Compression test question

5

54mainline

Junior Member
I picked up a 54 Mainline with a 239 Yblock last year. Everything but the car’s interior is completely stock. I’ve been having trouble starting it when the engine is warm so I decided to start poking around under the hood thinking it needed a tune up.

I replaced the (filthy) spark plugs and leaned out the carburetor since the plugs were black from the engine running too rich. I did a vacuum test so I could set finish adjusting the fuel mixture, idle, and timing. The needle on the manifold pressure gauge was shaking violently between 18 and 20psi. That scared me so I did a compression test. Here are the results:

Cylinder 1: 98 psi
2: 48 psi
3: 90 psi
4: 80 psi
5: 97 psi
6: 90 psi
7: 82 psi
8: 81 psi

At first, I thought cylinder 2 was my only problem since it’s so much less than the others until I looked up the specs on this engine and found out that the compression should be 130 psi so I’m way low on all 8 cylinders.

Before I launch into a huge job that I’m not mentally or financially prepared for, does anyone have any advice where to start? What could be causing the loss in compression and how would you go about trying to fix it?
 
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I suggest you remove the valve covers and ensure that your rocker arms are all adjusted correctly. Who knows? you may find the issue with cylinder #2 that way.

Your overall numbers are of course low, but that is just indicative of the wear and tear of your 239 over the years. If you can figure out cylinder 2, and if the engine does not burn oil too much blowing it out the tail pipes, I say run it as is until you feel like rebuilding it. If your vacuum gauge is correct then you have decent vacuum for numbers like that and that kind of action on the needle indicates the valve.

On the carb running rich, how did you "lean it out"? Was it just rich at idle or did you have to go down a couple of jet sizes?
 
Thanks for the reply. I figured I had a problem with the valves on #2 given the shaking vacuum and the low compression, but I wasn’t sure if the low compression on the other cylinders pointed to something worse. Now I have to figure out how to adjust the rocket arms. I’ve never done that before.

As for the carb, I just put the mixture screws all the way in and backed them out by a turn as prescribed in the shop manual. The carb looks like it was recently rebuilt and it seems to be working ok so I didn’t want to open it up and screw around with the inside of it.
 
I don’t know much about the history of this car, so it’s impossible for me to know how long it sat. It has relatively low miles (65k) and 70 years old, so it’s probably sat for many years without being driven at some point in its life.
 
The fuel is most likely stale and has turned to varnish. It plugs everything up.

Also consider a fresh oil change to prevent any additional damage.
 
The manual will give you the valve adjustment procedure. If you’re not experienced with feeler gauges, this attachment is the method Walt Knuckles used and wrote up. Probably have to read it 3-4 times, and while you’re doing the first couple. Simple and straight forward once you “get” it. Just make sure you’re on the base circle of the cam.

http://www.vintagethunderbirdclub.n...id=2792e8fa770d07d9214ed3be4b2c3079&mode=view

As K said, replace the fuel. I’ve had modern fuel (ethanol) go bad in as little as 3 months in warm weather. Car wouldn’t idle, loss of the volatiles in the fuel. Drained it and dumped it in my modern EFI cars, ran just fine. Fresh fuel in the carburetor car, problem solved.
 
miker98038 said:
The manual will give you the valve adjustment procedure. If you’re not experienced with feeler gauges, this attachment is the method Walt Knuckles used and wrote up. Probably have to read it 3-4 times, and while you’re doing the first couple. Simple and straight forward once you “get” it. Just make sure you’re on the base circle of the cam.

As K said, replace the fuel. I’ve had modern fuel (ethanol) go bad in as little as 3 months in warm weather. Car wouldn’t idle, loss of the volatiles in the fuel. Drained it and dumped it in my modern EFI cars, ran just fine. Fresh fuel in the carburetor car, problem solved.
Click to expand...
This is really helpful. Thanks
 
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