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Quadrajet Questions - Idle speed solenoid

chalet2506

chalet2506

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My 72 Jimmy has had some issues for a while. Hard starts, stumbling/dying at low cruising rpms, then just flat out died a couple times on my way home about a month ago. The engine was replaced around 86. From what I've found on the intranet, looks like the carb is from the late 70's.

Carb #'s
17054923
2688

Pulled my q-jet yesterday and plan on sending it to Jet for a reman job. I don't know much about carbs and really don't want to rebuild myself. But, have a couple questions. Mostly just for my information.

What's the solenoid looking thing up front of the throttle linkage?

PC270011.jpg


Is this called a hot air or divorced choke? It's the kind that screws to the top of the intake. Also, what's the little yellow plastic diaphram connected to a vacuum port?

PC270010.jpg


Kind of hard to see in the pic, but right in the center are 2 black rubber eraser looking things. Is that the JB weld fix I read about? One of them is coming loose/partly gone.

PC270013.jpg
 
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After poking around looks like the diaphram is vacuum control for the secondaries and the electric solenoid on throttle is the idle speed control.

Not sure if I need the idle speed control. I guess they were used for various purposes including anti-dieseling and idle control during A/C operation. This truck has A/C, but I plan on removing it as it doesn't work and what would be the point of having a/c if it did - without a top. Doesn't look like the solenoid has functioned in a while, think I'll just take it off.

Edit> Maybe its a good idea to leave it on, but will it work when I remove the a/c?
 
It has been a really long time since I worked on a QJet, and I am fresh out of the swamp and really tired.
But, I'll take a stab on the solenoid. Maybe the other stuff tomorrow.

If its an antidieseling solenoid, which I think it is, you probably should leave it on. If for no other reason than you probably don't have any other idle control.

Now, in some aps, it was a load compensating device. It kicked on to give the engine a little more gas when the AC kicked on.
But, most of those were on low powered 4 bangers.

In general I would leave it on.
 
One nifty thing about the idle solenoid is you could wire it to a switch and step up your idle for whatever. The "choke pulloff" serves two functions. The first obvious one is that as your engine starts cold with the cdhoke engages, vacuum builds and draws in the can. The screw that is/should be in that thing adjusts how far the engaged can dislodges your choke allowing the engine to run properly. The linkage attaches to your secondary air flap. When vacuum is high the can is engaged(pulled in) and effectively locks out the secondaries. If you mash the gas, vacuum drops and the can releases at a set rate due to orifice bleeds. This stages in the secondary air flap/raises the secondary rods and then you have that signaturte quad rushing of air.

If you have the proper intake for that choke then there is no reason to change it. If you plan on going to a modern intake that might not have that pad on the manifold you will be better off finding the next generation of quad 79?? to 84ish for the easily switchable electric/hot air bodies. People tell you to stay away from CCC cars and I agree. However I think it was 84? 85 trucks had a pug on top front dpass. side that only changed the volume of the pump shot. CCC carbs have the 3 prong TPS plug on front drivers side.

The black thing should be a foam fix but obviously epoxy is a better fix and can be applied on all the pugs for good measure.

Doug Roe quadrajet books should be avaiable if you have a decent library and the internet will provide everything you need to know. if you do rebuild yourself the carb doctor kits seem pretty good. Make sure you get/buy a split seal to retain the power piston as advances kits dont come with that

Hope this helps
 
The solenoid is for A/C, when the A/C turns on it will kick it up a notch. The idle speed is set on the pass. side. I know you can't check them, and they're hard to replace. The idle tubes get clogged on the ends, and the idle air bypass get clogged with gunk over time. May just be time for a rebuild. Run the numbers and find out what year that is. The 70's carbs are the better ones since you have a divorced choke pull-off
 
Short for time so i cant find pics, but a divorced choke is like you have. A bimetallic element bolts directly(I think) to the manifold and heatsinks heat into the housing. A linkage goes to your carb to move the choke cam. The next gen. carb ha a housing on the carb itself that either houses an electric coil or pipes air in from manifold to move choke arm.

Regards, Kenny
 
The early quads had a divorced style, thats what they called it I am sure. Where it mounted on the pass. front of the carb. The style I have mounts on the pass. rear of the carb and operated the choke and also the secondary butterflies. Weren't as efficient and were known to cause problems, but worked ok. I actually have two rods running from mine and does a push/pull motion. Yours the bumps the choke open when you give it gas, but when you go WOT, it forces the butterflies to open during a low vacuum situation.
 
i had replaced my q-jet with a reman unit from fleabay, it didn't have the solenoid. The old one didn't fit.

I've run it for years, with a.c., no problem. I just bumped up the idle a bit.
 
chalet2506 said:
How is the solenoid wired? If I remove the a/c system will it even operate?
Click to expand...


I have to look, but iirc, the same signal kicked on the solenoid that engaged the compressor...I'll look tomorrow.
 
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