1955 Ford Thunderbird - No vacuum from Advance Port - HELP!

[soundquest]

So I have been trying to troubleshoot the no-vacuum issue at the vacuum advance port on my carburetor. I am able to blow air through the port and into the venturi just fine, however, when I have the carb hooked up and running, and put the engine under load, I am not getting ANY vacuum or suction off of that port... not even the slightest. However, I discovered in removing the carb this time to further examine things, that the spark control valve was not screwed in all the way (almost looked like the rebuilder just got the threads started and then got distracted and forgot to finish the jub of screwing it in all the way. So I snugged it down (there is an 0-ring that it is supposed to compress to make a seal). So my question is, will this restore my vacuum now (I don't want to put the carb back together until I have fully examined it while it is off). The other thing I was made aware of was that there is supposed to be a little red check-ball underneath the little brass screw in the underside of the carburetor. Since my brass screw is all stripped out by the previous owner or the rebuilder, I cannot remove it to inspect that check-ball. Looking for some advice on this. Would really like to get my vacuum advance working properly as I just had the vac. advance diaphragm rebuilt and want to get this engine running properly (don't we all...). Thank you.

[dmsfrr]

If I knew anything about the guts of the carburetor I'd tell you what it needs, but I don't.

When Sal C. sees your post I'll bet he has a good answer for you.
https://www.fordbarn.com/forum/member.php?u=5934

[scicala]

The small plastic check ball was eliminated in mid-year production 1955. If you carb is Holley number LIST 997 or 998 it should have the ball. If it's LISt 1077 or 1078, it never had the ball installed.
The spark valve not being tightened is more than likely your problem. There isn't supposed to be an o-ring for the seal. Just a fiber round gasket.
If you can't get the brass screw out that retains the ball, then maybe just sray some gummout type car cleaner in the channels behind the spark valve and blow it out with compressed air. It's usuall pretty clean in the vertical channel the plastic ball is in if it's an early '55 carb. If still no vacuum, you may have to drill out the stripped brass plug to clean it out, then just seal the bottom of the hole where the plug was with a channel plug or lead shot.

Sal

[soundquest]

I am very much looking forward to his input and greatly appreciate your tip of the hat. Not many folks around anymore that really understand these carbs and their different iterations. One more pertinent detail -- this is an automatic. Here is a pic of the underside of the carb where you can see how badly the screw is stripped out. Aaargh!



https://drive.google.com/file/d/1-St...ew?usp=sharing

[soundquest]

Wow- looks like you were typing while I was typing Sal and we cross-posted! Thanks very much for the incredibly helpful and detailed reply! Fantastic info. I will check on the things you suggested (starting with carb model number to determine if check ball is there in the first place). I am hoping the the loose spark valve was the cause but I will be thorough in determining this while I have the carb out. I will post back with the results. Many thanks!

[scicala]

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Also, there will be no vacuum at the distributor port of the carb, until the throttle is opened slightly, to expose the hole in the throttle bore. The vacuum will vary from almost 0 to a max of 4" depending on engine RPM and throttle position. 4" of vacuum would be at high RPM and heavy load.

Sal

[soundquest]

Sal - it is list 1078, so no ball! Fantastic. Do you think the rebuild place would have forgotten to clean the channel behind the check valve? I think I am almost ready to just reinstall it and see how she fares. Have you ever seen that passage behind the brass screw get totally clogged? Also, since the vacuum is so low, what would be a good crude method for testing such a low vacuum threshold (without a SUN machine and other fancy equipment)? Should I just Go-Pro mount a camera in there looking down at the vacuum advance diaphragm rod to see if it moves?

[scicala]

It sounds like you should be good to go as long as the spark valve is fairly new and tightened all the way. Of the hundreds of these carbs that I have done, there may have been maybe 15 or 20 that I've seen with any dirt in that channel, and never saw one totally clogged. The main concern would be if you had the plastic ball in there, since it's almost the same diameter as the channel, and needs to bounce back and forth between venturi vacuum and manifold vacuum. No ball, no worry.
You can check for ignition timing advance and/or vacuum at the port with a vacuum gage, or use a timing light on the crank balancer. You would need to have someone in the driver's seat, and one person under the hood. Put the car in drive or reverse with a heavy foot on the brake so the driver can open the throttle slightly (brake torque). That should be enough to show some vacuum and/or timing advance. Can also pull out the parking brake and block the tires for more safety.

Sal

[soundquest]

Think I am making some progress (hoping so anyway). About how far should it advance under light load in reverse approx? I have the timing dialed in at about 10 degrees advanced, and when I have my assistant give it some gas, I can see the timing mark move toward me about 5 degrees approximately (standing on the passenger side of vehicle looking at the timing marks with timing gun in-hand). To test it, I disconnected the vacuum advance (but forgot to plug the carb side vacuum) and then put it under load again, and the timing did not move at all. So is this proof-positive that my vacuum advance works now?

