under the hood. I put the regular black fuel line into the can. I think fuel line is 2 different rubbers, 1 inside the line and 1 outside with a "fiber" in between. Anyway the fuel line turned my fuel filter black from the dissolved rubber....
The other day I decided to change it. It had some floaters. Then I remembered that when I put my tank back together I used a piece of fuel line to take the place of the old fuel pump. So the new filter is not black yet but I think it will happen eventually.

After the filter, it ran rough for the rest of that day, really rough, all day. The next day it ran like butter all day and has since, no problems at all. I actually forgot the truck was running because it was so smooth and quiet with zero vibrations. Sounded like a prius. Which was odd but I aint gonna complain.

But I like the idea of a throttle cable. I think I'll do that.

 

As near as I can work out through observation on mine it happens this way. The groove(s) that can get worn into the rods causes the rods to "flop forward" a bit further than the "normal" position at idle. This seems to hold the butterflys closed more than they should be at idle. That means the idle mixture is now more rich than it needs to be & the engine has a hard time completely burning all the fuel. This makes sense to me as I have noticed that sometimes when I get completely off the gas pedal with the truck still in gear, it will sometimes pop at the tailpipe. That is telling me there is unburned fuel vapor coming out of the engine, through the exhaust system, then the very hot fuel vapor hits open air at the tailpipe & then burns. A sure sign of an over-rich condition. I had a hard time understanding this at first, as this is usually incorrectly called "backfire" when it is actually "afterfire".

When I totally take my foot off the gas pedal and sometimes the rods do not drop into the groove, there is no popping at the tailpipe. That's what led me to look for causes other than the engine. I'm sure the problem could still happen with a sticky throttle cable, sticking near the end of travel near the idle position, but it seems to be much less likely with a cable system. I've never heard of this problem on a cable equipped vehicle before. Maybe that why Toyota ditched the old rod system for cable in '79 onwards.

What kind of gas are you running? Ethanol blend? If you were running ethanol blended gas and still had the older type of fuel line on the truck, the black specks are no surprise. The older type of fuel line was not made to handle ethanol fuels, and ethanol degrades the rubber that way. I had the same problem with black specks showing up in the filter, and even had some build up in my carb's idle circuit. I replaced all the rubber fuel line with new line rated to handle ethanol blends and have had no trouble since. Sounds like you had the same issue with a clogged idle circuit but it sounds like it cleared itself. Lucky you. I had to take out my idle mixture screw, fuel cutoff solenoid and idle jet access plug to get to as much of the idle circuit as possible, for repeated carb cleaner spray-out & compressed air blow-out to get it cleared.