Author Topic: 1956 Cadillac new engine - poor vacuum?  (Read 261 times)

Online carguyblack

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  • Name: Chuck Dykstra
1956 Cadillac new engine - poor vacuum?
« on: November 14, 2017, 02:23:57 PM »
I don't have a vacuum gauge and I know that'll be the first question. But I am going about this symptomatically. First, the carb has been not able to bring the idle up or down using the air screw, Secondly, the dash pot on the throttle linkage barely gets pulled in at all when the engine runs. It is new and I put a vacuum puller directly to it and it sucks in fine. Lastly, the idle up solenoid for the a/c pushes out fine when the a/c is engaged but when it sucks back in after the power to it is off the plunger seems to wobble in and out slightly as the car idles. The vacuum doesn't appear to pull it in solidly. That part is new as well. With the former engine before the rebuild, the carb (which has been restored) worked marvelously, the idle up solenoid to my knowledge didn't bounce around at all, and I'm not sure about the dash pot, since I just bought a new one. I have replaced all the rubber vacuum lines and double checked all the connections. The vacuum wipers work but not real good on the shut off - they don't want to return to the rest position very badly.
Does this sound like low vacuum being the fault of the carb or what am I missing? I don't want to send the carb back only to find out that it is fine with something else being the culprit. Meanwhile, the car isn't running very well and the exhaust smells bad too, not like a good running new rebuilt engine should. Thanks for listening and any suggestions.
Chuck Dykstra

Re: 1956 Cadillac new engine - poor vacuum?
« Reply #1 on: November 14, 2017, 02:36:58 PM »
Check the following:

1. Ignition timing

2.  Vacuum level.  If low after a rebuild and no leaks can be found, perhaps the pushrods are keeping one or more of the intake valves slightly open.

3.  Compression test.  On a 56 engine, the compression should be around 175 or more.
Art Gardner


1955 S60 Fleetwood sedan

Online carguyblack

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Re: 1956 Cadillac new engine - poor vacuum?
« Reply #2 on: November 14, 2017, 02:58:33 PM »
Art,
On your second suggestion, would a compression check pinpoint a bad push rod issue as you indicated?
The ignition timing was set according to the manual. Should I do otherwise?
Thanks for replying!
Chuck
Chuck Dykstra

Offline J. Gomez

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Re: 1956 Cadillac new engine - poor vacuum?
« Reply #3 on: November 14, 2017, 04:44:03 PM »
Chuck,

First grab a vacuum gauge..!   ;)

If you have a bad rod it would definitely affect the compression on that cylinder(s) either the intake or exhaust side, also you would be hearing some tapping noise while the engine is running.  :(

One note at this stage which I came across while putting my engine back together; the rocker arm mounting post (4 posts) are not the same length between the hole were the rocker shaft goes end the ends (top and bottom). On my hasting to get the heads bolted I had two of the post reversed (180 degree turn). When everything was back together I did a compression test and two cylinders had 0 zero compression. It took me a while to notice the difference between the rockers compressing the valves. The one side with the incorrect mounted posts the valves were about compress when the rocker were in the fully up position.

Also, as a second note you may be aware of the difference hydraulic lifters and rods that were available on the 365. Im not sure if you have the original 55/56 hydraulic lifter/rod place back or if you interchanged them with the newer ones?  The old and new were not interchangeable they have to be change in pairs. Just a tech hint.

Have you removed any of the vacuum hoses from the engine (specially the one feeding the wiper/washer vacuum manifold at the firewall) and plug including the one going over to the M/C vacuum chamber valve? Did the A/C fast idle speed-up valve changed?

Just something to get you busy, but for sure a vacuum gauge would be your best bet to read the vacuum.

Good luck..!
J. Gomez
CLC #23082

Time is irrelevant when you work on your classic car, eventually you will enjoy it..!

