problems w/76 idle and pickup

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F

FJmoneypit

10 Year
Joined
Mar 28, 2011
Threads
44
Messages
258
Location
Wilmington De.
A little background on my 76 I bought this summer and the issues I am having.

The idle has always been high and erratic and in my attempts to correct this I have ran into some issues. PO had installed an HEI w/vacuum advance. The advance was coming in as soon as she was started. I rerouted the vacuum, line to #6 on the VSV per my Haynes manual. This seemed to correct the advance coming in as soon as I started her. I put a vacuum gauge on her tonight and #6 does not pull any vacuum even when I rev the motor? Is there a better source to pull the vacuum from?

While I had the gauge out I tested the vacuum at the port located top front of carb. I'm lucky if it pulls 11 inches. I also tested the port on the manifold directly in front of the carb and this pulls 17 inches. What should my readings be at sea level (delaware)?

I have rebuilt the carb but have a question about the vacuum choke pull off. Should this activate as soon as you start the engine? How would you adjust the fast idle screw if it does not line up with the stop? It lines up fine as long as it is not activated. What is the purpose of it anyway if you have a manual choke?

I really cannot enjoy her until I get these issues resolved. Should I just de-smog her? She passed inspection fine.

John
 
Howdy! I am pretty sure that the 76 had vacuum retard on the original carb/dizzy. There is no way to get the right vacuum signal from the original carb without doing some serious surgery on it. Ya gotta be pretty brave, and carb knowledgeable, before you can start drilling holes in the carb. You might have better luck finding another carb from a year that had vacuum advance from the factory. John
 
It'd be easier to install a stock dizzy with elec ignition. Desmogging correctly is not simple-nor is drilling a ported vac hole for vacuum advance.
 
FJmoneypit said:
While I had the gauge out I tested the vacuum at the port located top front of carb. I'm lucky if it pulls 11 inches. I also tested the port on the manifold directly in front of the carb and this pulls 17 inches. What should my readings be at sea level (delaware)?

I really cannot enjoy her until I get these issues resolved. Should I just de-smog her? She passed inspection fine.

John
Click to expand...

I'd be cautious about doing de-smog if most of it is there and in working order.

At sea level you should be able to tune to 20-22" at idle speed. We're talking manifold vacuum here--below the throttle plate. Using the brake booster hose is a good way to ensure you're testing full manifold vacuum.

Do you have a stock carb? It may have been a vacuum retard setup. My '77 is vac advance, but I don't know when they switched.

Don't wait to enjoy your truck until it's perfect. Working through these issues is half the fun. Enjoy it now. :)

JD
 
The fast idle screw should not touch its lever unless the choke is engaged.
When you pull the choke cable, the fast idle screw goes up against the lever and it takes over control of the idle speed from the regular idle speed screw. Watch the linkage while someone pulls the choke cable and it'll be obvious how it works. The way I adjust it is to start the engine cold with the choke engaged and then adjust the fast idle to a bit above the minimum rpm where it runs smoothly. No magic.
 
That's my problem, the fast idle screw rests on the perch with or without the coke out and the motor off. Once I start the motor and vacuum is applied to the choke cutoff(?) and it retracts the rods attached to it. The fast idle screw moves toward the motor and is approx. half a inch from the pedestal. Once I can get the idle and choke issues fixed I'll deal with the dizzy issue. Any suggestions, I've compared my carb with the pictures and the linkage appears correct but something has to be out of wack. Help

John
 
edwjmcgrath said:
You sure you are adjusting the right screws? Pictures might help.
Click to expand...

I'm with edwjmcgrath. I think you have your adjustment screws mixed up because it sure sounds like you're talking about the throttle positioner screw--that is just what it behaves like.

To avoid frustration, take a look at the diagrams in the manual, or post a few pics to make sure we're all talking about the same screws.

JD
 
I am pretty sure I am adjusting the correct screws (but I could be wrong).
I used the search button and pulled up a picture and directions here on mud.
My 40 is a 2/76 and my carb is 6 A 28 (Jan 28 1976?). The Fast idle adjustment is slightly higher and attached to the bottom of the arm shaped like a boomerang?And the Idle speed adjustment screw is located so that it connects with the bottom of the sideway "U" shaped arm? Is this correct? Also what is the screw that goes into the base of the carb on the firewall side. I know the air/fuel adjustment is valve cover side.
I would gladly post pic's but not my strong suit. Also did some more visual checking and 3 of the 4 bolts that attach the Intake to exhaust are missing. The breakdown in the Haynes shows a gasket between them but this would cause an exhaust leak versus an intake leak, correct?

