Holley Vacuum Secondaries

NickRCarter · #1 (**permalink**) 11-13-2004, 02:20 PM

Hi there, I have a SBC 383 with 750 Holley. The cam is an Isky Mega hyd race cam, Canfield heads and MSD 6AL. The engine is in A GD 427 weighing 1086 kilos.
So far I have been able to get the secondaries to operate, despite using the lightest spring (yellow). The diaphragm is in tact and the system seems to operate correctly as far as I can tell by operation of the rod and diaphragm by mechanical testing ie the secondary butterflies are free to move once the primaries have been throttle advanced by say a 1/4, and the air/vacuum route to the primary venturi seems OK.
Can anybody give me any pointers as to why the secondaries are not moving at all! I have confirmed this with the use of a paper clip on the actuating rod.
Nick

tpiini · #2 (**permalink**) 11-13-2004, 06:14 PM

Are you saying that you've DRIVEN the car at WOT and confirmed that your secondaries do not open? I'm guessing that is the case based on your "paper clip" remark, because you know they won't (or at least, shouldn't) open by just revving the engine while parked.

If so, does the car run out of steam on top or is it still pulling hard through the expected rev. range? The thing about vacuum seconds is that they'll only open when the engine asks them to.

NickRCarter · #3 (**permalink**) 11-14-2004, 12:52 AM

Yes the car is being tested on the road, during which WOT is used in 2nd and 3rd. The car has plenty of power and response during the initial aceleration, especially when the throttle is moved with steady progression. However, if given a quick throttle up you can feel a lack of response and does not acelerate as you would expect it to bearing in mind the 450+ HP and the weight of the car. I have also tried to load up the engine by doing short early gear changes.
Bit of a pain really. Otherwise it runs smooth and never misses a beat.

tpiini · #4 (**permalink**) 11-14-2004, 07:31 AM

Hmmm...

When you stab it, does it immediately fall flat, or does it pickup for a slight moment, then go flat? In either scenario, does it then start to accelerate well after the hesitation? I'm wondering if your accelerator pump circuit is a problem.

Again, I'm not discounting the possibility that your secondaries are really not opening, but I'm wondering if maybe they're opening too fast instead.

Any chance your paper clip check is faulty? I'm assuming that you're putting it on the rod that comes out of the bottom of the vacuum pot, but doesn't that rod go DOWN as it opens the seconds? I'm trying to picture the vacuum second Holley without walking across the street to the garage. But I'm trying to think of where you could put a paper clip to confirm that the seconds are not opening.

NickRCarter · #5 (**permalink**) 11-14-2004, 07:42 AM

No they are definitely not opening. The paper clip is at the top of the rod. When the diaphragm is retracted by vacuum this rod goes upwards. So any actuation would move the clip downwards. The only other way it could move down is by vibration, which would only confuse matters, but so far its stayed put.

Initial aceleration is good and smooth, but wot is definitely lacking.
Cheers Nick

tpiini · #6 (**permalink**) 11-14-2004, 07:46 AM

Okay. I couldn't remember if the rod pushed or pulled.

I'm at a loss. If you have the lightest spring in place and you definitely have vacuum to the diaphram, I only have question marks circling my head.

Time for someone smarter than me to chime in...

Tom

trularin · #7 (**permalink**) 11-14-2004, 07:53 AM

First, it's a cheby. Dump the engine and get a Ford.

The thread does not have enough information to really help.
What is the carb number?
What work has been done to the carb?
What work has been done to the engine?
Was or has the card ever run correctly?
Is it a new or used carb?
What is the vac of the engine at idle and at 2500/2800 Rs?

There is a cork or black material gasket that interfaces the vac sec to the throttle body. Is that gasket in place and sealed to work? Makes all the difference in the world.

With the vac sec off the throttle body, can you activate the diaphram with a minivac? If not the seal/diaphram/pan is/are bad.

What are your capabilities? Sometimes a fresh face looking at the same problem may be able to see an error. Can you have a friend look it over that is knowledgable... and can spell?

NickRCarter · #8 (**permalink**) 11-14-2004, 08:52 AM

Was there anything in particular you wanted spelt in English !

trularin · #9 (**permalink**) 11-14-2004, 09:04 AM

Big-Foot · #10 (**permalink**) 11-14-2004, 12:49 PM

Tpiini is right in that you should not be able to get the secondaries to open when the car itself is not under a load.

I can't figure out what you are trying to do with a paper-clip..

Vacuum secondaries, when working properly, will operate totally seemlessly. In other words, you will not "feel" them open, but believe me they will open when the vacuum signal drops and the engine is under load. If you suspect that the secondaries are not opening, the best way to test it is to lock the secondaries by disconnecting the vacuum link and then drive it. You will now be on a bonafide 2bbl carb.

The lighter spring will allow the secondaries to open faster and you could experience a bog if the spring is too light and the RPM is too low when you romp on her.

There is one other thing that can cause the secondaries to not open properly - there is a choke interlock on the right side of the carb. Make sure that this interlock is fully retracted when the choke is open.
When the engine is warm, you should be able to manually open the throttle all the way which should then allow you to manually lift the vacuum link rod to the diaphragm which will open the secondaries. If they will not open, you may have a problem with the diaphragm.

