[vicgto]

Back in September, I was at the track and I was approached by TA Man who I had never met before. I'm glad I did meet him as he is very knowledgeable, since then I have reconnected with the PY Pontiac Forum. Ernie noticed the Holley vacuum secondary 3310 sitting on top of my RPM intake and suggested an HP 950 would probably improve my ET. Last week I picked up a used HP 950 (80676). Although I have not installed it yet, I am reading up on the idle speed and mixture tuning. The consensus seems to be to set the primary throttle plate with a square idle transfer slot (about 0.020" reveal) and leave it there, then do the final idle speed adjustment on the secondary throttle plate.

My question is would it not make more sense to have equal primary/secondary throttle valve openings as the idle circuit system is the same (primary vs secondary) on these carburetors?

Doing all the adjustment on the secondary side could result in too much or too little of the idle transfer slot exposed.

Has anyone tried setting the primary/secondary throttle valves equally and if so how did it work out compared to setting the primary throttle plate with a square idle transfer slot (about 0.020" reveal) and leave it there, then do the final idle speed adjustment on the secondary throttle plate.

Thanks
Vic

[Tom Vaught]

Well Vic, glad to see you back again.

1) For many years Holley had a 2 circuit idle system on the front of the carb and a very tiny fuel purge circuit on the secondary side of the carb. This helped the carb from gumming up on the secondary side. Barely worked but it was there for a reason.

2) Then about the mid 70s, some people decided that they could do a better job of keeping fresh gas in the secondary side of the carb bowls by adding a secondary idle system THAT YOU COULD ADJUST. MORE OF A MARKETING DEAL VS ANYTHING ELSE.
The open plenum and divided intake manifolds still worked fine with the 2 corner idle system. Dual metering blocks vs a metering plate where you could screw with the settings externally and really foul things up.

3) The Primary Idle Screws were set to 1.5 turns out from seated.
BUT the Secondary "Stale Fuel Purge Circuits" on the secondary side were fixed.

3) Now with the change to a 4 corner idle carb set-up you gave the guy twice as many things to screw up on that circuit.
Then later the Idle EZE carbs allowed you to close the rear carb throttle blade back to almost touching the bores but then
people thought you needed to do the same with the primary circuit so the Primary Circuit got all screwed up.

4) Today most people understand the requirement to put the Primary Blades in the right location on the Primary side of the carb.

5) Along the way the BIG 455 and 454 engines got small when the big 535 and 600+ cid engines replaced the "Small Stuff".
So again you need more idle airflow and that means larger blade openings for the 4 carb barrels.
Drilling holes in the carb blades only works for a given range of carb size.

6) Don't assume that every carb must have the front of the carb and the back of the carb exactly the same for the carb to perform properly during idle and off-idle, as well as at WOT.

7) Historically, unless you have a really big engine you do not need a lot of airflow from the carb to maintain a proper idle. Some of this Idle Eze stuff was just a copy of the Carter AFB carbs with the Idle Air By-Pass system Screw and drillings. But any way you can sell carbs, the marketing guys will say you need that stuff.

Hope this helps.

Tom V.

[PAUL K]

I wanted Tom V to answer first as he is a carburetor expert. Here are a few more thoughts;

We set the primary throttle plates as mentioned with just a square of the transition slot showing. Convert the secondary side so the idle can be adjusted on the car. If it is not like this already you can buy a kit, or you can install the throttle shaft stop set screw upside down to accomplish this task. Most likely your idle speed will be too high with all four transition slots showing. You want to reduce the idle from the secondary side, then double check the accelerator pump arm adjustment. If the throttle plates have the idle air holes drilled in them they will probably need to be plugged.

It's worthwhile to try but depending how your 780 is set up, your stall speed and weight of your car will determine what gains will be seen with the 950. I think an 850 vacuum secondary would be the hot set up for your combination, especially if your converter is a bit on the tight side and car is a little heavy.

[ta man]

I always square my front slot and use the rear for idle adjustment. Having a square front slot ensures good street manners from idle. I never touch my front idle speed setting after it is squared. Also my 950 hp throttle blades have been swapped to solid blades. I could not get my idle low enough with the bypass air holes.

[vicgto]

I was actually concerned with the holes in the throttle plates when I first picked up the carb, although I know this has been done to improve idle quality on engines with very long duration camshafts. For those that have plugged the holes, was this done by welding, plugging with solid rivets or with screws.

[PAUL K]

The HP 950 carburetor's originally came with those holes. We have had luck filling them with Epoxy, I've seen guys weld them or use a rivet.

