Quadrajet Fast Idle Problems

[landy]

I've got a 4-barrell q-jet with factory non-electric choke setup. With the onset of cold temps, the choke system doesn't seem to be working correctly. On cold startup, it starts fine and the idle speed kicks up. However, the fast idle won't kick down when the engine reaches operating temp. I can manually move the lever on the psgr side of the carb to get it to the correct idle speed, but when the throttle is moved, it kicks back up. Eventually, after a 1/2 hr or so of futzing around, it will idle at normal speed.

On a related issue, by manually moving the rod that's connected to the choke thermal coil on the psgr side of the carb, the choke butterfly opens fully. If left on it's own, it'll open almost fully, but not all the way. I don't see an obvious adjustment for this. Maybe it's in the steel box housing that contains the coil?

Lastly, the vacuum valve on the thermostatic air cleaner doesn't function correctly. The damper valve doesn't move with vacuum applied to it. Can this unit be replaced?

Thanks!

[TBONE1964]

Can you take a photo of your set up and down load it? Do you have a choke coil in the intake or is it on the carb. The difference is the one on the intake has a small rectangle shaped piece of tin over the top of it and a rod that comes out and hooks to the chock linkage. The one mounted on the carb is round and may have a screw driver slot in it. Either way, it does not sound like the coil is retracting when it gets hot. This may be because it is just old and wore out or, you are not getting enough heat to it to make it retract (unwind and release the choke.

It is normal for the choke Blade (plate) to not open fully. It should be almost fully open when the engine is at normal operating temp. When it is cold, it should close fully and open about 1/8 of an inch with the help of the vacumn advance.

Hope this helps.

[landy]

It's got the choke coil on the intake. So guess I'll try a new one and see if that solves the problem.

[geezer#99]

You've got a couple possibilities. Your heat riser valve in the exhaust is not working or the crossover port in the intake is partly or fully plugged with carbon. The rod that hooks to the carb that you pulled on can be bent slightly to make it longer and straighter so when the bimetal coil expands it allows the choke to full kick off the high idle cam.
I used to make my own rods from coat hangers for rods that couldn't be adjusted longer.
Easy to make.

[TBONE1964]

Originally Posted by TBONE1964

Can you take a photo of your set up and down load it? Do you have a choke coil in the intake or is it on the carb. The difference is the one on the intake has a small rectangle shaped piece of tin over the top of it and a rod that comes out and hooks to the chock linkage. The one mounted on the carb is round and may have a screw driver slot in it. Either way, it does not sound like the coil is retracting when it gets hot. This may be because it is just old and wore out or, you are not getting enough heat to it to make it retract (unwind and release the choke.

It is normal for the choke Blade (plate) to not open fully. It should be almost fully open when the engine is at normal operating temp. When it is cold, it should close fully and open about 1/8 of an inch with the help of the vacumn advance.

Hope this helps.

What I meant to say on my last post is vacumn pull off, not vacumn advance. If the vacumn advance is working properly, it should pull the choke plate open about 1/8 of an inch after the truck is started. You can test it with a vacumn pump.

[Longhorn 70]

Fast idle cams on QJets can get....gooky. They basically are cams that gravity pulls off. So...

Start the engine when cold and make sure the choke is engaged.
Let it warm up but don't hit the throttle.
Make sure the choke pulls off, if not give it gentle help to fully open.
Watch the fast idle cam and see if it moves out of the way when you hit the throttle.

If it doesn't fall freely right away with no coaxing, look for gook in the retaining screw hole.

Best of luck.

[Wrenchbender Ret]

Sometimes you need to make a shield to keep the cold air from blowing back on the choke. Like Geezer says the passagways in the manifold are probably blocked with carbon. The choke should be wide open when hot. You might lower the fast idle speed with the adjustment screw.

[68gmsee]

The link to the choke can also be adjusted (formed, bent) to adjust the choke plates. Another thing to consider is metal fatique. The choke coil might need to be replaced if it's the original.

I'm not sure about the breather air intake control valve but the dealer may still carry them or at least let you know what the GM nomenclature is. Many years ago, I bought one from them but for the most part mine have been disabled. Not needed here.

[landy]

Other than LMC, where might I look for a new choke thermostat? I tried NAPA, but the counter guy couldn't find it in the on-line system.

[geezer#99]

I'd just leave it alone or make the rod longer.
Or check here for a coil.
http://quadrajetparts.com/rochester-...ing-p-380.html

[Stocker]

I couldn't find one on the NAPA site, but Oreilly's had it. http://www.oreillyauto.com/site/c/se...make=Chevrolet