NickRCarter · #11 (**permalink**) 11-14-2004, 01:57 PM

Thanks Randy,
I shall check that out tomorrow. With regards the paper clip, this tip was taken from the Holley site. It is placed around the actuating rod at the top against the base of the diaphragm housing. If the secondaries operate it can be seen afterwards in how much the clip has moved down the rod.
Nick

NickRCarter · #12 (**permalink**) 11-21-2004, 05:45 AM

Its taken a time for the weather to allow me to play. I checked the linkage, as far as I can see the secondaries are prevented from opening until the throttle is advanced about a quarter open, then they are free to move, being closed again by the closing of the throttle linkage. Other than that the secondaries are free to move.
I have replaced the quick change top back to stock and installed the yellow spring. I was also able to check the vacuum passage to both primary and secondary venturis by use of a stethoscope made from a length of small diameter rubber hose and operating the actuating rod before reconnecting to the secondary spindle.
Out on the road its performing much better with a 0-90mph time of 14 secs although it should be better than that. I also found that the secondaries are now opening but only about 1/4" on the actuating rod. So not quite there yet.
Nick

tpiini · #13 (**permalink**) 11-21-2004, 05:58 AM

Hey Nick,

If they are opening about 1/4 of the way, that's likely all the engine is asking for. When you have a vacuum secondary carb, the only way that the secondaries will EVER open up all the way is when you've sized the carb too small for the application.

I'd continue to play with springs, squirters, and your stopwatch and let the engine dictate what it really wants from that carb. But it sounds like it is working properly.

Tom

NickRCarter · #14 (**permalink**) 11-21-2004, 09:00 AM

Thanks Tom,

I'll look at the main jets etc and perhaps fine tune the timing. Then test with the performance meter I have. Not looking for stunning off standing times, as I respect my drivetrain, more interested in the 10mph segment times from 20 - 100mph.

Cheers,

Nick

NickRCarter · #15 (**permalink**) 11-23-2004, 09:10 AM

Update. I have tried as a temporary measure, the bolt trick in the linkage between the throttle and secondaries link. I did this in such a way that the secondaries only open in the last 1/4 of the throttle movement. Took it out and it went like a DingBat. Very much faster 10mph segment times. As a result of this I noticed that the secondaries had still not opened fully. I then found that since the build, the throttle cable has stretched very slightly and the steel post that holds the cable outer at the carb had shifted. This had resulted in losing the last 1/4" of throttle movement on the pedal.
As soon as the weather improves I will test the car to see if the secondary problem has been cured. Fingers crossed !!!

Cheers, Nick

Big-Foot · #16 (**permalink**) 11-23-2004, 09:33 AM

Nick - The bolt in the linkage trick is not really helping you, although it does give more of a kick in the pants feel.
Here is the reason you get the kick feeling;
When you are opening up the throttle gradually, the carburetor's circuits are all interacting with the task of enriching the fuel mixture to handle the increase in air. You transition from Idle to main-metering circuits through the Acceleration Circuit (accelerator pump) - your power circuit (power valve) helps by enrichening the main metering system to cover-up any flat spots after the acceleration circuit drops out. When you open the throttle far enough to engage the bolt in your linkage, you force the secondaries open early which causes a momentary "lean" condition (slight to major bog) - the carb's secondary main metering circuit fights to start flowing fuel to richen the fuel mixture back up - suddenly it is flowing fuel and VIOLA you get the kick in the pants that you did not feel before!
Measuring the acceleration with a stop watch and distance between a properly tuned vac secondary carb and one that has the "bolt" trick, will always show that the properly tuned carb will win. Tom is right - keep working with the springs, squirters and accelerator pump cams and you will have superior performance.

NickRCarter · #17 (**permalink**) 11-25-2004, 09:56 AM

Problem solved !!!
Whilst doing the run referred to in my previous post where I used a bolt to force open the secondaries after 3/4 throttle had been applied. I noticed that the paper clip on the secondary actuating rod attached to the diaphragm had only moved 1/4"
So I checked and double checked everything again on the carb and discovered that the throttle cable since installation had "grown" very slightly and coupled with post holding the outer cable at the carb having slightly moved on its fixing, full throttle was being prevented by about 1/4". Just enough to effect the flow through the venturi and degrade the vacuum being generated for the secondaries.
Today a run out showed the secondaries opening FULLY with the associated vast increase in performance.
Thanks everyone for your inputs, which have been greatly appreciated and helped a Guy on a sharp learning curve.
Now where's that performance meter..................?????
Cheers,
Nick

Big-Foot · #18 (**permalink**) 11-25-2004, 11:15 AM

That's great news Nick! Now - buckle-up and hang on!!!

tpiini · #19 (**permalink**) 11-25-2004, 06:18 PM

It always turns out to be something simple, doesn't it?

Michael C Henry · #20 (**permalink**) 12-21-2004, 06:52 AM

If it's any help now. Quick Fuel Technology has a vacume secondary housing that has a needle valve incorperated in the housing.It's to control how fast the secondaries open. I've seen a newer version in the Car Craft mag that has the needle valve screw incorperated into the anodized cover .I'm using the older Quick Fuel Tech vacume secondary housing with the needle screw in the housing it's self,and a quick spring change cover.I sent my carbs to quick fuel Tech and had them redo them ( two 600 cfm 1850 holleys) . I had ordered the new housings earlier but hadn't installed them. They installed a small tube into the carbrueter main body , that accsess vacume from the primary away from the ventury wall edge under the booster.You'll see it in my gallery, it's not very large . They said they installed the purple springs. The way I understand it's the spring that determines when and the screw determines how fast the vacume secondaries open . They describe the operation should seem smooth and not a hesitation and a surge .