What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

What is the best way to tune a 4bl 1978 quadrajet?

wazzabie

wazzabie

1/2 ton status
 Premium
Joined
Apr 17, 2001
Posts
2,922
Reaction score
447
Location
Washington State
I got the K5 running recently. First time is ran and drive in 10+ years. I would like to dial in the quadrajet. It has no problems at startup and eventually idles curb at about 700 rpm. I believe I have to hook up a vacuum and try to get high vacuum as I adjust the two mixture screws? Any tips and instructions is appreciated.
 
Yes. You need a vacuum gauge to read vacuum while you fiddle with it. Primarily you get one idle mix screw tuned to the maximum vacuum you can achieve, then leave it alone while tuning the other the same way. Also a good idea to smoke out any vacuum leaks first.

Before you start, figure out the best screwdriver to get at each screw. There can be a lot of crap in the way on more emissions laden vehicles.
 
Polymath said:
Yes. You need a vacuum gauge to read vacuum while you fiddle with it. Primarily you get one idle mix screw tuned to the maximum vacuum you can achieve, then leave it alone while tuning the other the same way. Also a good idea to smoke out any vacuum leaks first.

Before you start, figure out the best screwdriver to get at each screw. There can be a lot of crap in the way on more emissions laden vehicles.
Click to expand...
As I move the mixture screws will this change the idle RPMs?
 
yes, it will/should increase rpm. Then you can set the curb idle (off choke) with the idle speed screw. also you can adjust the timing the to help set curb idle.
After the adjustments, snap the throttle allow return to throttle stop, look down the primary's for fuel dribbling. If you see fuell dripping/dribbling out the center venturi back out the idle speed screw till it stops. If this causes your idle to be to low inspect for vacuum leaks and repair, add a little timing advance, and or readjust the idle air screws
 
Thank you for the help.

Where should I attach the vacuum gauge on the carb? Port A, B, C or some other location?

carb.png
 
A works. Any manifold vacuum source. Even the brake booster and the HVAC vacuum would do.

edit: I certainly hope that is not your carb in the picture
 
Not my carbs but it looks like these carbs.

Is 1-3 ported vacuum or vacuum advanced? Would vacuum advanced be on the HEI distributor?

carbs_l1600_83f94708a78c1e0d7a24b60152618a279819e0fe-jpg.3600580
 
#2 manifold vacuum is what you want the distributor hooked up to. Manifold vacuum will give the proper signal for how much advance the distributor needs. Manifold vacuum is at the base of the carburator, or the intake manifold itself. Ported vacuum is above the base of the carburator, and is for accesories like cruise control or the EVAP system.
 
3 is ported vacuum, 1 is manifold vacuum. 2 is PVC vacuum, (Manifold) but I wouldn't disconnect the PVC to do vacuum reading adjustments, since the pcv is a calibrated vacuum leak.
 
Wes Harden said:
3 is ported vacuum, 1 is manifold vacuum. 2 is PVC vacuum, (Manifold) but I wouldn't disconnect the PVC to do vacuum reading adjustments, since the pcv is a calibrated vacuum leak.
Click to expand...
If #1 is mounted to the base of the carb, then yes it is manifold. It is hard to see from the picture what the #1 port is attached to.
 
1-ton said:
If #1 is mounted to the base of the carb, then yes it is manifold. It is hard to see from the picture what the #1 port is attached to.
Click to expand...

#1 is not on the base plate. It is on the lower part of the carb body. So is #1 still manifold vacuum?

s-l1600.jpg
 
Ya'all are going to make me go out and prove #1 is manifold vacuum ???? I suppose I could the 77 has been started in a month.
 
You must log in or register to reply here.

Latest Posts

Top Bottom