mikeb

On my '76, I'm wondering if the manifold vacuum pressure is enough to operate my headlights properly.
They currently go up rather slowly, and will not retract on their own.
I do not have any vacuum gauges presently.
Currently, vacuum hose from manifold (circled green) leads out to brake booster with a diverter to the
headlight vacuum tank (front frame rail tube) tube circled in orange.



Now, to get the headlights to retract, with engine running, I need to pull the top hose off the actuator relay, and suck on the tube until
the headlamp clicks out of the full-extended position (only slightly down-ward). I then hook the tube back
up to the relay, and the headlamp closes. It requires quite a bit of human vacuum to get the downward motion started, more
than the current vacuum supply provides.
Not sure what my problem is, but I feel quite a bit of vacuum is required to get the units out of their "locked in up position".

I also wonder if I should be using manifold vacuum as shown in above picture, or should I be
using carburetor vacuum from the front of the carb? (Red circled area on carb which is plugged closed)
(

One oddity: When I remove the main feeder to the storage tank, and plug the end of the tube, my idle speed drops about 100 rpm.
Does this mean I have a leak at the storage tank?
Thanks for looking.

bmotojoe

Personally I would look see if you had a pipe plug dead center of rear of carburetor and run a 3/8" vacuum rated hose dedicated to the Brake Booster.
Then use your manifold fitting to feed your lights and HVAC if equipped.
The Headlight relays, not actuators take around 5-7" of vacuum to change state, so if your lights are opening and closing slowly may indicate a relay problem or a vacuum signal to the relay problem.
The vacuum fitting on the front should go to the PCV,

gjohnson

Make sure the vacuum over ride switch is pushed in.If its out, lights stay up

mikeb

Thought I mentioned it, but I don't see it.
Both the pull down switch, and headlight switch are functioning as designed, except no retraction.
I tend to agree with bmotojoe's post that my problem is likely in the relays.
How does one test them?
I have 1 new actuator, and 1 old (original?) actuator. The new actuator is slower
than the old unit, so I'm guessing it's most likely the actuator relays.
Would like to test them before buying new units. May just buy all 4 actuators and relays.

bmotojoe

You will need a vacuum gauge and hand vacuum pump...
First I would test what is the vacuum signal via gauge at the relays, and if the diaphragm of the relay will even hold a vacuum.
With the headlight switch pulled what is the gauge reading on the small hose on top of the relay?. It only takes 5-7" of vacuum to shift the internal spool but the diaphragm of the relay should hold and maintain 15+" of vacuum for several minutes.
That's where I would check first.
If the relays hold a good vacuum then I would then move on to either try to rebuild/repair them or replace them.
Repairing them depends on the internal spool condition and the replacement relays are a crap-shoot.

Peterbuilt

Here's a 'how-to' on your headlight system.:

http://www.corvette-101.com/vacuum.htm

Just test each part as you go step by step and you will have the lights working 100% in a few hours.

4-vettes

Not seeing anything for PCV. I know some run with a breather cap in both valve covers, Although not optimal. I don't see that as well. Brake booster should have its own dedicated vacuum line to base of carb. PCV should be on other side of carb. Vacuum for headlights and HVAC should come off manifold.
Not having a vacuum gauge is a much bigger issue than the brand of carburetor your running.
You are pretty much dead in the water without tools.
As stated above, you need a vacuum gauge and a vacuum pump. These are inexpensive tools.
Then and only then can you move forward.
As per the drop in idle speed. Yes sounds like you have a leak somewhere. You need to test. you need a couple inexpensive tools.

mikeb

No PCV on this set-up, just a breather on either valve cover. There is a small vacuum port on the right side of the carb supplying vacuum advance to the distributor.

Anyway, time to get a few gauges and start testing. I plan on changing the brake booster to a dedicated line
off the carb front port, and the headlight vacuum lines off the manifold port, then test the system per using Peterbuilt's link.

Thanks again for all the tips and tricks.

mikeb

Just went out to measure for new hose for PB booster, tested the check valve on the headlight ports, and the valve is open both ways.
Not good. Went to the local Napa, no luck, so I need to order the part. Should get it early next week.

Peterbuilt

Hold off on ordering parts and make 1 order with all the parts that you need so you don't get charged for shipping several times.

Actuator repair kit:

mikeb

Using the linked list, I traced the leak to (2) parts, the check valve, and the left relay actuator.

Replaced both relay actuators one actuator I believed to be OEM yesterday, and headlights now operate as the should.

Thanks again for the link. It was of great heap.

scottyp99

Ahhh, I like stories that have a happy ending! I am happy this worked out for you. Although.....one thing I'm still kinda scratching my head over. Vacuum pressure? Seriously?

Scotty

derekderek

what is wrong with calling it vacuum pressure? Manifold Absolute Pressure is the new term for it. if you wanna call 40 years or so old new. 29.92 inches of vacuum is pure vacuum. 14 inches is around half an atmosphere. 7 psi approx.

scottyp99

29.92 inches of mercury is normal atmospheric pressure, not "pure vacuum". Anyway, right or wrong, it just hits my ear wrong. It's like if somebody said, "Who turned on the dark?" instead of "Who turned off the light?"

Scotty