flat acceleration issues on a 1956

[jgreene]

My car is a single Carter Carb, which has been rebuilt several times.  I have changed out the vacuum advance several times, and currently have an adjustable Accel from Summit Racing.  I don't know if it has a high enough HG to it.  If it didn't, what would the symptoms be?  The fuel pump is new, and pumps at almost 6 pounds.  The inside of the gas tank looks brand new.  The points and condenser are new, the spark plug wires are fairly new.  The coil seems fine, as I installed a new one, and it made no difference.  I have sprayed stuff all over the engine for vacuum leaks.
The car will accelerate off ok at light pressure, but step on it just a little more, and it flattens out.  Step on it a little more and you can feel the advance come on like someone just flipped a switch (I assume the mechanical is coming on).  It of course isn't smooth like it should be.  I drove a friends '57 and the engine followed the accelerator exactly under light throttle.  My cars engine number is over 92,000 and there was a change late in the year to a 2544S (air conditioned) carb, could the problem be with the metering rods?  I am obviously reaching here, but a dyno shop couldn't figure it out, nor could an experienced mechanic.  Any help would be greatly appreciated.
Jim Greene

[The Tassie Devil(le)]

A flat spot could be a number of things, but an inoperative or poorly operating Accelerator Pump will cause a flat spot.

Like it runs lean, just at the point that the air-speed isn't fast enough to "pull" more fuel through the Ventuties, even though the throttle plates are open.

Bruce.

[John Washburn CLC 1067 Sadly deceased.]

Jim,

Have you thought about the distributor? The vacuum advance is for low speeds, but the centrifugal advance may not be functioning correctly due to dirt and old grease on the springs in the distributor or the distributor plate could be bad?

My guess would be to check the centrifugal advance springs and plates to make sure they are not stuck with age and old grease.

The Johnny

[bcroe]

Make sure the timing chain is tight.  Bruce Roe

[jgreene]

Thank you all of the responses.  I hadn't thought about the timing chain--I thought it had been replaced about 20 years ago, but frankly I am not sure.  I will also check the accelerator pump on the carb as well.  I had the distributor out of the car and centrifugal advance weights seemed fine, but I will check them again.  Is there a difference in the spring strength, on the advance weights, from one side to the other?  Wouldn't weak centrifugal advance springs make it advance too fast?  I think that came on newer Cadillacs than a 1956.
The only way I know how to set the leanness/richness on the car is with the idle air adjustment screw, and the two idle port screws on the front of the carburetor.  I have looked at the approximate settings in the shop manual, and used that as my guide.

Also, how do know where the vacuum advance cuts back, and the centrifugal advance kicks in?  I have tried to figure it out from the charts in the manual, but it doesn't work.  I don't think I fully understand when the centrifugal takes over (such as at what RPM.)

Also again, does anybody know of a good dwell meter that doesn't conk out in months after a few uses?
Thank you for reading this.
Jim

[Doug Houston]

I once had a funny experience with the '56 I had back in the sixties. Of course, I wasn't trying to use it as a racing sled, but it had lost its normal responsiveness. It was a 62 convertible.

The problem was that the flywheel part of the harmonic balancer had broken loose from the hub, and had shifted, causing the timing marks to be in the wrong place. No matter what I'd done to get it to tune properly, nothing ever worked.

It seems that this was a sort of chronic problem on the '56 Cadillacs, and I learned of it a few years after I no longer had the car.

Maybe you're unusually lucky, or there's something else that hasn't come out here, but that transmission won't put up with a lot of kicking around. A performance transmission, it was not! A rugged transmission,  also, it was not.

[John Washburn CLC 1067 Sadly deceased.]

Jim,

The vacuum advance is for slow speeds and is activated by the vacuum (in hg) in the carburetor. The vacuum advance starts to open 6.5" to 8.5" hg (or begins to move the arm). Around 14" to 16" hg the vacuum advance is about 18 degrees advanced. So you can see that vacuum drives this control at low speeds.