[Tom Vaught]

Most times, with the help of someone who knows carburetors like Shaker455, you can send them the base plate and the 1-11/16" diameter blades (with no holes can be swapped into the baseplate. Also the "950 carb" is really a 750/780 cfm carb like tons of #3310 carbs and you can find a doner baseplate and remove the blades yourself if you have the correct screws to be installed after the swap.

Not big on rivits or welding them shut but have seen some plates epoxied.
I buy swap meet carbs for $15-20 all day long and buy screws in volume (5000)
so easy for me to swap parts. Each throttle blade should have a number on the blade.
I will look for some non drilled blades and maybe we can get them out to you.

Tom V.

[TCSGTO]

The few times I tried having the secondary slots exposed at idle there was an off idle hesitation that couldn’t be tuned out.
Do like everyone on here suggests and square the primary and set idle speed with the secondary plates. A small turn on the secondary screw makes a big difference in idle speed. Usually 1/8 to 1/4 turn at most from closed will do it.

[ta man]

I should have a set of solid blades if you need them.

[vicgto]

Thank You Ernie and Tom for offering the plates. Wow what a tight knit group, I didn't expect to see such generous offers. If either of you have them I will take you up on that, I will gladly E-tranfer you the shipping costs. Please let me know if you have a set.
Thanks guys, much appreciated

[vicgto]

Originally Posted by PAUL K

I wanted Tom V to answer first as he is a carburetor expert. Here are a few more thoughts;


I think an 850 vacuum secondary would be the hot set up for your combination, especially if your converter is a bit on the tight side and car is a little heavy.

Paul, I figure this information may help someone. I actually did a comparison with a 750 vacuum secondary vs an 850 vacuum secondary this fall. I jetted the 850 80/86 vs the 750's 78/86 so I'm not sure how much that affected the results. The 750 performed slightly better, nearly 1/10 second and 0.5 mph quicker. The 850 had 1- 9/16 venturi and 1-3/4 throttle plates with an electric choke. The 750 had 1-3/8 venturi with 1-11/16 throttle plates and the choke housing milled off.

[PAUL K]

Originally Posted by vicgto

Paul, I figure this information may help someone. I actually did a comparison with a 750 vacuum secondary vs an 850 vacuum secondary this fall. I jetted the 850 80/86 vs the 750's 78/86 so I'm not sure how much that affected the results. The 750 performed slightly better, nearly 1/10 second and 0.5 mph quicker. The 850 had 1- 9/16 venturi and 1-3/4 throttle plates with an electric choke. The 750 had 1-3/8 venturi with 1-11/16 throttle plates and the choke housing milled off.

That is good information. Is your carburetor the 750 or 780 3310? You've referrred to it as a 750 and a 780 in your signature. Or was the 750 in the test you mentioned a third carburetor? It'll be interesting to see how the 950 HP test goes.

[Tom Vaught]

All 950 HP carbs and 1000 HP carbs were rated (for advertising purposes to compete against Barry Grant's claimed air flow numbers.

If you test a 950 HP carb at the same 20.4" water test pressure as the 780 vacuum carb and the 750 DP carb, the cfm will be with-in a few cfm of the old 750/780 carb airflow numbers.

Same deal with the older 4781 850 cfm double pumper carbs and the rare Corvette Vacuum Secondary cards. 20.4" of water you will get 850 870 cfm not 1000 cfm.

20.4 was used forever before the Barry Grant days and he changed the carb flow test pressure to the higher 28" of water test pressure for marketing purposed because Smokey said the HP was closer on his dyno using the 28" test pressure vs dyno testing.. Again an advertising game. And again people like to brag about "leroy" being bigger in their pants/ carb.

Tom V.

[n20ta2]

Originally Posted by PAUL K

I wanted Tom V to answer first as he is a carburetor expert. Here are a few more thoughts;

We set the primary throttle plates as mentioned with just a square of the transition slot showing. Convert the secondary side so the idle can be adjusted on the car. If it is not like this already you can buy a kit, or you can install the throttle shaft stop set screw upside down to accomplish this task. Most likely your idle speed will be too high with all four transition slots showing. You want to reduce the idle from the secondary side, then double check the accelerator pump arm adjustment. If the throttle plates have the idle air holes drilled in them they will probably need to be plugged.

It's worthwhile to try but depending how your 780 is set up, your stall speed and weight of your car will determine what gains will be seen with the 950. I think an 850 vacuum secondary would be the hot set up for your combination, especially if your converter is a bit on the tight side and car is a little heavy.