Re: 1956 Cadillac new engine - poor vacuum?
« Reply #4 on: November 15, 2017, 07:44:37 AM »
Art,
On your second suggestion, would a compression check pinpoint a bad push rod issue as you indicated?
The ignition timing was set according to the manual. Should I do otherwise?
Thanks for replying!
Chuck
Yes, a compression check should reveal if one of the valves is being held open all the time.  If so, that cylinder will read rather low compression.
Art Gardner


1955 S60 Fleetwood sedan

Offline gary griffin

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Re: 1956 Cadillac new engine - poor vacuum?
« Reply #5 on: November 15, 2017, 09:54:09 AM »
I would basically consider the entire vacuum system suspect and run a pressure check on it. Easier to hear a pressure leak than a vacuum leak .   Our cars and prides and joys were not really designed for easy maintenance 61 years later.  Only a couple of PSI  of pressure and  listen carefully. If you are losing pressure you have a leak.
Gary Griffin

1940 LaSalle 5029 4 door convertible sedan
1942 Cadillac 6719 restoration almost complete?
1942 Cadillac 6719 (parts car) (Gone)
1957 Cadillac 60-special (Needs a little TLC)

Offline cadman56

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Re: 1956 Cadillac new engine - poor vacuum?
« Reply #6 on: November 15, 2017, 10:56:09 AM »
I will only say this.  Get a vacuum gage first and see what you get.  If vacuum is low and steady you have late ign timeing (Shop manual setting will give poor performance and low vacuum-from experience with many 56 365 engines.), incorrect push rods, or a vacuum leak.  If it is jumpy check for valve problems. 
Which lifters were used?  Later yr. lifters require shorter push rods.
Good luck.
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

Online carguyblack

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  • Name: Chuck Dykstra
Re: 1956 Cadillac new engine - poor vacuum?
« Reply #7 on: November 16, 2017, 09:01:45 AM »
A qualified mechanic friend is coming over tonight after work to test the vacuum as well as compression. I just want to make sure we do the vacuum test right. Do I unplug and test each inlet: 1. the advance on the distributor 2. the wiper cluster at the firewall 3. the top of the manifold that goes to the vacuum canister 4.  the brakes  5. anything else?
There are a couple of hard line connections directly to the carb base, too. One of which is the idle speed advance for the a/c.  And the dash pot on the throttle linkage at firewall.
There just seems to be so many areas for potential leaks. I did do an ether test when everything is assembled and the engine was running. I sprayed everything everywhere and got no changes in engine idle anywhere. No obvious culprits.
Chuck Dykstra

Offline cadman56

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Re: 1956 Cadillac new engine - poor vacuum?
« Reply #8 on: November 16, 2017, 11:27:17 AM »
There should be two vacuum lines on the bottom of the manifold on the firewall, one from the vac. pump (passenger side) and one from the intake manifold (driver side)
The vac hose from the intake manifold should have a tee with a hose going to the throttle relay.  To check engine vacuum connect the gauge hose to the tube that goes to the intake.  This isolates all possible vac leaks and you see what just the engine is doing.
If the vac reading at this point is low and steady your valves are probably ok.  At this point I would play with the timing by increaseing it to between 8-10 deg advance.
If vac reading is jumpy I would suspect valves or maybe misfiring.  Then run your compression test to isolate which cylinder is not correct.
Remember the throttle relay pulls the throttle back with vacuum applied and movement will be very little.

The AC throttle speed up should not allow vacuum to to pull the throttle open until the AC is engaged and only then.  You probably should disconnect that line and plug it while checking vac.

Did you purchase the lifters/push rods for your engine rebuild?  If not try to find out what was installed.
I wish you the best.
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

Online carguyblack

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  • Name: Chuck Dykstra
Re: 1956 Cadillac new engine - poor vacuum?
« Reply #9 on: November 16, 2017, 01:14:04 PM »
I bought everything from Terrill's taking their advice on it all. I bought the whole store and got everything matching, so I didn't have some old and some new together. Sure hope it's not in the engine internals!
Chuck Dykstra

Offline cadman56

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Re: 1956 Cadillac new engine - poor vacuum?
« Reply #10 on: November 16, 2017, 05:10:41 PM »
That's good.  Just always ask when trouble shooting.  Hoping the issues are only minor adjustments.
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820