Holley 750 vac secondary spring change [Archive] - Chevelle Tech

: Holley 750 vac secondary spring change


bracketchev1221
Sep 9th, 10, 11:50 AM
My Nova has a 750 vac on the 350 in the car. The engine is pretty mild, with a .480 cam, performer rpm intake and 1 5/8 headers. First question is, what is the stock spring for the secondaries, and does anyone really see a gain by opening the secondaries sooner. I have a feeling the carb may be too big already, but setting wide open to be around 6500 on a 5-5200 engine may give it a little more power. If I'm right, the secondaries don't fully open now until like 8000 rpm.

The Old Reliable
Sep 9th, 10, 12:12 PM
My Nova has a 750 vac on the 350 in the car. The engine is pretty mild, with a .480 cam, performer rpm intake and 1 5/8 headers.

First question is, what is the stock spring for the secondaries, and does anyone really see a gain by opening the secondaries sooner.

I have a feeling the carb may be too big already, but setting wide open to be around 6500 on a 5-5200 engine may give it a little more power.

If I'm right, the secondaries don't fully open now until like 8000 rpm.

What's the RPM limit of your engine? Hyd. FT cam & std. springs?

How are you assessing when your secondaries open (or don't open)?

bracketchev1221
Sep 9th, 10, 12:20 PM
The car doesn't have a tach in it, but based on driving it, I would say it's dead by 5500. It's a small cam, you really can't even hear a lope in the idle. They are small springs and press in studs, so the engine isn't much. Stock, this was a 350/300 L-48 car. The engine was just freshened, so i don't expect it to be more than 325 hp at best. I know the vac secondary carb will open as much as the engine needs, to a point. Just wondering if letting them open a little more will make a difference.

BowtieAaron
Sep 9th, 10, 12:23 PM
factory is either the plain or black springs, i cant remember.
black doesnt open until IIRC 7000+ on a SB and 6000+ on BB
and plain doesnt open fully until 5600 IIRC.

these are all by holleys specs. i will check when i get home.

i can feel a seat of the pants difference going from my 3310 to 4777. it hits the tires so much harder. i used to never have traction issues in 2nd, now i do.

but yes there is a differance in when secondarys open. i cant tell you how much, but when it bogs, put the next heaviest one in.

aaron

bracketchev1221
Sep 9th, 10, 12:25 PM
factory is either the plain or black springs, i cant remember.
black doesnt open until IIRC 7000+ on a SB and 6000+ on BB
and plain doesnt open fully until 5600 IIRC.

these are all by holleys specs. i will check when i get home.

i can feel a seat of the pants difference going from my 3310 to 4777. it hits the tires so much harder. i used to never have traction issues in 2nd, now i do.

but yes there is a differance in when secondarys open. i cant tell you how much, but when it bogs, put the next heaviest one in.

aaron


This was basically what I remembered about these carbs. Haven't used one in 20 years:D But that was another idea I had that maybe the car would run BETTER with a 650 dp on it instead of a 750 vac carb. I think it's the plain spring which doesn't really open to quickly, but I know over carbing the thing will slow it down too.

GRN69CHV
Sep 9th, 10, 12:37 PM
It's a 4 speed car with a fair amount of gear out back? If so, doubt the motor can load the carb enough to make it work right. My experience with vac sec carbs is auto with low stall or stick with high gears, they work well. Higher stall converter (3000+) or stick with 3.73+ gears, it will be hard to get a vac sec to work right. Regardless of cam, 350-300 motor, I'd be looking at no more than a 650DP.

BowtieAaron
Sep 9th, 10, 12:43 PM
quick change springs with chart..

http://www.holley.com/data/Products/Technical/199R8219-2.pdf

3310
http://www.holley.com/data/Products/Technical/199R9934-3.pdf

aaron

bracketchev1221
Sep 9th, 10, 12:48 PM
It's a 4 speed car with a fair amount of gear out back? If so, doubt the motor can load the carb enough to make it work right. My experience with vac sec carbs is auto with low stall or stick with high gears, they work well. Higher stall converter (3000+) or stick with 3.73+ gears, it will be hard to get a vac sec to work right. Regardless of cam, 350-300 motor, I'd be looking at no more than a 650DP.

Yup, 4 speed, and 4.11 gears with a 28" tall tire. That's why I think the car is running on just primaries. I thought a 650 dp would be good, but like any other typical "project" it needs a lot of other stuff before a new carb. I've already spent $650 in parts just to go through the brakes and add frame connectors.

Dave427
Sep 9th, 10, 12:59 PM
They come with a plain (silver) spring from factory.
If you have the quick change cover changing the springs is easy.
For your car keep going lighter until it bogs.
I have had best luck running the purple spring, its the next lightest from stock.

