oprahrider

I have an original 585 cfm square bore holley on my 66 SB 327/300HP Roadster. I have replaced the diaphram, changed the purple spring to the yellow spring (weakest one they have) and the secondaries still don't open until almost redline. And then they open only partially.

You can work the secondaries by hand after opening the primaries approx 1/3 of the way. When you open them in this fashion they work great. The engine RPM's leap up to redline very quickly.......

I run the stock cam and the vaccum pressure seems more than enough. I was told that the diaphrams for these carbs come with the long OR short shaft that attaches to the butterfly pivot shaft. It had a long one on it so I replaced it with another long shaft model.........BUT......I was wondering if maybe it was mistakenly put on there in the 1st place, instead of the short one..

As always your assistance is appreciated.

clem zahrobsky

make sure the little cork seal is between the main body and the diaphram housing and also make sure that the holes into the primary and secondary venturies are open as these supply the vacuum to the diaphram.

oprahrider

The cork seal is good

But I didn't know about the holes in the venturis providing the vacuum. What holes are these?......not the ones where the fuel comes in?......

clem zahrobsky

these holes are the one that connect to the hole where the cork seal goes. does the diaphram housing have a ball bearing in the vacuum passage,if it does try it without the ball bearing.

ghostrider20

This may seem rudimentary, but what method are you using to determine if the secondaries are opening or not? I ask because they will not open, free revving the engine. I am sure you knew this, just eliminating it as a possibility.

If not, I use a paper clip. I slide it on the diaphragm rod and slide it up, flush against the vacuum pod and then go for a run. When you get back to the garage, note how far the paper clip was pushed down the rod. I use this method and check the RPM in 1000K intervals 2K, 3K, 4K etc. With the 30-30 cam I tuned to have them wide open around 4500 RPM.

A finely tuned carburetor will not make a bog sound when the secondaries open. They should come in gradually. I know you feel a small punch when they are forced open, but you are actually feeling the car pull less, then regain power. If the secondaries are forced open or open to soon, a single pump holley with one squirter will not have enough pump shot to cover up the momentary vacuum loss. You can increase the pump nozzle size, but then you usually get too much of a shot during normal driving.

The plain spring is usually a good starting point for spring selection.

What gears do you have in the rear? When I changed from 3:36’s to 4:11’s I increased the pump shot size a tad and added a small set screw to the secondary slide follower to force them open at near WOT. The combo seems to give good throttle response. The setscrew is set so at WOT the secondaries are open about 10 degrees. I coupled it with the lightest spring and let the engine vacuum do the rest.

A good literature piece is "How to build Horsepower Vol. II", "Carburetors and Intake Manifolds". [Vizard]

Mark

Kid_Again

..yup, you're getting good advice....BTDT

..IF the secondaries are not opening, you have a vacuum leak...check the seal around the secondary diaphragm, the little cork seal, etc

SMI-FST

One thing I ran into was the vac. passage had become gunked up. Make sure there is a good seal between the diaphram and the carb body. Then put a piece of tape over the port in the secondary venturi and drive the car. Sometimes this secondary port bleeds off the vac signal before the secondaries get to the open posistion. If this cures the problem, you can permantly plug the passage. Worked on my 65.

TheOman

There it is! You can rev the beegeebers out of the car standing still and the back side of the carb is not gonna open. If it is working and you cannot feel a big bog on the road the spring setting must be just about right. There should be no stumble just a smooth transition to 4 BBl operation.

JohnZ

Manifold vacuum has nothing to do with opening the secondaries - the vacuum signal that acts on the secondary diaphragm is generated in the primary venturi by air velocity passing by a pickup in the venturi (and enhanced later by a similar pickup in the secondary venturi when the secondaries start to open). They won't open revving the engine in the garage; the engine must be under load in order to generate the required venturi vacuum signal to the diaphragm. If the vacuum port from the venturi pickup to the diaphragm housing (through the cork seal where the housing bolts to the main body) is plugged, or if there's a vacuum leak at the cork seal, they won't open at all. :thumbs:

oprahrider

Well I did not know that the secondaries won't open at WOT with no load on the engine. I remember the Q-jet carb did. Totally different system I guess.

