Carburetor Idle mixture screws?

This can be difficult. These are hardened metal caps over the mixture screws in the baseplate of the carburetor (front). We used to take a small metal punch and pop hole just behind the length of the caps to help drive the caps out.

No it's not too bad, I was saying the caps are hardened so you can't drill though them, yes turn the carb over to do the job.

Those were installed at the factory after they were adjusted for proper mixture. You have to cut or crack them to get them off, then they are discarded.

After a rebuild, it is a normal part of the procedure to adjust the mixture screws. The plastic caps were intended to prevent tampering with what was already adjusted properly at the factory. Conditions are different after the rebuild, so those caps are discarded.

Idle speed is not related to the mixture screws. If you hold the throttle blade open a little, you'll see if you work the choke blade by hand, a linkage pulls up on the idle speed cam. An idle speed adjusting screw rests on that cam and is used to set idle speed. The choke needs to be wide open, then, that cam needs to drop down to the low idle position. It can't do that while the throttle is released because the idle speed screw is putting pressure on it. That's why you must tap the accelerator pedal momentarily to let that cam drop down.

Sometimes those cams get gummed up with grease and oil residue, and need to be cleaned with Carburetor Cleaner from a spray can, but that shouldn't be a concern now, right after a rebuild.

I should back up a moment and start from the beginning. When the engine is cold, a thermostatic spring is trying to pull the choke blade closed, in preparation for starting. The idle speed screw is holding the cam from moving. That makes it impossible for the choke blade to close. You must press the accelerator pedal just an inch or two to let the choke blade close. Normal procedure is to press the accelerator further, one time. That "sets" the choke, and fuel sprays from the "accelerator pump" into the intake manifold for priming. That spray is needed because liquid gas does not burn. Only vaporized gas can burn. The goal of that sprayed-in gas is for a high enough percentage to turn to vapor by the time it gets into the cylinders for the engine to run. Most of that gas goes out the tail pipe, wasted, as a liquid, until the engine warms up.

As soon as the engine starts running, a vacuum-operated "choke pull-off" pulls the choke blade partway open through a short linkage. How much it pulls the blade open is critical and is adjustable. Opened too much when the engine is still cold will cause a lean stumble when trying to drive, or the engine may stall. Open too little, you'll have black smoke from the exhaust, or the engine may stall from being flooded. When you have a choke pull-off on the side of the carburetor, the linkage is in the form of an "S" bend. The adjustment is made by squeezing together or pulling apart that bend to make the linkage shorter or longer.

Many GM carburetors used a black plastic disc on the side of the carburetor for the choke pull-off. They have the thermostatic spring and the choke pull-off built into that one assembly, and on newer models, there could also be an electric heater built in to get the choke opened faster. Those will have one wire attached to the side of that disc. The disc is held on with three screws. Loosen those screws just enough to allow the disc to be rotated. That is how that style is adjusted. There's usually writing molded on the side with arrows that tell you which way to rotate it to make the choke open more or less / sooner or later.

When you don't have that black plastic disc, the thermostatic spring sits in a well in the intake manifold. A "heat riser" valve in the passenger-side exhaust manifold closes to force all the exhaust gas from that side to flow through a passage through the two cylinder heads and the intake manifold, then out the left side. That hot exhaust gas passes right under the thermostatic spring to warm it and cause it to open, and it warms the base of the carburetor to prevent icing in cold weather. It used to be quite common for that passage to become blocked with carbon. The symptom was after running okay for a few minutes, the choke wouldn't open all the way. A lot of black smoke came from the exhaust, and the engine ran very poorly. The electric assist on newer models helped solve the choke opening, but it didn't help with warming the base of the carburetor.

Related to that, the snorkel on the air filter housing has a vacuum-operated "motor" that opens a valve in cold weather so warmed air is drawn from around the left exhaust manifold, through a tube, into the air intake to help with getting the liquid gas to vaporize.

The first photo below shows the Quadrajet carburetor uses a combination of the things I described. The blue arrow is pointing to the vacuum-operated choke pull-off. Instead of the "S"-bend linkage, a screw, (green arrow), is used to set how far the choke blade opens. The red arrow is pointing to the cover over the thermostatic spring. The three screws are loosened just enough to allow that disc to be rotated. Just to the right of the red arrow, you can see a group of little notches in the housing. Those are to indicate where the disc is set. Adjustments are very small to make a big difference. When an adjustment is needed, usually one or two notches is enough. There's no terminal on this one for the electric choke assist. If there had been one, it would have been just above or to the right of the red arrow.

