I finally got around to rebuilding my Quadrajet 4MC this afternoon. Everything went well from what I can tell, with my limited carburetor experience. The car starts amazingly well, which was one of the main issues I was hoping to resolve. However there is one new issue I am seeking opinions on.

I adjusted the idle mixture screws with a tach and a vacuum gauge hooked up, the car had only been running maybe 5 - 10 minutes, but long enough for it to come off of the fast idle. It was running great, about 15 inches of vacuum and idling around 800 rpm in park.

I pulled it out of the garage and was about to take it for a test drive when the idle dropped way low, around 200 rpm or so but it keeps chugging along without stalling, until it is put in gear.

It seems like once everything was thoroughly warmed up, the idle went to hell on me. If I give it some gas it'll come back up until I let off the throttle again, but seems to run rough under 1000 rpm.

Any suggestions of what I should look at first? Seems to mainly be the idle circuit that's messed up, and mainly when warmed up.

Thanks,

 

So it idled that low the whole time after you left the driveway? I would reset the idle screws and try the adjustment over again. Did you adjust each screw equally? its important not to have one screw out 3 turns, and the other 2... I usually do two adjustments. The first one after getting the most vacuum, then set the idle speed. Make sure you goose the throttle after each adjustment of the idle speed screw so that you know its not hung up. Then after the speed is set, I redo the mixture screw adjustment to make sure its pulling the most vacuum at the appropriate idle speed. Give it another try because it doesn't sound like it could get much worse.

 

Wait. Are you telling us that you adjusted the carburetor without the car being warmed up?

And specifically, did it come all the way of the fast idle, or just of the first step?

The first step in the instructions says the car has to be warmed up before adjusting the carburetor or any other tuneup procedure.

Go back and do it again.

- Eric

 

There are a number of steps of fast idle, it sounds like you adjusted it with at least one step left to go. Take the car for a ride to fully warm it up then start over. Also I've learned that when adjusting idle screws wait 30 seconds or more between adjustments for things to "settle in" before the next turn of the screw.

 

What they said^^^^^^^^^^!

 

I always re-tighten, ALL screws, after 1st warmup.
Squirt the choke linkage with a little WD-40, too.
Turn the idle up a little, and let it warm up!

 

It was all the way off fast idle. I admit I rushed the tuning though. I will do it again today after giving it more time to warm up, as well as re-tighten all the screws/mounting bolts.

The manual says set the idle mixture screws 4 turns out and adjust from there, yet I've read on here 2 turns. What seems to work for those of you with this carb? I will likely follow the manuals 4 turn recommendation.

It just seems odd to me it would idle so poorly warmed up after a rebuild with very similar idle settings as it had before the rebuild.

 

The only thing that I can say is possibly the linkage was in a bind or it was on the hi idle setting. Did you set your idle with it in gear, somewhere from 650-750 rpm???

You can start from whereever 2-4 turns with idle fuel mixture!! I usually start at 2.5! Then start turning them in.

 

Turns out after the first warm up the air horn screws and carb to manifold bolts came loose some. I re-torqued them and gave the idle mixture another tune up and it runs like a champ!

Thanks for all the suggestions.

 

Great!

And you did it all by yourself, without spending a lot of dough.

Enjoy your ride!

- Eric

 

Thanks! I wish I just checked the bolts last night, but it was getting late, cold, and I was getting hungry.

Cost about $30 for the kit and float. Haven't had much auto carb experience, just smaller weedeater and lawn mower carbs.

One last question, what sort of vacuum reading is normal at a warm idle (in park). I'm getting just under 15 inches. Before the rebuild I think it was more like 12.

 

It could have been worse, you could have been wet too!! Glad you got it fixed!!! The higher the vacuum the better! Is it steady?