Vacum Secondaries on Holley 600 in a 77 '2

[likesoldjunk]

This weekend, I put a 4bbl carb/manifold from an 83
mustang into my 2.

Everything seems to be working OK, except I'm not
convinced the vacum secondaries are opening. They're
certaintly not if I just rev the engine in the shop.

If I remove the solenoid from the Carb, I can't sense
any vacum at the carb. I've cleaned all the passageways,
and to be honest I can't really see how it's supposed to
work in the first place. It draws venturi vacum from the
primaries, with a vent to amplify it from the secondaries
once they open. Seems to me it would just vent from one
to the other.

Any experience or insight ?

 

[COBRAIIW]

my 600 didn't open full secondary until i was under a load.more vacuum required.

 

[Wart]

Could try a lighter spring.

 

[likesoldjunk]

COBRAIIW

How could you tell it was opening at all ?

 

[cobra_2_76]

I had thought about installing a micro switch around the vacuum can to sense if they were opening. Hook it to a temporary light in the interrior for testing, would then also tell you what RPM it opens. Switch would ground or unground the light depending on how you wire it.

Also with the car off make sure secondarys can open. PUsh the throttle to full position and see if you can then maually open the secondarys.

Also you can unscrew the cap on the vacuum can and look for tears in the diaphram.

Dave

 

[likesoldjunk]

Done all that, except for the switch.

I with the throttle open I can move the
secondaries. with the solenoid off I can suck
on it to make it move. (don't get any ideas ;^))

I was thinking about puting a little bolt in
the linkage so they are forced to open and
see if I can tell the difference.

Maybe that head/cam combination just
doesn't draw enough air to open them...

 

[85MustGT]

You will not see them open unless its under a load. You should hear/fell them open.

 

[cobra_2_76]

If you have a cam that doesn't create much vaccum then you need get a secondary spring kit and put a lighter spring in there to get them to open with less resistance.

 

[likesoldjunk]

So.. you all, understand that this is not manifold
or spark vacum, but a separate vent/port that
draws venturi vacum from about halfway up
the venturi.

I understand the concept of a lighter spring, but
this is an otherwise stock '77 302, with a
stock 4160 from an 83 302. Should the Cam/
heads be differet enough from a 77 to an 83
to impact airflow that dramatically ?

 

[85MustGT]

So.. you all, understand that this is not manifold
or spark vacum, but a separate vent/port that
draws venturi vacum from about halfway up
the venturi.

I understand the concept of a lighter spring, but
this is an otherwise stock '77 302, with a
stock 4160 from an 83 302. Should the Cam/
heads be differet enough from a 77 to an 83
to impact airflow that dramatically ?

The stock spring should be very close and should open the secondaries. You are correct it is pulling ported vacuum.
You can tell if your secondaries are opening by keeping them shut with a spring or other means. Test drive the vehicle. If performance is worse with the secondaries held shut, then they were opening when operating normally. Winging the engine in neutral does not work! Vacuum secondaries operate off engine rpm and load, not just rpm, and there just isn't enough airflow in a no-load situation to open the vacuum secondaries.


http://www.bob2000.com/carb.htm

 

[COBRAIIW]

COBRAIIW

How could you tell it was opening at all ?

Change in sound,plus sudden acceleration.

 

[a351Must2]

I've always been told that if you can feel the secondaries open, the spring is too weak.

Otherwise, a stock II 302 with a 600cfm 4160 Holley will not open the secondaries all the way with the normal vacuum secondary spring, but they will open far enough.

 

[likesoldjunk]

On my way home last night, I stopped by the
speed shop and bought a Holley spring kit, and
a neat replacement top for the solenoid that
allows you to change the spring without taking
off the choke and solenoid. I'll let you know
monday what I learn.

 

[Dano78]

Yeah you're definately not gonna want all 600 cfm to dump into that stocker 302, especially if it's a 4160 Holley since you cannot alter the rear jetting. The 302's in the late 60's came from the factory with 450 and 465 cfm 4bbls (the Autolite 4100 models) and they were solid performers. Of course most the guys would get the Hi-Po cam and add the 390 FE 4100 Autolite carb (rated at approx 575-615 cfm) and it'd make a heck of a difference.

You're on the right path with the springs though. But like a351Must2 said, you should NOT feel the transition if the secondaries are set correctly. This is the area where GM's Thermo-bog carb earns it's nicknames.

You can put a tiny bolt in the slot where the secondary linkage slides before opening, to artifically open the seconaries. I have done this. It does work, but like most mechanical secondary carbs, its got a nasty bog when you mash it to the floor but even moreso, because that particular Holley was never designed to be run like a Mechanical, therefore not having a temporary enrichment circuit.