Idle Mixture or Choke?

[monkeyspunk79]

Hello friends. My 76 C20 had a dying Quadrajet (leaky plugs patched a long time ago with the wrong epoxy) and so I splurged and bought a brand new 1902 Edelbrock Quadrajet replacement last year. It ran well for about 6 months and then started to cough and chug upon warmup. I also think its not putting out the power it should once warm. Its the divorced choke style unit just like OEM. Idle mixture screws are set out of the box, I didn't mess with them. The truck aircleaner still has that stove/preheat thing attached to the exhaust manifold and its 100% original under the hood with the exception of new plugs, wires, and hoses.

Choke thermostat isn't broken and appears to work...but it still coughes black / blue smoke and its rich enough to where I think its thinning out my oil. I also take 15-20 minutes to get to work and at stoplights I notice a random puff of black or blue until I'm almost at work. Not cool.

So last night it was 12 degrees when I left work so I pulled the air cleaner off to observe what is happening. The choke butterfly is where it should be when its cold. Almost closed but still open a touch. The truck was still running like garbage UNTIL I opened the choke butterfly up. I can open it wide open when cold and it improved the idle a ton, raised the speed to where it should be, and stopped chugging out smoke. I let it warm up like this and it was a dream the rest of the way home.

Since I'm not really good with carbs, does this mean my idle mixture is too rich? Why would opening up the choke on a cold start get me a better performance? I'm an EFI guy and I'm trying to work through this. Thanks for whatever help you all can offer.

[Grim 82]

A new carb will rarely be set right out of the box. When you get it at operating temp and the choke is open try setting your idle mixture to achieve maximum vacuum and go from there.

[eventhorizon66]

I can open it wide open when cold and it improved the idle a ton, raised the speed to where it should be, and stopped chugging out smoke.

Very rich idle.  Typical of a remanufactured carb.  You are fuel washing your rings and wearing them prematurely.

Warm the engine up.  Hook up a vacuum gauge to the full manifold vacuum port (it should read strong vacuum as soon as the engine is idling).  Turn one of your idle screws clockwise until you hear the engine speed drop then back off about a 1/2 turn (until the vacuum reaches it's max).  Do the same with the other screw.  When finished, if the idle speed has increased significantly, readjust it to the speed you want and then repeat the procedure.  I'm not sure if this is the "proper" way to do it, but it works for me.

If this still doesn't help, adjust your choke pulloff to open the choke a bit more when the engine catches.   The choke pulloff is the little vacuum canister on the passenger side of the carb (usually in the front, sometimes in the rear).  There is an adjustment screw on it.  With the engine cold, try turning that screw to open the choke plate until the engine idles properly.  It should only need to be cracked "a hair" to "a bit" (that's my precise measurement ).

[monkeyspunk79]

Thanks for the advice guys. I figured it was too rich if I could prop the choke open and she runs like a top. I'll admit I have ignored this since putting the carb on last winter. It ran better than before and I just called "good enough" for way too long. I'll get a vacuum gauge and see if I can try it that way since it sounds simple enough. Can I use the vacuum from a fitting on the base of the carb that's currently blocked, or from the carburetor itself?

Thanks again. 

[Dr_Snooz]

Turn one of your idle screws clockwise until you hear the engine speed drop then back off about a 1/2 turn (until the vacuum reaches it's max).  Do the same with the other screw.  When finished, if the idle speed has increased significantly, readjust it to the speed you want and then repeat the procedure.  I'm not sure if this is the "proper" way to do it, but it works for me.

That's the way that Edelbrock says to do it. If you are going for daily driver fuel economy, then turn them the other direction until the idle speed drops and back of 1/4 to 1/2 turn.

[eventhorizon66]

That's the way that Edelbrock says to do it. If you are going for daily driver fuel economy, then turn them the other direction until the idle speed drops and back of 1/4 to 1/2 turn.

No, turning them in clockwise then backing off the way I described should give the best lean idle for economy and minimized engine wear at idle.  Turning them counterclockwise richens the idle.  I don't think any Q-jets were ever reverse thread on the idle mixture screws like some emissions Holleys.

Can I use the vacuum from a fitting on the base of the carb that's currently blocked, or from the carburetor itself?

You can use any vacuum port that shows full vacuum at idle.

[monkeyspunk79]

UPDATE: I followed the advice and got a vacuum gauge from Harbor Freight for a whopping $12. Did the idle mixture adjustment and made sure she was getting between 18 and 19 inches of vacuum after the adjustment, which shows good on the gauge. Drivability is the same, and best of all my cold start is BEAUTIFUL. Choke kicks in and brings the idle up nicely, and once I kick down the fast idle its no more black smoke!!! Only condensation and the usual exhaust smell. I also changed the oil & filter and we should be in good shape for most the winter. Little less wear & tear on the rings & is always a good thing.

This helped me out a great deal, thanks again everyone. It helped me learn the right way to dial in a carb and this will serve me well with helping my old man dial in his '47 Chevy this spring. This forum is great!!

Now I hope to use a little less gas, too since its up to $3.09 here