Forgive my ignorance as its been a number of years since I have messed with a carb but would the primary adjustment be accomplished with the carb off the car or it it possible installed? I Have a billet 1050 4150 fst carb with duel idle screws and im fighting the a/f meter trying to get a clean idle and keep the car running. This might possibly be why If I have my transfer slot to exposed?

[PAUL K]

Originally Posted by n20ta2

Forgive my ignorance as its been a number of years since I have messed with a carb but would the primary adjustment be accomplished with the carb off the car or it it possible installed? I Have a billet 1050 4150 fst carb with duel idle screws and im fighting the a/f meter trying to get a clean idle and keep the car running. This might possibly be why If I have my transfer slot to exposed?

I don't know of a way to square up the transition slots with the carb on the car. We always set both, the primary and secondary on the bench and start from there.

[Tom Vaught]

1) You remove the carb, carefully dump out the gas in the bowls (turn it upside down and let it drain), work the primary and secondary throttle levers a few times to get rid of the
gas in that accelerator pump circuit.

2) I drink coffee in the morning so I have a Folgers Coffee red plastic container (that I have rinsed out) and use it to hold the "turned over carb" on the bench or outside the house or garage.

3) Now I look at the Primary Blade Position vs the Transfer Slot and adjust the Primary Idle Screw for a "SQUARE WINDOW OPENING as related to the Slot vs Primary Blade Position. Then that screw never gets moved again.

4) Next is the Accelerator Pump adjustment.
Open the Primary Throttle Lever/shaft/Blades to "full open" and check for an additional
.025 to .045" Accelerator Pump Travel before the Pump "bottoms out" (no more movement left).

You do not want the Pump Geometry (at full open) to ever rupture the Accelerator Pump
diaphragm. Bad Fire risk then.

Then you adjust the Secondary Blade Position by flipping the set screw slot over to where it can be adjusted from the top when the carb is on the intake.

3/4 Turn Out on a 4 corner carb idle mixture screws to 1 turn out on each screw.

AS FAR AS THROTTLE BLADE REMOVAL and INSTALL goes.
a) Remove the baseplate from the carb.

b) Using a file that fits inside the throttle blade opening CAREFULLY remove the swedged over Brass Mounting Screws so that each screw can be easily removed vs binding on the throttle shafts.

c) Write down which way the blades were installed in the baseplate BEFORE messing with the screws or removing them. Remove one pair of screws at a time (for ONE BLADE).

Hopefully TA Man or I can find a non drilled hole set of blades for your carb.
Do not disassemble the carb before you get the blank Throttle Blades.

Work the throttle linkage several times on the baseplate to confirm no binding
BEFORE Re-installing on the mainbody of the carb.

Tom V.

[PAUL K]

Originally Posted by Tom Vaught

All 950 HP carbs and 1000 HP carbs were rated (for advertising purposes to compete against Barry Grant's claimed air flow numbers.

If you test a 950 HP carb at the same 20.4" water test pressure as the 780 vacuum carb and the 750 DP carb, the cfm will be with-in a few cfm of the old 750/780 carb airflow numbers.

Same deal with the older 4781 850 cfm double pumper carbs and the rare Corvette Vacuum Secondary cards. 20.4" of water you will get 850 870 cfm not 1000 cfm.

20.4 was used forever before the Barry Grant days and he changed the carb flow test pressure to the higher 28" of water test pressure for marketing purposed because Smokey said the HP was closer on his dyno using the 28" test pressure vs dyno testing.. Again an advertising game. And again people like to brag about "leroy" being bigger in their pants/ carb.

Tom V.

Good info Tom. I'm more curious if the OP is using the original 3310 with down leg boosters. A bit surprised a 3310 with straight leg boosters would out run a 850 vacuum secondary on his combination, but the choke housing and air cleaner choice may have contributed to the performance difference.

[vicgto]

Originally Posted by PAUL K

That is good information. Is your carburetor the 750 or 780 3310? You've referrred to it as a 750 and a 780 in your signature. Or was the 750 in the test you mentioned a third carburetor? It'll be interesting to see how the 950 HP test goes.

Carburetor is 3310-01 which Holley shows as 780 cfm in their numerical listing chart.

[n20ta2]

Good info thanks a bunch! Fst was no help on the phone with my situation

[Tom Vaught]

3310-01 should have been a duplicate of the Chevy 396/427 Corvette carb.
A very good carb and non drilled throttle plates are easier to acquire.

A bit confused with topic talking about a vicgto carb and your 3310-1 vacuum carb.
Most of the info is common but need to post which advice I am giving to which carb owner.

Tom V.