In the second photo, the purple arrow is pointing to the idle speed screw. On this carburetor, the accelerator pump is adjusted by putting the linkage in a different hole in the pump's lever, (orange arrow). There were normally three holes, but there's only two in this photo. If the pump was set to take a stroke that was too short, you'd have a miserable stumble and hesitation when trying to accelerate, even very gradually. If the stroke was adjusted too long, engine performance would be okay, but it wasted gas every time the accelerator pedal was pushed down. This is the lever for the pump that sprays the priming squirts in when you press the gas pedal to start the engine.

Another adjustment has to do with when the secondaries open. Opening too soon wastes gas. Opening too late causes a lag in power when more is needed for passing or going up hills. This adjustment is easy to do, but hard to describe. It involves bending the linkage at the light blue arrow. You bend the top of it forward or rearward. The top of it pushes on the spring-loaded lever, (yellow arrow). The more the link is bent forward, the further you must press the accelerator pedal before the secondaries start to open. Adjusting them to open later can increase fuel mileage at mid-throttle where the extra power isn't needed.

To address your question about having to press the accelerator pedal twice when the engine is warm, most likely too much gas was going into the engine when it was stopped, and that created a too-rich condition for the next start-up of a warm engine. Pumping in more gas next to the base of the carburetor gives it a chance to vaporize by the time it gets to the cylinders. The engine needs that gas to be in vapor form to burn. A potential clue here is the engine starts rather easily when cold, without having to set the choke.

It is also possible the engine was running too lean when stopped last time. Little fuel is left in the intake manifold. Since cranking takes place at a very low speed, fuel is not easily drawn into the cylinders. You need a lot more liquid gas to ensure a high enough percentage vaporizes to get the engine to run.

Okay, if it was running okay before the rebuild it may just need the idle speed reset. Base idle is set on the side of the carb where the accelerator pump is, there is a screw there that you adjust to set the base idle. However, you will be making multiple adjustments to get things correct. First start the car and let it warm up. Now make sure the carburetor is not on the fast idle cam. Next set the base idle with that screw. Next you will need to adjust the idle mixture screw to get it closer than adjust the base idle, then the mixture again until you get it into the correct settings.

This might help:
https://www.youtube.com/watch?v=sBMckb5PGZY

Thank you, Steve. I apologize for the delay. Ran into four Goodyear tires leaking through the sidewalls, (one original on truck, and three replacements from an auction), a flat tire after sitting just a half hour in a Walmart parking lot, (9 degrees below zero), driving five miles home twice on five pounds in one tire, (no internet at home, so I sit by Walmart to use theirs), and now I'm down to my last replacement wheel / tire other than the spare. Have a friend with a body / repair shop where I can switch tires, but he is in northern Michigan playing with snowmobiles. If this last replacement tire gives up like the rest have done, please don't panic if it takes me a day or two to reply. I really don't like winter!

The idle screw Steve mentioned is the purple arrow in my photos. He can answer more questions if I'm slow in replying.

If it's running too lean, you may have a hard time getting the idle speed down to where it should be. You also have to look at the height of the float. If that is too low, the mixture will be lean. It can also make it a little harder to start the engine because fuel has to be pulled up higher to get into the venturis before it goes into the intake manifold. Engine vacuum does that, and vacuum is really low during cranking. To overcome that, it may be necessary to pump the accelerator pedal, even when the engine is warmed up. A potential clue to that can be the engine refires easily right after it is stopped, when warm, but it's harder to restart after sitting for a few minutes. While sitting and warmed up, residual fuel in the intake manifold has a chance to evaporate. Upon restart, new gas has to get pulled in first.

Another thing to be aware of is there was a somewhat common problem with plugs in the base of the carburetor leaking. To form some internal passages, they need to drill two of them in different directions, from a common starting point, then they close up the exposed hole with a soft metal plug. When a plug leaks, the engine usually still runs fine, but once stopped, fuel leaks from the float bowl into the intake manifold. That might create an excessively rich, or flooded condition, along with the need to crank long enough to refill the float bowl, then some gas vaporizes in the venturi for the engine to fire up on. Remember, liquid gas doesn't burn. Even though a cylinder may have a lot of liquid gas in it, as long as it doesn't short out the spark plug, the cylinder will still fire once enough vaporized gas is in it.

The clue to a flooded condition is you will need to hold the accelerator pedal down a little to keep the engine running roughly, until it clears up within a few seconds.

The fix for a leaking plug was to scrub the area clean and wash it, then seal it with a dab of epoxy.