I think I found the source of the problem I'm experiencing with my 1979 Pontiac Catalina. If you recall, I asked why I'm having a very high idle when the engine is hot. I drove the car around to get it nice and hot, then came back home and put it in park. The RPMs revved up like they have been doing once the load of the transmission is removed and stayed there. I recorded a video of the idle with the air filter cover off. The choke is not shutting when the engine is hot. You'll see in the video that I manually adjust the choke to about where I'd expect the choke to sit when it's closed, and the RPMs come way down, close to what I would call a "normal" idle on a hot motor, and it runs well.
I traced the linkages on the choke, and I found an arm that self-extends toward the linkages to the choke, but it is not connected to anything. Is this thing supposed to be connected to the choke linkages somehow?
Choke Mechanism
Engine Running
P.S. Is that prominent suction sound I'm hearing evidence of a vacuum leak or is it just the engine breathing?
The choke linkage should be connected to the thermostatic spring, which is in the round housing to the right of your hand in the first video. As the engine heats up the spring operates the choke linkage. The black cover on the spring housing adjusts the choke richer or leaner.
Should there be anything attached to that odd lever looking thing? It's connected to a vacuum line, but it's not connected to the linkage.
If I understand the part you're talking about, just to the right of the choke coil housing, it appears like it's involved in controlling the idle speed but it's hard to be sure. Late 1970s carburetors have all kinds of crazy emission-control gadgetry and I haven't worked on one of those GM carbs.
The arm doesn't look like it does anything when it's extended, which is interesting. It's a choke pull-off per google. I might have to take it to someone who knows carburetors much better than I do, lol. I assume that really high idle isn't normal. It sounds like you're pushing on the gas, as you heard in the 2nd video. The car jumps when you put it in gear if it's hot, which probably isn't great on the transmission.
No, it's not normal. Hot idle on that engine is probably supposed to be something like 700 rpm. You've already observed that the choke is not working properly unless you manually manipulate the linkage, so something is amiss whether part of the linkage is messed up or the thermo spring is way out of adjustment or not working. On one of my old cars I recently installed an electric choke because the heat tube going to the thermostatic spring housing rotted away.
Not sure if this is related, but I have noticed if you touch the pedal soon after a cold start and don't really push on it like you would to accelerate onto a road, like if you're pulling forward slowly in a parking lot, it will stall. Only does that within the first minute or two of a cold start.
If it weren't for the fact you're saying the fast idle never comes down I'd say it was because when you touch the throttle the fast idle cam is being released prematurely while the engine is still cold. (Remember, to take the engine off fast idle you do have to "blip" the throttle so the fast idle adustment screw is pulled away from the cam so the latter can move down a step.)
It sounds like your carb is getting hung up on the fast idle cam. The sucking sound you are hearing is your throttle plate partially open and trying to transition. The fast idle cam is supposed to hold your throttle butterfly open until the coil warms up and opens the choke plate. The it comes off and returns the throttle plate to the idle position. Check the linkage between the fast idle cam and the throttle plate. Squirt some WD40 on the shafts inside and outside the carb body to make sure they are rotating properly.
Exploring the area this morning, I adjusted the thermostatic spring slightly and twisted it toward lean. I found a couple vacuum hoses that were disconnected and reconnected them. When I started it cold, it started and stalled, so I backed off the spring a bit. It runs slower. I sprayed carb cleaner all over that area of the engine and the idle slowed. I found a hanging 3" long vacuum hose with no obvious connection. Some hoses are cracking, so starting tomorrow I'm going to replace vacuum hoses one at a time. I ordered the OEM service manual on CD from RockAuto, so that should help out quite a bit.
Good that you ordered the shop manual, there are so many vacuum hoses and emission-control gadgets on those cars you really need it. Also there should be a vacuum hose diagram under the hood somewhere unless it's gone missing.
I started replacing the cracked vacuum lines I found yesterday, and the idle has come down noticeably. It turned out that the loud sucking sound in my video was a vacuum leak, not a problem at the carburetor itself. I did find a sticker for the vacuum lines under the hood, but it's not 100% clear where things go. I believe a few lines aren't situated correctly, so I'm gonna look at the manual when it comes. I fixed the vacuum leak for now by putting a piece of hose on the barb and threading a screw into the open end. After doing that, it's still somewhat jumpy when I put it in gear while it's hot, but it didn't chirp the tires this morning.
Can you develop a vacuum leak in an EGR valve or near/ on the housing? I kept exploring the engine while it was running, spraying seals, hoses, etc. with puffs of carb cleaner, the engine slowed when I hit the area around the EGR valve, spraying the base seemed to point to a spot. The vacuum line into the valve is brand new, it's one of the ones I replaced today.
Sounds like you are making progress. There are a lot of vacuum-operated gadgets on late 1970s cars for emission control. It's not surprising to find one involved in controlling the idle speed.
The EGR valve or its gasket could very well be leaking. A vacuum tester can be used to see if the actuator is working or if it is leaking. On a car that old everything has to be treated as suspect.