The automatic advance, centrifugal, starts adding advance (1/2 to 2 degrees) around 600 RPM, 4.5 to 6.5 around 1100 RPM, and maxes out around 9 to 11 degrees at 1650 RPM.

Hope this helps.

The Johnny

[dplotkin]

Hey! I wonder if Mr. greene & I have the same problem? I have low vacuum & no idle, cannot find the leak, timing checks OK, but if the marks are wrong.....??

Dan

[John Washburn CLC 1067 Sadly deceased.]

Now is the time to get the vacuum gauge out and see if it will help indicate the problem.

Also, I had the same problem as described by Doug. But it happened when I had the dampener rebuilt, the guy moved it. So I timed the engine using a vacuum gauge. Then marked the dampener to show where the correct timing was.

The Johnny

[J. Gomez]

Jim,

Here are the references John stated above..!

http://www.modifiedcadillac.org/documents/Multiple_Years/55-64%20Motors/tn/55-64%20Motors-5.jpg.html

Since the vacuum advance was a one year only it is extremely difficult to find a match for the 1956. I’ve also try the same adjustable vacuum advance and found the adjustments were off with no tension on the spring about 8hg on the low end and about 18-19hg on the high end. I even try to place a stopper ring to stop movement at around 16hg but the low end was a problem.

Maybe adding a different spring could solve the problem but that mean dismantling the unit and trying different springs..! Maybe when the weather turns around..!

[John Washburn CLC 1067 Sadly deceased.]

Jose,

The other alternative is have your old vacuum advance rebuilt. Costs around $100.00.

The Johnny

[J. Gomez]

Jose,

The other alternative is have your old vacuum advance rebuilt. Costs around $100.00.

The Johnny

John,

Thanks for the 411..!

Unfortunately in my case I have a generic aftermarket unit, I wished I had the OEM.

I bought the adjustable one as it looks similar to the OEM type with the hex front hoping to get it close into the range, but no cigar.

Thanks

[John Washburn CLC 1067 Sadly deceased.]

Jose,

Have to look may have a core for you. If interested will look. You have to pay the shipping.

I still have a few new ones for my 56.l

[bcroe]

I had a lot of dampers of that era fail.  You can make a timing mark that doesn't depend on the damper.  However the damper can still come completely apart.  I finally fixed that permanently with FluiDampers on my 5 cars. 

As for timing curves, I had great luck with an electronically programmable unit based on a common crank trigger unit, needs no distributor.  See on PHOTOBUCKET   

     http://s93.photobucket.com/albums/l71/bcroe/

(that is a lower case "L"71, not an upper case "i"71)

  click on a Sub-Album   (CRANK TRIGGER IGNITION)
  click on a picture to enlarge, or
  click on "view as a slide show"
Bruce Roe

[jgreene]

Thx for all of the great suggestions.  In regard to how I drive it; I don't hot rod it, it is at very light throttle that it flattens out.  It takes off just fine if you push down on it, just the light throttle that is a problem.  The transmission works fine, and I know they were very weak and would not handle abuse.  Apparently they fixed all of the problems in the '57 transmission.  I will look VERY closely at the mechanical advance and make sure there is nothing making it hang up.  No one had any knowledge on the springs being of different strength, so I will assume they are the same.

Question on the vacuum advance curve---the adjustable Accel advance seems like it only goes to about 12 HG.  Is that high enough to handle the advance needs, or do I need to shave off some metal to get it to go farther?  The original VA I have is the original part that was only used for about a month.  They made a change to 111 6103, I believe before the actual year 1956.  Anyway, I am also looking for a correct original to have rebuilt.

I will also look at the suggested website.  I will work on the car when I got some time this weekend.  I have also got to buy a decent dwell meter and tach.
Thx to you all
Jim

[John Washburn CLC 1067 Sadly deceased.]

Jim,

The springs on the arms for the centrifugal advance are the same.

The Johnny

[J. Gomez]

Jose,

Have to look may have a core for you. If interested will look. You have to pay the shipping.

I still have a few new ones for my 56.l

John,

Thanks for the offer, you have a PM from me..!

