I'm running a 403 with a stock 4 barrel Quadrajet, on cold starts such as when I'm going to work it idles very poorly and slow for 3 minutes (I timed it) and then fast idle finally kicks in. Is that normal? I'm sure it still needs some idle air adjustments as I'm continuing to smooth out the idle, would that also cause this or is there a different problem I need to hunt down? I did some work on it, but that was just changing gaskets, I never messed with the adjustments. This is also my first carbureted car so I'm not familiar with it.

 

Not normal. On a cold engine with a properly working choke, the first press of the accelerator should allow the choke plate to snap shut under spring tension from the choke thermostat, this will pull the vertical rod from the choke plate upward. The vertical rod is attached to the high idle cam which should have just moved to a high idle step Upon starting, the vacuum choke pull off should open the choke blade approximately 1/8". The car should stay on a higher idle until it is warmed enough to fully open the choke blade vertically and the accelerator is tapped to get the high idle cam to drop back down.

The short answer to how long should it take is immediately upon start up.

 

From what I've seen of it pushing the throttle down does close the choke plate, eyeballing it looks to be open the appropriate amount. Over time it opens up, I'll have to look at it again but maybe the fast idle is only engaging when the choke is open fully. When it gets to that stage and starts fast idling it's almost like a jet engine powering up. Seems like I need to look at the fast idle connection.

 

Check out my video here which shows the operation of the fast idle cam. I think it shows very well how the cam works, even if it's of an older design. If you go to the 1 min mark you can see how the cam is engaged when you first hit the throttle. I was showing it on a cold engine so every time I would hit the throttle, the cam would snap into place.

The operation is slightly different if you have an electric or hot air choke but the principle is the same. The notches on the fast idle cam hold the throttle open a bit to enable the higher RPMs.

 

Thats a great video, if it's not engaging until it's virtually off choke maybe it's not positioned in the right spot.

This leads to a separate but related question, for the '78 M4MC model is the fast idle supposed to come off on its own or is it required to tap the gas pedal every time?

 

This sounds more like an inop choke pull off to me. It has to crack the choke enough after to start up to let enough fresh air in to idle up. If its not opening it'll chug and run poorly. I'm sure the throttle is on the fast idle cam. That can't just fall into place.. the three minutes is probably the choke spring starting to open.

 

Anyone I've seen the throttle must be tapped/opened to get the pressure off of the high idle cam so it will drop. The cams can get sticky and sometimes need to be freed up.

Try putting yours on the second step of the cam with a warm engine to see if it revs higher, the high idle screw may need to be adjusted.

 

I will make a short video of it when I start it up to go home, and check out what it's doing. I'll get back to you on this either tonight or tomorrow morning before work when I have some time to upload it.

Ok, good to know. I can try figuring out how to do that this weekend. Thank you.

 

Ok, I pulled off the air cleaner, set the choke on the throttle by hand, and somehow except maybe a bit of excessive cranking everything suddenly worked perfectly fine. The choke opened up at a reasonable rate, there wasn't the bogging like it was doing. Before it had a consistent 1 minute of chugging before fast idling, then it turned into 3 minutes of chugging, then unlike this morning it suddenly decided to work ok. Besides demonic possession the only explanations I can think of is either the vacuum brake is starting to go bad, or somehow that part of the mechanism is binding and by taking the air filter off this time I bumped it back into place. Here's the video i shot, there is quite a bit of wind noise from the fan unfortunately as i was trying to get a good shot of the carburetor in action.

 

Was the air cleaner cranked down overly tight or was a vacuum hose possibly getting pinched by the air cleaner?

 

As far as I can tell the vacuum line hasn't been pinched, but maybe the air cleaner was tightened too much. How tight is it supposed to be?

 

Is the OP talking about the fast idle cam circuit, or the engine beginning to idle fast?

When one starts a quadrajet car stone cold, it will idle around 1000 rpm out of gear, but will be on the fast idle cam. This is because the engine is cold and fuel is not atomizing well. After idling for a few minutes, the engine will begin to race at 2000 or so rpm if left alone. This is because it is on the farthest fast idle cam step and is unattended. Normally, simply using the throttle would take it off. This results in the common behavior of blipping the throttle to get it off the cam, or at least the highest step. So, I would say it would begin to run fast on the fast idle cam at about 5 minutes, and fully so at 10-15 at idle. Faster if driving, but it falls off as needed when driving. This is why I think the quadrajet is the best carb out there; it behaves almost like a fuel injection.

 

Nah man, EFI behaves almost behaves like a Q-jet..

 

X3, a dialed-in Q-jet runs almost like fuel injection.

 

I'm not sure the exact RPMs it was chugging at but it seemed the same or lower than regular park idle speed. This morning instead of completely removing the air cleaner I loosened the screw. The results were similar to the video last night, maybe slightly less well. It took a minute or less to start racing, not 3 like before. I also noticed the engine ran significantly better on the drive to work this morning compared to yesterday morning when it was slow starting.

 

When's the last time you changed the air filter?

 

Earlier this year when I got it on the road for the first time, it's brand new. It seems what's been happening is I thought the air cleaner needed to be tightenened down way more than it did, which pulls the housing down onto the choke pull off mechanism causing it to bind.

 

Is the air cleaner stock?

 

There is supposed to be a gasket between the air filter housing and the carb. If that is missing that might cause your problems

 

As far as I know.

It was missing when i got it, in fact the air cleaner was in the trunk. I did get a new one and put it on but it was very thin. Certainly wouldn't have added much space under the housing.