Dazed and confused

[Bill Braun]

Shortly before the server decided to go into White Screen of Death mode, I posed a question about the missing automatic choke in my new (to me) '67 big block. Uncle Joe and Carl Rose were kind enough to respond (thanks guys), but due to the WSD, I was unable to ask a follow-up question. The number on the air horn of the carb is 4779-B or 4779-6, which, according to Holley's website, coincides with model 4150. Close examination of my AIM shows that the choke bell crank attaches to the throttle plate, which in turn attaches to the choke butterfly. According to the AIM, however, there should also be a vacuum assist, which likely has the function of pulling the choke butterfly open slightly after sufficient manifold vacuum is available. This vacuum assist is also missing. I realise this is a fairly prosaic problem, but can anyone give me an idea of what this is actually supposed to look like? Are these parts available or am I better off just installing a hand choke?

Thank you.

Bill Braun #33186

[Jack H.]

Re: Dazed and confused

The item you're describing is called a 'pull off'. It's vital to automatic choke operation in cold weather. System uses vacuum from carb (little/none on initial start) to 'crack' open the choke valve as the engine does its cold start and begins to come up to speed.

As the block heats, the thermostatic system (choke stove) gradually opens the choke and steps down the curb idle in lock-step. BUT, it takes time for heat to develop and propagate. The pull-off system's job is to intervene and open the choke plate a tad RIGHT after initial start, before heat has actually developed. Without it, and presuming truly cold ambient, the engine will either balk on initial start (with choke plate too wide) or start then run rich (maybe too rich and stall) until the engine actually warms up....

You can read about the system in the HP Book on Holley carbs in the NCRS merchandise library. You can get replacement parts from scrap yard carb cores and/or through Holley authorized dealers.

[G B.]

Holley Double Pumpers

Bill -

The Holley on your '67 is a 750 cfm "double (accelerator) pump" 4150. This street/competition model is not designed to use a vacuum choke pull-off like the assembly line 4150 Series Holleys. When new, a List #4779 carburetor comes with a manual choke mechanism only. If you want the choke to be functional, you have to buy and install a pull cable into the passenger compartment.

[Wayne Kever]

Re: Dazed and confused

I expect Jerry's right about your carb. If I were you, I would get along without a choke until you can get the right carb and choke setup. Hand chokes are a PIA and look hokey. Plus, you probably don't get the Vette out on really cold days too often.

- Wayne

[Bill Braun]

Re: Dazed and confused, but less so

Thank you for your responses. I was hoping I could simply unscrew the manual choke bits and bolt on an automatic, but this now seems unlikely. I intend to use the car in nice weather only, so I may be able to do without the choke. At the moment, with the temperture hovering around freezing, the car starts with no trouble, but I have to keep 'goosing it' a little to keep it running. Ain't much in the way of swap meets or auto boneyards here (Switzerland), so I'll have to get in touch with Holley.

Bill Braun #33186


C'mon Summer!

[G B.]

Converting to auto choke

Bill -

You have the necessary screw holes in the side of the main body to convert your 4779 to an automatic choke with a fast idle cam. However, I believe the primary throttle shaft in your carburetor is the wrong length for a bolt-on swap to a mechanical automatic choke. Of course you could change the shaft or buy a new baseplate, but that is an awful lot of trouble and expense.

I suggest you buy a Holley electric choke conversion kit for your 4779-6. An electric automatic choke will bolt up with no modifications to the carburetor. The Holley part number you want is 45-224. Summit Racing Equipment, (800) 230-3030, sells this kit for about $40.

Holley electric chokes do not give a perfect fuel mix when the engine is cold. If you adjust them to be rich enough when the engine first starts, they will stay on too long. If you adjust them to go off as soon as the engine is warm, they are too lean initially. But I have to admit, they do work better than a manual choke.

[Bill Braun]

Re: Converting to auto choke

Jerry -

That's an avenue I hadn't considered, but it sounds like a viable - and reasonably economical - solution to the problem. I'll call Summit and see what they can do for me. Thanks!

Bill Braun 33186