Just did a compression test

['31 Patina]

I have a '31 coupe with a stock engine. I just did a compression test and here are my results...

#1 = 55
#2 = 55
#3 = 48
#4 = 55

Can someone tell me if these are bad results? I've been searching other threads on compression testing and its seems 60 is the normal?

Should I be concerned about #3 only being 48?

Thanks for the help!

[Special Coupe Frank]

Most sources state that a fresh, stock Model A engine should pull 65 psi on compression test ( engine warmed-up, all spark-plugs out, throttle tied wide-open ).

There should be no more than 7 pounds variance between all readings.

The consistency between cylinders is more important than over-all pressure.

Sounds like you engine might be getting a little tired, but should still be usable ? Certainly doesn't sound horrible.

Did you try shooting some oil in each cylinder and re-checking ?

A couple squirts of engien oil through the spark-plug hole, towards the driver's side fo the car will seal-up leaky rings for a few revolutions... if the compression reading with oil comes-up over the "dry" reading, that indicates worn rings.

If the compression does not improve after the addition of the oil, that suggests leaking valves and /or perhaps leaking head gasket / crack in block or head.

How does the car run ? Does it smoke / use a lot of oil ?

['31 Patina]

Originally Posted by Special Coupe Frank

Most sources state that a fresh, stock Model A engine should pull 65 psi on compression test ( engine warmed-up, all spark-plugs out, throttle tied wide-open ).

There should be no more than 7 pounds variance between all readings.

The consistency between cylinders is more important than over-all pressure.

Sounds like you engine might be getting a little tired, but should still be usable ? Certainly doesn't sound horrible.

Did you try shooting some oil in each cylinder and re-checking ?

A couple squirts of engien oil through the spark-plug hole, towards the driver's side fo the car will seal-up leaky rings for a few revolutions... if the compression reading with oil comes-up over the "dry" reading, that indicates worn rings.

If the compression does not improve after the addition of the oil, that suggests leaking valves and /or perhaps leaking head gasket / crack in block or head.

How does the car run ? Does it smoke / use a lot of oil ?

Thanks for the input Frank.

I did the compression test when the motor was cold and not hot. Does that make a difference?

I didn't try squirting any engine oil into each cylinder but I'll give that a try. I just installed a new head gasket so that should be good.

The car doesn't smoke and doesn't use a lot of oil. I'm trying to find the answer as to why the engine idles fast, runs hot and runs lean. I had the carburetor and distributor rebuilt and timing is right on but for some reason I can't get it to idle down. I can turn the idle adjustment screw on the carb all the way in or out and there's no difference in idle. So I'm trying to figure out if it's the carb that's my problem or maybe a valve.

Another question I have, which might be obvious so I apologize if it is, but I have the carb off the engine right now. Would having the carb off affect the results of the compression test?

Thanks for the help.

[Pilotdave]

Carb on or off should have no affect on the compression test as you are measuring compression which occurs with both valves closed. If the carb were on the engine, you'd want the throttle wide open so that air flow reaches the intake valve.

[Jim/GA]

Originally Posted by Pilotdave

Carb on or off should have no affect on the compression test as you are measuring compression which occurs with both valves closed. If the carb were on the engine, you'd want the throttle wide open so that air flow reaches the intake valve.

Everything you say is true, but you really want the engine to be close to normal operating temperature when doing the compression test, which requires running it, which requires a carb.

The symptoms you describe could be caused by the slightly low compression in one cylinder if you have a leaking head gasket or a cracked head or block. Do a block test where you sniff for combustion gasses in your coolant while the engine is running. Easy to do and quite definitive.

[Kurt in NJ]

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Look for a vacuum leak---manifold gasket--worn throttle shaft--wiper pipe???

If you have the carb off the engine hold it up to light with the idle speed screw backed off, it is possible that the throttle butterfly valve is not centered ( light on 1 side, touching on other), or worn (edges of butterfly next to throttle shaft)

To center loosen the 2 small screws on the throttle shaft, tap the butterfly some with a screwdriver as you push the throttle arm toward the closed position --while holding the lever closed tighten the screws ---hold it up to light ---if perfect almost no light will be seen anywhere.