[soundquest]

Also, you brought up a good point Sal -- the possibility of the rebuilder putting the plastic ball in there and it getting stuck when no plastic ball is needed in this model. I guess the only way I am going to find that out is by drilling out that stripped out brass port screw.

[scicala]

It sounds like the timing is advancing like it should. You won't get a lot of advance without driving the car and putting it under a heavier load.
As long as it's advancing and drives good, I wouldn't worry about trying to take the brass plug out to see if a ball is in there. I would guess there is no ball, unless the rebuilder told you there was.

Sal

[rotorwrench]

Look up information about the Holley Load-O-Matic spark control system. It is operated by venturi vacuum and not manifold vacuum as more modern cars were.

The nylon ball was in some carbs to give some vacuum at idle with the throttle closed. Once the car is a road speed all of the vacuum acting on the distributor is from the port in the venturi. This system doesn't work at all like the more modern vacuum spark controls used after 1956 in Ford products.

[scicala]

It's a combination of ported manifold vacuum and venturi vacuum, all depending on the throttle position/manifold vacuum and RPM. Not just venturi vacuum.

Sal

[soundquest]

Just an update on this. I decided to do some more testing to see if I was getting full vacuum on the advance port. I did this by hooking up a transparent vacuum line to the port, putting the car in reverse (assistant in the driver seat, blocks behind wheels, etc) and having my assistant apply the gas pedal to simulate a load. I then placed my transparent vacuum tube in a small quantity of water and observed some pretty weird vacuum behavior. It would only suck up the water at a specific RPM, and even then, it would only suck it up several inches or so and then all of a sudden just stop. It would only "clear" the line and enter the carb after several flutters of the throttle (and only under a very weak load, not under a higher load with more throttle applied). I appreciated this small quantity of water entering the engine as it helped to decarbonize things a bit (black carbon splatter out of the tail pipe, the slightest amount).

Anyhow -- possible that spark valve is defective or not sealing properly? Possible that I have an obstruction beyond that seized/stripped access screw? Or should I actually try to drive the car with this same test setup (vacuum hose stretching into passenger seat so I can observe suction while I actually drive it down the road). Thanks.

[rotorwrench]

They call the system a load-o-matic for a reason. A car isn't under load until it's either pulling a heavy load or it's going up a long hill.

I would be more concerned whether you have the correct carburetor to work with the correct distributor for the specific year and engine application. The load-o-matic worked but I would rate it as just barely adequate as far as how well it worked for spark control on the Y-block. It worked better on the flatheads than it did for the overhead valve engines. This is why it was replaced in 1957 after a year of unsuccessful tinkering with the system in 1956.

Get yourself a parts catalog and find all the 1955 Thunderbird maintenance information you can to insure you have the correct components. The vacuum diaphragm can has to work as well. Holley also had their own information on these carburetor and distributor combinations.

Most of the spark control systems work off the principal that engine manifold pressure (vacuum), whether it's modified or not, loses vacuum pressure as the throttle is opened up under load. This acts on the distributor to retard the spark a bit while under load. As soon as the manifold pressure gets back to normal then the spark advances back to normal running position. The amount of degrees it changes isn't all that much but it's enough to prevent spark knock and improve mileage a bit.

Since the load-o-matic doesn't have any centrifugal advance, it has to provide all of the advance through the vacuum diaphragm. It is very important that the vacuum system be completely sealed up from the port in the venturi to the diaphragm. If it leaks anywhere in that circuit then it doesn't function well.

The system that came out in 1957 has both centrifugal and vacuum to operate the spark control. A lot of folks look for the 1957 or later Y-block combination to replace the earlier set up but they are getting harder to find as time goes by.

[dmsfrr]

Originally Posted by rotorwrench

. . . I would be more concerned whether you have the correct carburetor to work with the correct distributor for the specific year and engine application. ....

The original '55 T-Bird carburetor should have an intake manifold mounted choke thermostat and a 9510 part number with an - ECJ - prefix and a Holley "List" number of: 997-1, 998-1, 1077, 1077-1, 1078, or 1078-1.
Some of these List numbers are for factory replacement carburetors.

An original '55 T-Bird distributor will have a tach drive cable fitting as part of the cast lower housing and an aluminum tag fastened to the upper side, with an FEA (12127) E or F part number.

As you probably know...
the '55 (& '56) distributors can only advance the ignition timing based on the correct vacuum signal from the '55 or '56 carburetor. (see this link). https://www.ctci.org/cracked-exhaust-manifold/


https://www.ctci.org/shop/restoratio...-set-110-58fc/

https://www.ctci.org/product-categor...nical-manuals/
.

[soundquest]

Wow! All I can say is... YOU GUYS ARE EXTREMELY HELPFUL! I can't thank you enough. I have some work to do in making sure I have the correct and matched equipment on my car.