John
 
FJmoneypit said:
I am pretty sure I am adjusting the correct screws (but I could be wrong).
I used the search button and pulled up a picture and directions here on mud.
My 40 is a 2/76 and my carb is 6 A 28 (Jan 28 1976?). The Fast idle adjustment is slightly higher and attached to the bottom of the arm shaped like a boomerang?And the Idle speed adjustment screw is located so that it connects with the bottom of the sideway "U" shaped arm? Is this correct? Also what is the screw that goes into the base of the carb on the firewall side. I know the air/fuel adjustment is valve cover side.
I would gladly post pic's but not my strong suit. Also did some more visual checking and 3 of the 4 bolts that attach the Intake to exhaust are missing. The breakdown in the Haynes shows a gasket between them but this would cause an exhaust leak versus an intake leak, correct?

John
Click to expand...

John here is a pick of what should be a very similar carb.


image-715072400.webp

The screw at the bottom of the "boomerang" is the choke breaker or (throttle positioner) it can be adjusted by disconnecting or pinching the vacuum line to the choke breaker (closest to the brake booster). You can find the vacuum by following the boomerang arm up to the linkage piece running horizontally to the breaker itself. With the engine running, pinch the vacuum line and you will see the boomerang move that adjusting screw towards the throttle lever. If you rev the motor while pinching the screw should catch on to the lever and the idle will be 1200 rpm when this is properly adjusted.

The fast idle is actually the screw at the bottom of the "U shaped linkage". Have someone pull the choke while you watch and you will see how it works. I set mine to about 1500 rpm when cold, FSM says 1800 but that just seemed high to me.

The mystery screw at the base is actually your idle speed adjustment screw. This is the one to use when establishing idle speed, FSM says 650 +- 50.

The mixture screw determines how much fuel is released from the idle port below the primary valve. There are several good threads for adjusting this using a tach or vacuum gage in the FAQ. Hope this helps.
image-715072400.webp
 
77mustard40

Ok, that clarifies things a lot better. No wonder I have not been able to get it straight. I'll give that a try and see how it works. While I am not new to working on my own cars this is my first 40 and even with the issues I am having it is awesome. And thanks to everyone else that replied, it is nice to know that you gut's/gal's can be counted on for assistance.

John
 
Making Progress

Thanks to you all and special thanks to 77Mustard I appear to be making some progress here. Now I am dealing with a vacuum leak at the base of the Intake manifold where it connects to the exhaust manifold. Pulled carb and it appears PO had the same issue and attempted to fix it. Cleaned area up and laid a layer;p:p of JB Weld. Just need to wait for it to set up and hopefully another issue fixed. Thanks again guy's/gal's.

John
 
Agree with pinhead. Do you want a glob of jb stuck in one of your valves? Fix it right.
And...the intake manifold usually cracks there when the exhaust diverter valve is stuck. Fix that too when you pull the intake manifold.
 
Thanks for the heads up guy's
I guess I'll be pulling the intake shortly.

Just a question, JB is supposed to handle up to 600 F, how hot does the exhaust mainfold get?

John
 
Hot enough to crack an intake manifold!
Exhaust gasses can be way higher than 600. The other problem is the difference in expansion rate.
Even if the jb holds together, it will lose its bond with the manifold and break loose.
 
Success, finally

After trying and trying to fix my idle issue I think I got it.
Apparently I need to pay more attention to how the gaskets go when I rebuild the carb.:doh:
Now instead of trying to feed fuel off of the primary it is actually using the idle circuit, what a difference.
Vacuum is now steady at 20, and i can get on to others things I still need to do on my baby.

John
 
Good to hear. IIRC someone else recently had the same issue and it was caused by the same problem. Gasket upside down when rebuilding their carb.

I did the same thing once doing a BMW head gasket. How was I supposed to know what "oben" means? I found out quickly when the oil pressure light didn't go out when I started it.
 
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