I think My 1969 Ford Galaxie 500 has bad piston rings. Just tell me If I have the right idea. I started the car, let it heat up, checked its idle as listed below. Then I would unplug one spark plug wire at a time and replaced it after checking the rpm drop. I figured, the furthest RPM drops, meant that cylinders piston rings ere in the best shape. In this case it was Cylinder 4. Am I correct in all this? Thanks for any help!

Idle 850 RPM
Without Cylinder 8 780 RPM
Without Cylinder 7 780 RPM
Without Cylinder 6 740 RPM
Without Cylinder 5 740 RPM
Without Cylinder 4 730 RPM
Without Cylinder 3 770 RPM
Without Cylinder 2 760 RPM
Without Cylinder 1 760 RPM

 

I recall doing a test similar to this referred to as a power balance test and it was used to pinpoint a cyl that wasn't performing up to par . Maybe I missed something there.
I believe the better check would be a true compression test of each cyl folllowed by a cyl leakdown test to confirm which area is responsible for possible questionable compression like valves or piston rings. Depending on the % leakdown and if the pressure is going out through the intake or exhaust valves or worn/poor sealing piston rings. Vacuum gauge readings told a lot too on those earlier engines.

 

Well here is something that I noticed. I used a small pressure gauge that only goes up to about 25 psi, and hooked it into the vacuum hose coming from the carburetor to the distributor. I hooked it up where the distributor should be and I only got a reading of maybe .25 psi. The only way the pressure went up is if I increase the idle speed! Shouldnt it be reading at least a few psi?

Also how do I do a cylinder leakdown test? I know how to do the compression test but whats the leakdown test? Thanks for the help! Feel free to check out my other thread right below this one if you would like!

 

The port on the carburetor that is connected to the distributor is usually located just above the throttle plate when in idle position, therefore very little vacuum reading until the throttle blade is moved to expose the port to engine vacuum then the vacuum causes the dist advance diaphram to advance the timing of the spark.
If you can try a google search for an explanation of a leakdown test. It requires a rather lengthy explanation and would save both of us some time.
What it does is; puts a controlled amount of air pressure into each cyl at TDC and where you hear air escaping is the location of the leak like in the intake for an intake valve, at the exhaust pipe for an exhaust valve and at the PCV opening on a valve cover for air escaping into the crankcase past the rings.