[R Schroeder]

This is interesting. Might not solve your problem, but........

http://www.gofastforless.com/ignition/advance.htm

[Caddy Wizard]

This sounds like a vacuum advance problem.  If you want to re-do the vacuum advance, you can do it yourself to be better than buying a new one (a NOS unit will have a 50 year old diaphragm and will fail shortly after you put it in service).  Here is my description of doing this myself.


My vacuum advance unit was completely shot -- no advance and the thing was a pure vacuum leak.  Fortunately, the vacuum unit on the modern-style '56 distributor is the same basic design that GM used for decades to follow. So while the exact fit unit is no longer available (spring rate and screw connection to vacuum), I was able to take my original unit apart and use the guts from a common later unit (that I also took apart). The diaphragm and its attached rod are EXACTLY the same as my original. The Echlin unit I got from NAPA for this purpose is VC1765. Cost about $10 -- works perfectly. My old unit is now absolutely as good as new.   The following is how I did it.


To do this, I clamped the old unit in a wooden vise, using a pair of wooden blocks to grip the stem of the housing.  I cut a V-groove in one of the blocks to receive the movable shaft (the actuator rod).  With the unit securely clamped in the vise, I carefully prised open the "can" using a small custom-made chisel.  I took a narrow pin punch and sharpened the end into a skew chisel and bent its shaft to work around the curvature of the canister a little bit.  Using this very narrow skew chisel, I started to work my way around the bent-over lip of the canister.  Once I had a little length of it prised up just a tad from the cover, I took pliers and bent it up straight, a little at a time, working my way around the cover.  Be careful, as at a certain point the cover will come loose and the spring will pop it up in the air.  Then it was a matter of cleaning the inside and discarding the old diaphragm/rod assembly.  I followed the same procedure to harvest the diaphragm/rod assembly from the new VC1765 and installed it in the old canister.  Then I buttoned it up by gently folding over the edge of the canister over the cover by gently tapping it with a very small hammer, again working a little at a time.  In the end, it looks just about perfect and works as good as new.

[jgreene]

Thx to you all for all of the great suggestions.  I believe I now have the vacuum advance issue solved.  I made permanent adjustments to the Axel vacuum advance, and it seems to work fine.  The centrifugal advance is also working fine, and I could find no vacuum leaks.  HOWEVER, the metering rods were not adjusted correctly!  I still don't know if I have them correct, but it certainly seems to have lost its flat spot.

It states in the manual that the metering rod lever is loosened to obtain a "slight bind" on pump countershaft.  I followed the directions, I think.  What is unclear is: is the bind that needs to be obtained with the metering rods all the way up?  Meaning with it at full throttle it puts a bind as it pulls up to the limit of the accelerator linkage.  When you push the metering rod link all the way down it gets temporarily stuck in that position.  What appears to be happening is there is a tab on the metering rod link which gets stuck in the tang (curved "c" shaped tab) that is hooked to the set screw.  So is it ok if it gets stuck down since the tang pulls it up when you step on the gas?  And should the little tang start pushing up on the metering rod link the second you begin to give it gas?

I know that this is a bunch of obtuse questions.  So far the advice has been very helpful from everyone.

Today I turned around the metering rod link, so it looks like the factory photos from Carter.  The flat spot is completely gone, and it is very smooth.  It is unbelievable what goes on out there in the mechanical world.  This was no small task, and I also discovered the floats were not exactly correct either.  I have no idea what small float miss-adjustments would cause, but my guess is not enough fuel in the bowls, or too much.  I also found out the gaskets have to be filed/cut to fit correctly, or they will break.  My next project is the vapor lock/lack of fuel in the bowl.  The fuel pump tests at nearly 6 pounds, and the gas tank is spotless.  I don't want to have to replace the fuel lines, but it may come down to that.  I have not figured out how to shove a copper wire through the windy part of the line that comes directly out of the gas tank, and I don't have a compressor.
The big headache I have had with the car for years is gone!!!!
Jim
1/15/11
Thank You!
Jim