Need Help with Holley Carb

[Dennis H.]

• Set the choke coil housing at 1 mark Lean
• With throttle in the open position and choke valve closed against unloader tang measurement is good using a 3/16 drill.
• Started car and set fast idle to 1500 RPM and let engine reach operating temp.
• Moved throttle and fast idle cam did not drop.

Couple of things that I discovered during all this is that the choke heat tube is only getting hot half way up the tube so I pulled the heat air tube it off and checked for vacuum at that point and had nothing. So it would seem that my problem lies with no vacuum at the choke housing pulling heat into the choke coil.

So I guess I have 3 things going on;

• Engine will not idle with the AC engaged.
• Engine seems to load up at 1500 RPM while driving
• Fast idle drop off not working.

Thanks for any help.

[Dennis H.]

Re: Need Help with Holley Carb

Throwing this back up top for one last go around.

[Joe C.]

Re: Need Help with Holley Carb

Dennis,

Much of your problem is resulting from your choke not releasing completely.
The choke heat tube is at atmospheric pressure....no vacuum. The tube is simply a closed, air convection system, beginning with filtered air from the air cleaner via a 1/8" rubber hose to the bottom of the exh manifold, thru the manifold, out the top and into the choke housing via a 1/8" steel tube. It should be hot all the way from the right side manifold, to the choke housing!
It sounds like there is a blockage inside the tube. Remove the 1/8" rubber hose to the air cleaner, and blow through it...........you should feel that air exit out the hole in the top of the exhaust manifold. If it's an original manifold, it might be that the air tube is corroded/blocked/restricted. The tube runs straight thru the manifold, and can be "rodded" out with a piece of bailing wire. THERE SHOULD BE NO PERFORATION IN THIS TUBE, WHICH WILL ALLOW EXHAUST GASES TO ENTER IT!!!!!
Let's begin there.

Joe

[Dennis H.]

Re: Need Help with Holley Carb

Joe

Thanks for the discreption on how the choke is setup. Don't know how I missed this but I did, I don't have the tube coming out of the bottom of the exhaust manifold and running up to the air cleaner. You know I have pulled this air cleaner off many times and seem where this hose hooks up and thought several times what GM might have hooked to that. I just checked my AIM and LICS and see what you are talking about.

I just blew air through the heat tube from the top side and was able to put my finger on the bottom side and it looks like the tube is intact. So I guess that is a good thing.

Can I install this tube without pulling the exhaust manifold for clearance?

Dennis

[Joe C.]

Re: Need Help with Holley Carb

Dennis,

If you plug the bottom of the heat tube, and blow thru the top, there should be no flow, meaning that the tube is not perforated.
Next, remove the plug from the bottom and blow thru the top, to be sure that the tube is unrestricted. Send a piece of baling wire thru it, to be sure there's no debris inside.

Once you get that done, then the addition of the lower steel tube will induce a convection current in the passage, sending heat to your choke spring. Completing the circuit with the rubber hose to the air cleaner will keep things clean, and prevent debris from entering.

Exhaust manifold should remain in place for addition of the tube. Right side "V" shield, if present, will have to be removed, as well as starter heat shield, if present.
Joe

[Jack H.]

Re: Need Help with Holley Carb

Speaking to the problem of the car not idling with the A/C system running, that one's a bit tricky. Even when these cars were sold new, dealership mechanics often 'boosted' the engine idle RPM a tad (100-200 RPM) on A/C equipped cars to compensate for compressor drag...

On used/restored cars, the situation can be compounded by the specifics of the A6 compressor. Corvette was rather unique compared to the rest of Chevy's car line. It had a small cockpit requiring less thermal transfer to achieve the same interior temp and it was a performance vehicle.

Soooo, the A6 compressor used on Corvette had an internal displacement of 10.8 cu in while the passenger car line used an A6 with a 12.6 cu in displacement. The big suppliers of remanufactured A/C compressors (NAPA, Four Seasons, Etc.) don't really recognize Corvette as being different and supply "one fits all" compressors for Corvette and Chevy...

