'74 lights stay up
Thread Starter
Instructor
Joined: Apr 2005
Posts: 137
Likes: 0
'74 lights stay up
I bought my '74 and it came with the lights in the up position. I've heard this is a common problem. Or viceversa will not come up. Bottom line, does this affect the performance of my engine? I cannot get my car to idle in gear. It simply turns off in gear. My question is does the headlight issue have anything to do with engine. I've done everything imaginable on the engine side (short of taking it to the Mayo Clinic) and am now wondering how big a role the lights play. Please advise.
Burning Brakes
Joined: Apr 2007
Posts: 1,028
Likes: 1
From: SOUTH ELGIN IL
This should be an easy Q to answer for many on the forum because it is such a common problem but I'll take a crack from a position of about 50% knowledge...
Your 74 uses vacuum to actuate the headlights up and down, that's why they only work (when in running order) when the engine is running. The vacuum that moves the lamps up and down is pulled from the engine. When I got mine it had fixed headlights (left) installed from a kit, check Corvette Central. Since I'm not looking for numbers matching/originality I am planning to pull a servo headlight system (ie a mid '90's camaro or TA) and use it for better reliability and switch back to the flip up lights.
As I understand the system vacuum is only pulled when raising and lowering the lights so when the car dies by putting it in gear it shouldn't be related. It could be any number of things, are you sure your carb is tuned propperly? Alternator is good?
See what everyone else has to say...
Your 74 uses vacuum to actuate the headlights up and down, that's why they only work (when in running order) when the engine is running. The vacuum that moves the lamps up and down is pulled from the engine. When I got mine it had fixed headlights (left) installed from a kit, check Corvette Central. Since I'm not looking for numbers matching/originality I am planning to pull a servo headlight system (ie a mid '90's camaro or TA) and use it for better reliability and switch back to the flip up lights.
As I understand the system vacuum is only pulled when raising and lowering the lights so when the car dies by putting it in gear it shouldn't be related. It could be any number of things, are you sure your carb is tuned propperly? Alternator is good?
See what everyone else has to say...
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Usually the reason the headlights stay up is because the "small" hose side is not recieving a vacuum signal.This would not cause much of an engine idle problem.
So it really depends on which portion of the system has the problem.
If you know where the 3 port check valve is then pinch off the hose between it and the manifold.If you hear an increase in RPM then you probably have a major leak.
Le Mans Master
Joined: Aug 2006
Posts: 7,427
Likes: 1,560
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Vacuum "power" for operating these systems is cheap, simple [in concept; the system is not "simple"], and reliable. Many have operated flawlessly for over 30 years. Because they are not understood by the average owner, they appear complicated and folks fear working on them. Buy a good "Vacuum System Diagram" and a vacuum repair guide for your year C3....there were differences about every year. Read the guide thru once or twice to get a good understanding of how the vacuum system works and what each component does. Buy an inexpensive vacuum gauge kit at your auto parts store and diagnose/fix your vacuum systems. Usually, the problem is hoses, check valve, etc.; sometimes one of the vacuum actuators or vacuum switches can be at fault. If all of your system parts are still on the car, it likely won't cost much to get it working correctly.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Here's a good site if you do have a vacuum leak http://www.corvette-101.com/vacuum.htm PG.
EXCEPT THE "VACUUM DIAGRAM" IS WRONG. 
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Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Have you checked to see if the green striped hoses have vacuum on them? The green has to have vacuum to go up.If you have vac on green and they wont go up then you probably have bad witches hat seals.http://www.ecklers.com/product.asp?p...3&dept_id=1476
Last edited by ...Roger...; Dec 9, 2007 at 10:35 PM.
