Quadrajet - High Idle

[jmabrao]

Hey guys I just rebuilt my quadrajet on my 83 GMC C2500, I started with the idle mixture screws at 1.5 turns out and it wouldn't idle. Turned them .5 turn out alternating left and right till I got the fastest idle speed.  But heres the rub, the idle will not go down very low and I can literally back the Idle speed screw out so it isn't even touching the throttle arm.  If i'm not mistaken you should be able to kill the engine buy turning the idle speed screw all the way out.  If I pull the vacuum advance the idle goes down around where I want the idle to be but with the advance hooked up it jumps up probably a good 4 to 5 hundred rpm.  Also I gunned it once and the secondaries stuck slightly open and I had to manually close it.  I'm thinking maybe the secondaries aren't closing all the way?

Thanks for your help

Joel

[Cliff Ruggles]

Did you rebuild the baseplate with bronze bushings and center/index all the throttle plates?

If not, that is the next step, assuming the rebuild was done correctly with good parts and correct settings......Cliff

[jmabrao]

I found out the cause of the sticking secondary, apparently the gasket I used I didn't realize had a little circular extension around the secondary which had came loose and jammed the secondary. But the baseplate seems very sold very little play and solid actuation. Idle speed screw is still backed out completely and it still idles a bit high.  What's weird is the idle mixture screws are kinda functioning as a idle speed adjustment, turn them in the idle goes down turn them out the idle goes up. Is it possible there is an internal vacuum leak? If it's not getting air from the butterflies it's gotta be somewhere else right?

[jmabrao]

Okay I may have found the problem, I unplugged and plugged the PCV port off and idle came down quite a bit so the PCV valve is acting as a big vacuum leak. I replaced the PCV valve and it does the same. If I'm not mistaken you need a PCV valve so I don't really know what to do. Its at a pretty good idle at the moment I just don't like that the idle speed screw isn't even touching the throttle linkage.

[Cliff Ruggles]

Just out of curiosity are you using the supply to the vacuum advance from the front of the main casting drivers side just to the right of the fuel filter housing?

Most of those are ported vacuum, not manifold vacuum as the vast majority of the later model 350 truck engines had such tiny cams in them that they didn't want, need, or idle down well with a lot of timing on them........Cliff

[jmabrao]

My particular quadrajet doesn't have ported vacuum I have found out.  The sticker on the air cleaner says to plug vacuum advance into the back of the carb baseplate on the passenger side.

[77cruiser]

Fast idle cam isn't hung up is it?

[Cliff Ruggles]

The rear port on the baseplate is usually for the rear pull-off.

Can't say as I have ever seen a vacuum advance plugged into that port, and never directly into it on later model cars and trucks.  They usually ran the supply vacuum thru other devices first to limit vacuum at idle and other vehicle operating conditions.....Cliff

[jmabrao]

I may have looked at it incorrectly it seems. Here is the sticker, I did remove the rear vacuum break.

[Cliff Ruggles]

The vacuum to the distributor was routed thru a check valve and delay valve, so basically it will act pretty much like ported vacuum even when supplied by the manifold vacuum source.

What unloads the choke on a cold start and holds the secondary flaps closed if you removed the rear pull-off?

Many of those carbs were rear only, some had a front pull-off, but in most cases it just unloads the choke but doesn't dampen the secondary airflaps to prevent stumble/hesitation/bog going quickly to full throttle.......Cliff

[jmabrao]

My carb has front and rear vacuum breaks, the secondary damper is attached to the rear break. So I ordered a new front and rear vacuum break, should be here in a few weeks. I have noticed a bog if you stomp the throttle too quickly.

[Cliff Ruggles]

I didn't see an order come in?

We modify the rear vacuum breaks as NOTHING currently being sold will allow the secondaries to open smoothly.  The ones currently being supplied so not have the correct restriction in them and will not prevent the secondaries from causing stumble/hesitation/bog when you go to full throttle.......FWIW....