View Full Version : carb question

06-21-02, 03:45 PM
At what point should my secoundaries open.

This is a brand new 750 cfm holley with vaccum secoundaries, I found that there was no screw installed to operate the linkage.

I've added a screw and nut but don't have a clue wheather to set it at a certain rpm or what?


Thanks guys,


06-21-02, 04:26 PM
It doesn't need any screws or nuts in the linkage - the secondaries are opened by a vacuum signal to the big diaphragm on the passenger side - you'll see the actuator rod sticking down from the bottom of the diaphragm to the lever on the secondary throttle shaft.

You can't tell by revving the engine in the garage whether they're opening or not - the vacuum signal isn't manifold vacuum, it's vacuum generated by air velocity through the primary venturi as it passes by a small pickup tube; this method senses actual engine load and only gives it as much fuel as it can use based on load and rpm. Great street setup.

06-21-02, 09:32 PM
Thanks John,

I had to go and ass-u-me again, though I actually only made one of myself this time.

My 302 has a screw on the linkage, thats why I figured I needed one on this carb. Both have vacuum secoundaries.

Speaking of linkages( is that a word?) my clutch linkage came apart tonight too!

I'm having soooo much fun.


06-22-02, 10:30 AM
Putting a screw in the linkage is sometimes seen as an attempt to make a vacuum-secondary carb into a mechanical-secondary carb, but they don't work very well that way, as that opens the secondaries immediately with no extra fuel, as the secondaries on a vacuum-secondary carb don't have a separate secondary accelerator pump and "shooter" (like "double-pumpers" do), and immediate full opening of the secondaries results in a huge "bog" before the metering system can react and supply extra fuel.

In the same vein, some guys remove the little check ball in the vacuum secondary circuit so the secondaries open faster, with the same result; the check ball is there to "soften" the opening rate of the secondaries to avoid the dreaded "bog". A lot of engineering development has gone into the Holley vacuum-secondary carb, and they work best when they're left alone, although you can "tune" the secondary opening rate by changing the strength of the diaphragm spring - Holley has a kit for that, and they also have a "Quick-Change" kit that replaces the upper half of the diaphragm housing so you can change the spring in about a minute without having to remove the entire housing from the side of the carb.

ALL Holleys used on Corvettes have always had vacuum secondaries, except for the original L-88's, which had an 850 "double-pumper" with no choke, and they were notoriously un-streetable by design.

My original/unrestored '69 Z/28 302 also uses a vacuum-secondary List #4053 800CFM Holley - they all did.

06-22-02, 10:42 AM

I envy you having a 69 Z/28 too.

All I've got left of mine is the drive train and the left taillight.

Mine was Hugger orange with black stripes and black top.

My carb was a 780CFM, are you sure your's is an 800CFM?

BTW, what heads are on your Z and what is the build date of your car. I understand there were alot of different parts used on the 69's.
Mine came with 492's.

I'll check my own dates later today.


06-22-02, 08:07 PM
Depends which spec sheet you look at - some call out the #4053 as a 780, some call it out as an 800. In any case, it's WAY over-carbureted - they had to build them that way so the SCCA Trans-Am racers could legally use the huge carb they needed to run constantly at wide-open throttle between 5000-8000 rpm to make power under race conditions. They run a LOT better on the street with a 600 Holley instead of the 780/800.

All '69 Z's used 3927186 heads; there were several blocks during the year, but the heads were always 186's. Mine was built 02D, the last week in February, and is a fully-documented, unmolested, unrestored 33,000-mile original car (and will stay that way) - fun machine!

06-22-02, 09:39 PM
Sounds like a beauty, but then again so does your 67 Vette.

Someday I'd like to pickup another one.