Holley 4000 Spark Advance Circuit

[doug7740]

My 1955 Thunderbird, with a Fordomatic transmission, had the carburetor rebuilt in the spring and it has been running without any problems until lately. The problem I have noticed is when I accelerate under light to moderate throttle, it will almost feel like the engine is puttering or holding back. If I accelerate from a stop there is no hesitation and the engine idles smoothly. Since it seemed like my problem was with the spark advance I checked the timing to make sure it was set at 6 degrees, but when I accelerated the engine there was no advance. I made sure the breaker plate was moving freely when I applied vacuum to the advance using a hand vacuum pump. If you put hand on the vacuum line at the distributor you can feel very slight vacuum, however when I put a vacuum gauge on the line there is not enough vacuum to obtain a reading on my gauge at idle or under acceleration. If you look into the small vent holes of the spark valve mounted on the carburetor, you can see the spark valve diaphragm moving under acceleration. Since I had an extra spark valve I put it on the carburetor, however the results were the same. The questions I have for the group is what is the vacuum reading at the distributor advance supposed to be at idle and when accelerating? Also can someone explain how the spark advance circuit of the Holley 4000 carburetor works?

Doug

1955 Thunderbird Blue

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[Flat Ernie]

The '55-56 Y-block distributors are load-a-matics and have ONLY vacuum advance. So, you may not see any timing change with a timing light as you rev the engine. And it doesn't use regular manifold vacuum, it uses a special ported vacuum signal that is almost inverse of the vacuum signal seen at the intake.

The load-a-matic is a crummy design and unless you're committed to originality, you'd be far better off swapping for a later distributor...

 

[doug7740]

I have the 1955 Holley 4000 Carburetor Manual, however reading the explanation of how the advance circuit works, there was still some confusion. This is how I think the circuits works:

There are two vacuum passages that supply vacuum to the distributor vacuum advance. The primary passage is at the venture (that’s why you have to set the base ignition timing with the distributor vacuum line disconnected) and the other passage is at the throttle plate.

At idle, when the throttle plate is closed, venture vacuum is higher and it raises the check ball in the vertical passage sealing off the than the passage to the throttle plate. This prevents venture vacuum from escaping through the throttle plate passage.

This is where I get confused; All of the vacuum to the distributor vacuum advance passes through the spark control valve. When accelerating and the vacuum at the throttle plate is high, the spark control valve closes the passage from the throttle plate to the distributor vacuum advance preventing too much advance or a controlled amount of advance.

Two questions I have are:

1. How much vacuum is present at the distributor vacuum advance vacuum line, at idle when the check ball is closing off the throttle plate passage?
   
   
2. How much vacuum is present at the distributor vacuum advance vacuum line, when accelerating and the spark control valve closes the passage from the throttle plate?
   

This time I used a different vacuum gauge and connected it at the distributor advance using a vacuum tee. At idle, I was reading 1.25" of vacuum and while driving the car I was reading maybe 1.5" of vacuum under load. Are these readings normal?

When I purchased the vehicle the original vacuum advance had a leaking diaphragm and I could not find a replacement, so I purchased a aftermarket replacement. Using a hand vacuum pump, I measured how much vacuum was required to just start to move the breaker plate in the distributor and I discovered that at 3.5" of vacuum the breaker plate started to move!

So now I need to know how much vacuum is required to just start to move the breaker plate on the distributor with the original vacuum advance?

I also have been checking with the different parts suppliers looking for an original vacuum advance for a 1955 distributor and no one seems to offer it. Does anyone know where I can purchase one?

Doug
1955 Thunderbird Blue

 

[Flat Ernie]

I can't answer your two questions. I can tell you that as manifold vacuum goes up, ported spark vacuum goes down and vice verse.

By putting a modern vacuum can on the distributor, you likely require too high of a vacuum signal to get it to advance at all. Most modern designs require high vacuum to advance and the ported vacuum is less overall than the manifold vacuum.

Once again, if originality is not critical to you, I'd toss that distributor for a '57 or later that uses both centrifugal and vacuum advance. The Load-a-matic design was from the early '30s and wasn't very successful back then either.

If, however, you wish to retain all originality, you need to find a vacuum can for a '55-56 Y-block

 

[doug7740]

Yes, I want to keep T-Bird original, it still has 6 volt positive ground and a generator. I found a few 1955 T-bird vacuum advances, (B5S 12370-A) available as NOS on eBay for prices around $150.

 

[Flat Ernie]

Good on you for keeping it original! I'm not an originality die-hard, but I respect the dedication and effort of those who are!

 

[Tater Salad]

Doug,
I see this is an old post. Im hoping you can explain your findings. Im about to drop the engine and trans back in, but would love to test the vacuum advance first. Did you find the correct vacuum pressure the load-o-matics should operate at?

 

[doug7740]

I was able to purchase a new old stock vacuum advance for my 1955 Thunderbird. After I installed the vacuum advance I tested it using a Mityvac and discovered that it took approximately .20 inches of vacuum before the advance started to move, and at 3.0 inches of vacuum the distributor breaker plate was at the maximum advance. With the vacuum line disconnected from the vacuum advance and the engine running at idle, I was reading 1.25" of vacuum at the vacuum line. I set the base timing to 6 degrees before top dead center and after the vacuum line was connected and the idle speed was set to 700 rpm, the timing was reading 18 degrees before top dead center. When I accelerated the engine I could see the timing advancing even farther. When I drove the car on the road I was amazed how well it performed. When I stepped on the accelerator only a quarter of the way down the engine performed as if I had the accelerator three quarters of the way down. I also noticed the temperature gauge was reading much lower too.

Its been almost three years and my Thunderbird still performs like it should.

doug7740
1955 Thunderbird Blue

 

[Tater Salad]

Thanks Doug,
I found my advance is not working properly. When line is plugged the shaft moves freely in and out. I saw a NOS on Ebay but elected to buy a feferb due to 60+yrs of life on the shelf or someone's garage.
I will test the new (to me) one with a vacuum pump once it arrives. Thanks again for the information.

 

[doug7740]

Just make sure the vacuum advance you get is the correct one. The 1955 distributor advance is very sensitive, it works in the low range of 0 to 3.5 inches of vacuum.

doug7740
1955 Thunderbird Blue