The larger internal displacement A6 compressor will have a slightly higher drag profile than the smaller Corvette version challenges the car's ability to idle smoothly at the original, factory specified, RPM. I think Delco Remy is the only source for drop-in replacement A6 compressors with the smaller, Corvette specific, internal displacement...

The trick in sourcing a replacement A6 is to see if the PN the supplier uses for his 'Corvette' compressor is different from what he lists for equivalent Chevy passenger car applications or the era. If not, you can pretty well assume the A6 has been remanufactured with larger, 12.6 cu in, internal components common to passenger car applications.

Most don't care. They simply dial in another 100-200 RPM at idle to handle the situation and enjoy the cost savings associated with 3rd party remanufactured A6 compressors...

[John H.]

Re: Need Help with Holley Carb

The airflow through the Holley 2818 hot-air choke system is a vacuum system; there's a tiny orifice between the choke vacuum piston chamber in the choke housing and the primary venturi that provides vacuum, drawn past tiny slots on the vacuum piston, to pull hot air into the choke housing to heat the bi-metallic coil that operates the choke and fast idle cam. If that tiny orifice is plugged or the piston is gummed-up in its bore, there won't be any continuous flow of hot air into the coil chamber. Diagrams below are from the Holley Carburetor factory service manual.

[Dennis H.]

Re: Need Help with Holley Carb

Thanks John, would this vacuum be enough to feel by putting your finger over the housing hole while the engine is running?

[John H.]

Re: Need Help with Holley Carb

Thanks John, would this vacuum be enough to feel by putting your finger over the housing hole while the engine is running?

Yes, it is, although it's not as strong a signal as you'd get from a PCV port or the full manifold vacuum port for the vacuum advance.

[Stuart F.]

Re: Need Help with Holley Carb

John;

If for some reason you eliminated the vacuum pull through the hot air tube, say by using a sealed electric choke conversion, would this loss of air bleed affect the mixture? I'm considering just such a test as I have all the parts, etc. My concerns were first; for the possible change to a richer mixture, and second; that perhaps the tube in the manifold may burn through w/o the flow of air through it. May sound silly, but the reason I want to test this is to try and cut down on the heat soak of the carb after shut down, and I figure eliminating the steel hot tube connections to the carb might help. I have a L-76 63 w/an AFB. I am also considering change to a fuel hose to the carb from the filter.

Stu Fox

[John H.]

Re: Need Help with Holley Carb

Stu -

Logic says that plugging that vacuum passage would eliminate the (calibrated) minor vacuum leak and enrich the mixture slightly, but I doubt if you could tell the difference without a wide-band O2 sensor reading before and after, and the difference might be so small that it would be within test variablilty tolerances.

[Stuart F.]

Re: Need Help with Holley Carb

John;

Thanks. That's about what I figured, but I thought I'd like an expert opinion on it first. I'll give it a try - anything to reduce the heat conduction/introduction into the carb.

Stu Fox

[Jack H.]

Re: Need Help with Holley Carb

One other comment... You said you initially setup the carb this way:

Started car and set fast idle to 1500 RPM and let engine reach operating temp.
Moved throttle and fast idle cam did not drop.

That means you guessed at the right fast idle adjustment. Idle RPM can drop 50% or so based on whether the engine is hot vs. cold and what ambient temperature is. The 65 Corvette Shop Manual Supplement instructs setting the carb's fast idle this way:

(1) Get the engine fully warmed up and the choke fully open.

(2) Open the throttle slightly and lift the fast idle cam up and close the throttle to catch the high step of the choke cam.

(3) Adjust the fast idle screw while it's on the high step of the choke cam with engine fully warmed to achieve 2200-2300 RPM.

Depending on ambient temperature, that would set the carb for an initial cold start idle of roughly 1100 RPM. You might be off a tad the way you did it resulting in the fast idle cam not releasing...

[Dennis H.]

Re: Need Help with Holley Carb

Thought I would update this a little:

Still trying to figure out why I don't have vacuum at the choke housing. I started the car today and can feel nothing in the way of vacuum. I pulled the housing and can feel vacuum at the carb so I'm guessing I have a leak between the housing and the carb. I did check the gasket on the housing and it is in place.

Still working it.