Dave

69-CHVL
Sep 9th, 10, 1:19 PM
Pretty sure the 3310's come with the black spring, the 770 SA's come with the silver/plain. My 454 went faster and faster as I used the next lighter spring. The white spring produced the best et/mph with a stick car no less. Went faster when I got a DP :D

Tom Mobley
Sep 9th, 10, 1:53 PM
carb is already on the big side for the engine/car combination. Opening the secs faster is not likely to be helpful. I'd be tempted to try the next stiffer spring, get better mid-range.

crazydavey
Sep 9th, 10, 2:30 PM
Ray I've had several cars with combo's very close to that. All of them ran much better with something in the 600-650 area (especially on the street). My auto's worked best with a simple 1850 but a stick would probably work fine with the 650 dp...3310 is too big for that combo...JMO...Dave

bracketchev1221
Sep 9th, 10, 2:34 PM
Ray I've had several cars with combo's very close to that. All of them ran much better with something in the 600-650 area (especially on the street). My auto's worked best with a simple 1850 but a stick would probably work fine with the 650 dp...3310 is too big for that combo...JMO...Dave

Yeah, that was my guess as well. Just figured if there was anything I could gain from a $12 spring kit for now I would do it. I even had the springs on the shelf and threw them out when I moved, because I didn't own a vac secondary carb.

crazydavey
Sep 9th, 10, 2:46 PM
yeah I just gave away all my springs too....that combo will be a lot more fun and responsive with the smaller carb. I tried many time to get the 3310 to work and it's just is too much carb....

mr 4 speed
Sep 9th, 10, 5:32 PM
All the 3310 carbs I used on 350 motors worked just fine. This is Chevy,Oldsmobile and Ford 351W
I usually leave the box stock jetting and always swapped the secondary spring for one of the yellow ones.Stock spring the secondaries will never open.
Last one I used was on an Olds 350 with the Performer cam and it ran mid/low 13's with a 3000 stall and 3.73 gears.
A 3310 is not too big for a 350..the LT1 motors used them from the factory (780 cfm) and all the other motors came with 750 cfm Qjets.
Now a 750 cfm double pumper might be a bit much for a mild 350 with highway gears and a tight convertor.

Rmchevelle
Sep 9th, 10, 5:36 PM
One of my cars has a similar combo with a 3310 and it runs amazingly, in fact, I can't imagine it could run much better.

Mike
Sep 9th, 10, 5:59 PM
I've never understood how a 750 vs Holley could be too much carb when GM used a 750 vs QJ on the 350.
.

trmnatr
Sep 9th, 10, 6:05 PM
My Nova has a 750 vac on the 350 in the car. The engine is pretty mild, with a .480 cam, performer rpm intake and 1 5/8 headers. First question is, what is the stock spring for the secondaries, and does anyone really see a gain by opening the secondaries sooner. I have a feeling the carb may be too big already, but setting wide open to be around 6500 on a 5-5200 engine may give it a little more power. If I'm right, the secondaries don't fully open now until like 8000 rpm.

If the carb is too big you want a heavier spring so it will open when the engine could use it

If the carb is too small such as 650 on a 454, you would want them to open fast as the engine is big enough to pull signal without a bog, on an engine where the carb is too big you want to delay them opening so it will have signal when they open

Now if they open too fast you will need a LARGER squirter, not uncommon to see 45 to 56 squirter on them in a performance application to cover the secondary's opening too fast

trmnatr
Sep 9th, 10, 6:07 PM
I've never understood how a 750 vs Holley could be too much carb when GM used a 750 vs QJ on the 350.
.

The butterfly allows more signal IMO on the QJ when you mash the peddle

IMO a 750 vacuum seconday holley with 1 3/4" butterfly would be awesome

What is the QJ 1 15/16" or 2" on one end and the other is??????

trmnatr
Sep 9th, 10, 6:08 PM
Ray, someone has an adjustable pod so you dont have to play with springs, just turn the dial and it tightens or loosens the spring

I think it is Quick Fuel, I messed with this add on ONE TIME and it is very nice

The Old Reliable
Sep 9th, 10, 6:31 PM
Agree with all the factual & positive views
on vacuum secondary carbs as posted.

The most "universal" performance carb ever made: 3310, & its variants. --- :)

crazydavey
Sep 9th, 10, 6:33 PM
yes a 3310 will work but not as good as a 600-650 for Ray's purpose. Think about it....396/425HP, 11 to 1, soild lifter, big port heads, high rise intake..etc...came with the equivalent of a 3310.....428 CobraJet...735... the list goes on...two things I see the most, motors that are over cam'd and over carb'd....JMO...
Dave

trmnatr
Sep 9th, 10, 6:36 PM
yes a 3310 will work but not as good as a 600-650 for Ray's purpose. Think about it....396/425HP, 11 to 1, soild lifter, big port heads, high rise intake..etc...came with the equivalent of a 3310.....428 CobraJet...735... the list goes on...two things I see the most, motors that are over cam'd and over carb'd....JMO...
Dave

I dont agree about over cam'd, maybe not cam'd per the cars needs.

Yes a ton of cars have too much carburetor but then again some can use it with high stalls and high rpm. All depends on the combo in the end.

