Outstanding. Glad the problem finally turned up. Damn parts with no quality controls.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

PROGRESS!

Spent all day on the car.

1. Re-ran the compression tests on a cold engine with throttle wide open ---basically same results as yesterday's warm engine. Pressures ranged from 102--114 psi. Reinstalled the plugs. 

2. Temporarily removed the primary muffler. Took car for spin ---the crazy loud engine still sputtered at 2800-3000 rpm and wouldn't rev past 3100 rpm.

3. Next I decided to change out the condenser in the dizzy even though it's brand new as are the points, rotor, cap, wires, etc. The web is full of stories about unreliable modern condensers... Easy change, take her for a spin, and BINGO!!!! On separate accelerations the engine flawlessly rev'd to 3400, 3600, 3800, and finally 4200 rpm! I can't believe it ---a brand new basic, static, round tin foiled $7 part had needlessly awoken gnawing thoughts of engine rebuilding gymnastics. What a major relief!

I'll reinstall the muffler tomorrow and run it again. If it continues to rev OK I'll probably go ahead and begin the '57 Intake and Centrifugal dizzy swap. The low vacuum condition at idle persists---hopefully, in a sense, I'll find problems with the intake gasket(s). 

More to come.
 

Disconnect the return springs and wire the throttle open. Sticking a screwdriver down and wedging a butterfly is not in your best interest.

I wouldn’t get too hung up on the vacuum reading being a bit low if you get everything else solved and it runs well. I’ve had a lot of worn engines that ran ok and went a lot more miles. Including a couple I just kept adding oil to.


miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Good catch! I'll stab a long screw driver down the throat of the Teapot and try again. It's raining and 48F outside ---engine is much colder so it won't be an apple to apple comparison.

A compression test is performed with the throttle wide open.  If the throttle was left closed, the results will be lower than expected.



Lawrenceville, GA

Thanks for sharing the MotorTrend article. It states a 9:1 compression engine should develop about 9 x 14.7= 132.3 psi (at seal level). Factory built '54 317ci's had a 8:1 compression ratio therefore using MT's math should develop about 8 x 14.7 = 117.6 psi which is closer to my values (100-112 psi) and lower than the book's 135 +- 10 psi spec. Curious difference.

I do suspect my engine was relatively "cold" when I performed the test. I had warmed it up for about 10 minutes before beginning the chore of extracting the plugs. Pulling all (8) takes 45 minutes thanks to interplay of the frame, cast iron exhausts, inner fenders, oil filter housing, power steering pump, big hands, smallish and stiff uncooperative tools, weak magnetic hanging lights, etc. Felt like hell froze over by the time #8 fell out of the socket hitting my forehead and solidifying an adamant belief that "hot" engines must bump-up cold values by at least 5-10 psi!

So maybe my compression results aren't too bad after all...? Does anyone else have similar "Y" engine cranking compression numbers you can share? 

I don't readily have access to a leak-down tester. I'm toying around the idea of buying one but not before removing the exhaust pipe. I awoke this morning to a forehead swelling with the conviction that a restricted exhaust could contribute to low vacuum and lack of engine acceleration beyond 3,000 rpm. Backing off some lash on the valves is second on the list followed by removing the intake manifold. Further down the list is pulling the timing chain cover and inspecting for a worn chain.

More to come....

Compression Test
The compression test works by spinning the engine through its cycles with the compression tester in place of the spark plug in the cylinder. As the engine makes pressure, the gauge will move and hold to the highest point read by the gauge. Take measurements for all cylinders and then compare the readings to each other. A healthy engine will have a range of no more than 10 percent variance between the readings.Hot vs. Cold test: An engine that has been brought up to temperature will be better sealed and give a more accurate reading for the normal temperature range of the engine; however, a cold engine will give you the worst-case scenario because the metal has not expanded. Also, engine compression readings can vary depending on altitude and other factors, so it is more important to compare the cylinders to each other rather than the assumed compression number. If you find one or more cylinders are below the 10 percent range of the others, squirt a little oil down the cylinders and try again. If the pressure comes up significantly, the problem may be worn piston rings. If the pressure does not come up on retest, the issue may be with the valves or the head gaskets.

(or late valve and/or ignition timing).

Leak-down Test

The leak-down test measures how well the cylinder is sealing. This is done by inserting compressed air (around 100 psi) into the cylinder at TDC and then measuring how much is lost from the seals (there will always be some). This is done by two gauges on the leak-down tester.

SOURCE - https://www.motortrend.com/how-to/engine-diagnosis-leakdown-compression-test/

A restrictive exhaust should have shown on the vacuum gauge test.

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Back in the garage tonight after Christmas holidays and finally completed the Cranking Compression test on a warm engine.

Results:

Cyl1---100 psi
Cyl 2 ---105 psi
Cyl 3 ---107 psi
Cyl 4 --102 psi
Cyl 5 ---112 psi
Cyl 6 --106 psi
Cyl 7 --100 psi
Cyl 8 ---104 psi

The good news is a fairly tight range of 100 -112 psi but the Lincoln Shop manual calls for 135psi +- 10. "Y"ikes!

What do you guys think? Are these below spec cranking compression values the cause of the low vacuum at idle (14-15" vs factory spec of 18-21")? The numbers seem to indicate the rings weren't installed correctly or didn't seat well.

Tomorrow I'm going to pull the intake and check/pray for a bad gasket. 

One of the other things I noticed while under the car is that the single exhaust muffler (followed by two old resonators) is very old and rusty. So I'm also disconnecting the exhaust tomorrow morning on the Hail Mary chance that the exhaust is restricted. 

"Secondary throttle plates that are too far open, or poor alignment/seating of any of the throttle plates in the carburetor could cause some of your idle-related symptoms."

If the secondaries are indeed adjusted in this manner, it was trying to hide something,

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LOAD-O-MATIC THEORY - http://m571.com/yblock/loadomatic.htm

Back in the day, the procedure was to remove the DIST and set it up on a SUN MACHINE w/ MANOMETER. I would think it was rarely done as if the TECH stuck in a fresh set of points and condenser (flat-rate) and it ran well around the block she was good to go.

If once re-installed and there was a performance problem, you would go to the vacuum supply side diagnostics.

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