I have a 76 Ford F150 with 390 FE with 4 barrel carb with manual choke everything is stock but the problem is it doesn't seem to run quite right upon cold start, I have to rev it up to around 750-1000 RPMs or it idles really low and seems to start loading up and die, is there anything I can do to fix this problem. Also, I was wondering if not having the Vacuum secondaries working will hurt my fuel economy, the reason is when I rebuilt the carb I didn't have a diagram for the vacuum secondaries and haven't picked one up for it yet.

 

What size and brand of carb is it?

 

It has the stock Holley 4 barrel carb with list number 6947, I believe I was told in the past that it was a specially built carb for the Ford 390 and was told it belongs to the 4160 family I believe.

 

Not having the secondaries functioning is not going to hurt fuel economy. Likely help since you can't use them.
Looking at Holleys list number page, it is basically a 600 cfm Vacuum secondary carb. Very similar(not exact) to the 1850 you can buy new any place. It has all the Ford linkage parts.

Holley page:
https://documents.holley.com/techlib...al_listing.pdf

Is the choke plate set correctly? Is the high idle set correctly?

 

Secondary throttle plates need to be cracked open slightly for idle.

 

But as soon as the truck is up to operating temp it runs great idles at a good RPM and everything.

 

So that leads it back to choke setting and high idle speed correctly adjusted. If the choke plate is staying closed too much, it will be hard to keep running. If the high idle speed is too low, it will be hard to keep running without you adding throttle. Once both of those are set, it should start and stay running.

Go to page 9 and read a bit about proper settings and shows were the high idle screw is located:
https://www.jegs.com/InstallationIns...10-0-1850S.pdf

 

I'd consider switching to a cable choke instead of an electric one as well, especially if you live somewhere the temperature varies from -20 F to 100 F in a year's time.

It doesn't seem to matter what adjustment guide I use, as soon as the temperature swings 30 degrees, the adjustment is off again.

 

So true. I've always converted to cable for the same reason.

 

He said it was a manual choke carb.

If it's a factory Holley for a Ford it will have a Ford casting number stamped into the choke horn above the list number.

 

Good call - I didn't even catch that.

 

Since when ?

 

Since never, as far as I know. Secondary venturis have no idle circuit. If they need to be "cracked" for idle, that means the idle circuit is too rich on the primaries. Doesn't it?

 

Yea, works for me, but I would work on the idle circuit settings, not crack open the secondary's, that would only complicate setting the idle speed, possibly resulting in a "too high idle"

 

Holley does give a spec for the secondary throttle blades but I don't remember exactly what it is, primarily it cracks them open enough that the blades don't stick against the base.
On engines with big cams that need a fair amount of throttle blade opening to maintain idle some have open the secondaries a little more in order to close the primaries enough to get the blade below the transfer slot.
This helped to regain curb idle adjustment.