Agree about Ray's combo

crazydavey
Sep 9th, 10, 6:46 PM
I dont agree about over cam'd, maybe not cam'd per the cars needs.

Yes a ton of cars have too much carburetor but then again some can use it with high stalls and high rpm. All depends on the combo in the end.

Agree about Ray's combo

I think that can be the same thing Butch. When I was selling Hi Per parts I would try to keep guys from ordering the wrong cam but they lusted after the big lopey sound and would order it anyway. I'm not talking about what most guys on this board would spec out, I'm talking about the average Summit/Jegs shopper that really doesn't know much about combinations.....believe me there's a ton of them out there...;)

trmnatr
Sep 9th, 10, 6:51 PM
I think that can be the same thing Butch. When I was selling Hi Per parts I would try to keep guys from ordering the wrong cam but they lusted after the big lopey sound and would order it anyway. I'm not talking about what most guys on this board would spec out, I'm talking about the average Summit/Jegs shopper that really doesn't know much about combinations.....believe me there's a ton of them out there...;)

Oh I know lol

There was a given person that wanted the same stick I use. Only problem is Im 307/313 @.020 and 278/284 @.050. He has a 383 with 9.5:1 :sad:

I have heard the sound thing so much, look at the new line of "SOUND" cams out there.

Put one in a STOCK 327 with 237/249 or something like that. One of their largest flat tappets with the Nitride process

Most people need to go down 3-4 rows up in the cam books and 1-2 sizes smaller on the carbs.

The Old Reliable
Sep 9th, 10, 6:55 PM
Let us not forget:

A '67-69 302 with a mild cam
came with...a 780 Holley, w/vac. sec. :yes:

Tom Mobley
Sep 9th, 10, 7:07 PM
yeah, they did. and they were useless under about 3500RPM too. even with a 3.73. They did run good up top though, if you had headers on it.

crazydavey
Sep 9th, 10, 7:09 PM
11.25 compression....solid lifters...better springs....202 heads....high rise intake....6500+ motor...:D

sleeper
Sep 9th, 10, 8:12 PM
The yellow spring with a .028 squirter always seemed to wind up working best for me.

A good double pumper will do everything better.

-SS454-
Sep 9th, 10, 11:13 PM
How come holley doesn't have opening points for the white spring?

The other yellow spring I have in the kit that came with the car is a cut spring. Can clearly see the sign of wire cutters on it. Is this normal?

MEJ1990TM
Sep 10th, 10, 12:09 AM
I'm running a 3310 on a plain Jane 350, 2.64 ST-10, and 2.73's out back. That is one step up from the worst gear you could run with a stick shift. Even so this 3310 seems to work ok with it. It doesn't bog ,anymore, when I nail the throttle. It's got 31 squirters, stock 72 jets, and stock metering plate. I had a 4777 that I could never get to work right, so I used what else was laying around.

BowtieAaron
Sep 10th, 10, 8:08 AM
The yellow spring with a .028 squirter always seemed to wind up working best for me.

A good double pumper will do everything better.

purple spring and 37 squirted netted me my best ET to date..

I'm running a 3310 on a plain Jane 350, 2.64 ST-10, and 2.73's out back. That is one step up from the worst gear you could run with a stick shift. Even so this 3310 seems to work ok with it. It doesn't bog ,anymore, when I nail the throttle. It's got 31 squirters, stock 72 jets, and stock metering plate. I had a 4777 that I could never get to work right, so I used what else was laying around.

my 3310 i thought worked good. fired right up, thought it was responsive, until i put the 4777 on it.. even though i have some issues with the DP it works 100 times better for my combo.. and ihave more head and cam than a lot of these SB's came with from the factory..
the responsiveness of the 650dp is second to none vs the 3310. i will be going to the track next week, so i will see how it works out.

aaron

bracketchev1221
Sep 10th, 10, 10:46 AM
Well if anyone has a yellow or purple spring they aren't using and would like to donate it, send me a PM. Thanks.

MEJ1990TM
Sep 10th, 10, 6:51 PM
I do not doubt that the 4777 would have worked better for my combo. I got kind of fed up with not getting it to work right so I went with the 3310. I may give it another try eventually, but not very likely in the near future.

Tom Mobley
Sep 10th, 10, 7:19 PM
Well if anyone has a yellow or purple spring they aren't using and would like to donate it, send me a PM. Thanks.

So, you've already figured out your car is over-carbureted but you're still looking for softer secondary springs?

What's wrong with this picture?

bracketchev1221
Sep 11th, 10, 1:27 PM
So, you've already figured out your car is over-carbureted but you're still looking for softer secondary springs?

What's wrong with this picture?

Just looking to play with stuff. I can't afford a new carb right now with all of the mechanical stuff it needs. So I figured I would try it. What do I have to lose? I just replaced the complete rear brakes and still have master and power booster to go. Changed the pinion yoke and seal, added frame connectors and I probably have only driven 150 miles.