I took the carb off the manifold last night to find a metal gasket, then a thick paper gasket.....neither had "any cut outs" except for the 4 barrels and bolt holes.

But there is a slot approx. 4" across and 1/4" wide with a 3/8" hole on both sides of the slot cast into the manifold....There was carbon build up on the bottom side of the paper gasket where it covered these slot/holes.

So what is the slot for?......If the gasket covers it up I assume that it is defeating whatever purpose it was made for.

The underside of the carb has a slot that looks like it will more or less match up to the slot on the manifold.......

Sorry for all the questions but I am not the most mechnicallly inclinded guy in the world........I just figured out how the anvil works...Lol

clem zahrobsky

that slot is the heat crossover for the carb to help in cold weather. there should be a gasket on the intake with a stainless steel plate on top and then the carb set on top of the stainless steel plate. if you have a problem with the gasoline evaporating from the carb you can install steel expansion plugs in the holes at the ends of the manifold slot to cut down on how much exhaust heat the carb see. in later years this GM eliminated these holes in the manifold :chevy

65tripleblack

The secondaries will not open with no load. AFAIK, the only way to truly verify this, is to place the car on a dyno, and actually observe the butterflies under full load, at WOT.

If you have the diaphragm housing-to carb main body seal in place. The proper return spring (purple), and the secondaries can be manually opened when the primary lockout is disengaged, then you are probably good to go. I have no idea whether my secondaries are opening, because vacuum secondaries open very gradually, according to load. There is no such thing as getting "pinned" in the seat when the secondaries open. I can tell you that my engine (327/365) pulls strongly to 7500 RPM, so I know that if I had no secondaries, the motor would run out of breath long before that.

Try the following website to learn a bit more about carburetor CFM theory:
http://www.bgsoflex.com/auto.html

clem zahrobsky

the secondary throttle linkage on the vacuum diaphram has a wide open throttle stop that hits the aluminum throttle body base and if you put a little dab of grease where this stop hits the throttle body base,make a full throttle run,then check the grease for a mark from the wide open throttle stop. :chevy

Daffy2

Here's a pic of a gadget that I put on my carb to monitor the secondaries operation. I wanted something directly connected to the butterflies that could stay in place. It's not my idea originally, but I don't remember where I saw it.
The parts don't interfere with anything on my car, you would want to be sure that it can't hang up your linkage. Been on for two years, when I want to check the secondaries, I just push the yellow (foam rubber piece) to the front and go for a run.
BTW has anyone ever replaced the stone age cork seal with an o-ring ?

Daffy


[Modified by Daffy2, 9:47 PM 12/24/2003]


[Modified by Daffy2, 9:48 PM 12/24/2003]

Kid_Again

...it sounds like the gaskets were put on incorrectly...the metal gasket/shield sits next to the manifold to protect the carb base from the hot gases.....i would not take any chances, rebuild (or have it rebuilt) the carb and make sure the throttle base plate is not warped


[Modified by Kid_Again, 8:31 AM 12/25/2003]

JohnZ

Here's a photo of the "hot-slot" - plug the holes at the end with 7/16" freeze plugs or tap them for allen-head pipe plugs, and still use the stainless baffle; this design was abandoned in 1969 after the biggest carburetor safety recall in history. If the passages are left open, it can really do a job on your baseplate, and on a Q-Jet car, it can melt the lead plugs in the bottom of the float bowl and cause a fire (which is what caused the recall).


:steering:

oprahrider

Hey thanks for including the photo. It looks like a foolproof way of checking for secondaire openings......

You guys say that there is no discernable change when the secondaires open?. Now you got me wondering. Maybe they were working all the time.

I am more use to Q-jet spread bore carbs....
No question about if they were opening or not. Big hit and Big sound. And it would work in the privacy of your driveway......Lol

I ran one on my 67 RS/SS Camaro. It was a stock Small Block, 4 speed with 3.08 gears..........and to tell you the truth I think it would mop the floor with this 66 Stingray. My car just does not feel fast.

Thanks to everyone for your help.
It sure is nice to tap into the experience here.