Low compression on cylinder 2 & 3

A

A Model Man

Junior Member
I have about 5000 trouble free miles on a rebuilt standard Model A engine. Recently on tour the engine lost power but continued to run. Compression on warm engine is near 70# on front and rear cylinders but less than 50# on the two in the middle. Removed head, the gasket did not appear to be blown and valves were not stuck and appeared to be operating properly. Could not see any cracks in the head or block. Checked flatness of head and block with a straight edge and feeler guage and they do not appear warped. Filled headless engine block with coolant and there is no leak down of coolant. Installed new copper head gasket and torqued head appropriately to 55 ft/lbs. Rechecked compression and there was no improvement. I am puzzled. Anyone have a similar experience that might give me a next step to pursue?
 
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Did you lap and adjust the valves while the head was off? You could make a fitting and add air under pressure to a faulty cylinder and see if you can hear it escaping out the carb, other cyl., or exhaust pipe.
 
If you try the compressed air approach, don't forget to check if air is escaping through the crankcase vent pipe (oil pipe). It could be a bad ring causing the low compression.
 
J Franklin said:
Did you lap and adjust the valves while the head was off? You could make a fitting and add air under pressure to a faulty cylinder and see if you can hear it escaping out the carb, other cyl., or exhaust pipe.
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Lapping the valves may be my next step. I thought it unlikely to have sudden valve problems on adjoining cylinders simultaneously but I guess it could happen. They appear to be closing properly but appearances can be deceiving.
 
I’m thinking it’s in the head or block, possibly causing a gasket problem. Before tearing it apart again, do a leak down test. Pretty sure you’ll find a problem common to those two cylinders. Be sure and have all the plugs removed.
 
A Model Man said:
Lapping the valves may be my next step.
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I wouldn't try any fix until I knew what the problem is. You will only confuse the issue.

See post #2 and #3 for the initial diagnosis. If it comes up that air is leaking past the valves, I would adjust them first, then test again.
 
I totally agree with the post#7. Spending$$$ on parts and labor before exhausting available diagnostic procedures is asking for a ticket to do it again. Cylinder leakage test also known as leak down test has been around before the model A. If your new to this, OK good, it will teach you how to visualize the compression elements better than anything else. Do the searches and make the tool. You'll never regret it.
 
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