Quadrajet Carb

Ben Plunkett

New Member
Sep 3, 2020
16
Hey everyone,

I recently rebuilt the carb on my '79 Camaro and was looking for some help with tuning it. First off it has a hot air choke with a primary pull off, no secondary pull off. It also has the double D screws for the air mixture screws and was wondering if there are any common tools I may be able to use or would it be best to buy the specialized tool?

I was hoping you all might have some advice on where to set the choke, what rpm range I should be going for on startup and as it drops to idle speed, and how to go about adjusting the mixture screws?

Any other advice would be greatly appreciated too!

Thanks in advance,
Ben
 

Twisted_Metal

Administrator
Staff member
Lifetime Gold Member
Feb 26, 2004
31,197
Bloomington, MN
Get the proper tool for adjusting the mixture screws.

The type with the flexible shaft works well for reaching them when the engine is running and things are hot.
(No fun burning your knuckles on a hot intake manifold.)

Hot idle should be around 500 rpm on a “normal” engine.
(Some cams require a higher idle speed.)

Cold, (choke on) will probably be around 8-900 rpm.

Distributor timing is the first thing to nail down.

Then, set the mixture screws to where the engine creates the most vacuum. (Use a vacuum gauge.)

Lastly, adjust the choke so it engages when the engine is cold and disengages when it’s warm at the above mentioned rpm.
(There’s a high idle adjustment screw near the choke mechanism.)

You can then go back and make minor adjustments to the timing and advance mechanism in the distributor.
 

Gary S

Administrator
Lifetime Gold Member
Apr 14, 1999
24,514
Bismarck, North Dakota
Proper setting for the choke per GM manuals it for it to close fully, but lightly on a cold engine when the ambient temperature is 70 degrees F.
Choke pulloff setting spec is in carb manuals for your exact model and varies with model. Your carb kit used to rebuild it should have that setting.
 

Ben Plunkett

New Member
Sep 3, 2020
16
Get the proper tool for adjusting the mixture screws.

The type with the flexible shaft works well for reaching them when the engine is running and things are hot.
(No fun burning your knuckles on a hot intake manifold.)

Hot idle should be around 500 rpm on a “normal” engine.
(Some cams require a higher idle speed.)

Cold, (choke on) will probably be around 8-900 rpm.

Distributor timing is the first thing to nail down.

Then, set the mixture screws to where the engine creates the most vacuum. (Use a vacuum gauge.)

Lastly, adjust the choke so it engages when the engine is cold and disengages when it’s warm at the above mentioned rpm.
(There’s a high idle adjustment screw near the choke mechanism.)

You can then go back and make minor adjustments to the timing and advance mechanism in the distributor.

When I took the car for a drive I noticed I had little to no acceleration and it didn't change despite how much gas I gave it. Not really sure what's happening there.

I haven't adjusted the distributor at all yet, given the amount of grease and grime I can't even find the harmonic balancer to begin with. It also hasn't been touched in what I can assume is many years.

I tried buying a flexible double D screwdriver and it doesn't fit over the mixture screws so I have no real way to adjust them as they have basically no access when the car is running. When it's off I can just manage to get the end of a forked screw bit in there to turn them but can't really do that with everything running and hot.

Same goes for the fast idle mixture screw, very inaccessible when the car is running from what I can see. But my main priority is the choke.

I seem to be stuck on two things when it comes to the (mechanical)choke. The fast idle cam will either jump into a high position, closing the primary choke valve and upon starting , drop into high idle and open the valve a bit but will not open into the fully open position on curb idle meaning the secondary lockout lever doesn't move so I have no secondaries opening. Or, the valve will open fully when the choke is adjusted to the leaner side but will not close or go into high idle thus not being able to start or run at high idle.

Obviously I want both of these things to happen given that I need to start the car and also be able to use the secondaries but whenever I adjust the choke I can only get one or the other to happen.

I'm at a total loss here so any advice would be great.

Thanks everyone
 




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