If the plugs have been in a long time without being disturbed removing them can cause pieces of carbon to fall off ---these pieces can get caught in a valve and cause lower compression readings ---do another test --no oil ---there seems to be much worry about conpression ---I only have 55 ---my friend has 60 --it could just be a different compression gauge , I have seen much variation between gauges --as much as 20-30 lbs.

I have all the tools, but I have never checked my compression with a gauge ---but I can get a goood idea by how it cranks with the ignition off, or by how it pulls with light throttle from 7 mph in 3rd ---each power stroke can be felt ---if each feels it is making the same effort your compression is good --or at least all even.

With the hand crank (ignition off) and another person (to listen at tail pipe, or intake, or breather) ---a hiss out the intake on the same cyl that cranks over easier than the others --intake valve not seating ---more noise in the breather --time to question the rings/piston/cyl etc.---no tools needed.

[Mike V. Florida]

Originally Posted by '31 Patina

I have a '31 coupe with a stock engine. I just did a compression test and here are my results...

#1 = 55
#2 = 55
#3 = 48
#4 = 55

Can someone tell me if these are bad results? I've been searching other threads on compression testing and its seems 60 is the normal?

Should I be concerned about #3 only being 48?

Thanks for the help!

Something not running right? Why did you do the compression test?

[blownaway]

Originally Posted by '31 Patina

.

. I'm trying to find the answer as to why the engine idles fast, runs hot and runs lean. I had the carburetor and distributor rebuilt and timing is right on but for some reason I can't get it to idle down. I can turn the idle adjustment screw on the carb all the way in or out and there's no difference in idle..

Sounds like a vacuum leak, could be the answer to it running lean and lean means hot right?
Check intake gaskets, wiper tube as said and stuff like that. Worn throttleshaft could mean vacuum leak but the throttleplate could have the engine idle fast but wouldn´t make it run lean...
But i guess the heat could be bad cooling as well. My engine was all clogged up of rust in the water jacket wich made it bubble of steam in the radiator even if the water was about 50celsius...
Good luck!

OT:
Hi! I´m Olov from Sweden, my first post here.
I´m planning to build a banger for my -30 Coupe De Luxe and hope to be able to trade some info.
Regards!

['31 Patina]

Would there be reason to believe that there could be a valve thats not opening or closing all the way causing the lower compression of 48 psi in cylinder #3? Should I re-seat the valves?

I did a compression test because I was curious what kind of compression the engine has. I also did it to hopefully eliminate what could be causing the engine to idle fast and run lean. I'll check for vacuum leaks but when it comes to the wiper tube where should I be looking for leaks? I have a shut off valve where the wiper tube goes into the manifold. I have the valve screwed all the way to the right which I would assume is closed and no leak. Is there somewhere else I should look when it comes to the wiper tube? I recently installed a new exhaust manifold so that should be okay but I'll double check.

[Special Coupe Frank]

I agree with the others who are suspecting a vacuum leak.

The low compression on #3 could be a leaking valve, worn or stuck rings, or perhaps a leaking head gasket. A cylinder leak-down test would pin-point this.

A few months ago, I was going-over my car, and found the manifold-to-block nuts were loose - at least 1/2 turn on each one. I tightened them down, and the idle slowed-down, and the car ran perceptibly better.

You can use WD-40 with the little straw or Oil from a squirt-can to check for vacuum-leaks along mating flanges / gaskets: WD-40 will get sucked-in and cause the engine to speed-up a little for a moment, oil will seal smaller leaks for a moment, causing the engine to slow a little, and probably making a slight "slurping" sound as the oil gets sucked-into the leak.

I would start with disconnecting the wiper tube and valve and putting a pipe-plug in the manifold tapping, then making sure you do not have nay vacuum leaks between the intake and the block and between the carb and the intake.

Once you have established that the carb and manifold are leak-free, you can re-install / reconnect the vacuum wiper plumbing one piece at a time, and chekc that for leaks.

The wiper plumbing includes a steel tube that runs-up the door pillar, across the windshield header, and out through through the steel header facia, with a short rubber "jumper" hose to connect to the wiper motor. This steel tube frequently rust-out, particularly if the car has weathered / roof has rotted.


Compression tests are best done after the engine has been at at operating temperature for 15-20 minutes, when things have expanded and are at their "running" fit...