Compression test blues!

[kevindahle]

Recently I have been trying to get my Duster back in road worthy condition and while trying to pin point the cause of the irritating stumble in the motor, I finally did a compression test to find the number 1 and 7 cylinders only producing 75 and 90 while the others being from 140-155 in a mostly stock '71 360 I rebuilt years ago. I don't have a leak down tester but did put air to the cylinder by using a connecter used to hold the valves up for service, it is apparently the rings as air escapes through the valve cover breather. It puzzles me why two would go to hell, Broken ring? IDK just needed to vent. However in all the cylinders the compression was higher 15-20psi oiled vs dry.

 

[gtsdude]

Does it have adjustable rockers? Make sure they are adjusted correctly so you are not holding a valve open. If all is ok there, maybe pull the heads off, could be bad valves or head gasket also

 

[67Dart273]

An easy mistake is, with an intake open, air can blow through the PCV into the valve cover. Leakdown must be made with valves closed, so you must either do this at TDC, or else pull the valve gear so you can also do so at BDC Horrid Freight sells a tester for only 40 bucks. You can hardly build one for that.

 

[mech1nxh]

Recently I have been trying to get my Duster back in road worthy condition and while trying to pin point the cause of the irritating stumble in the motor, I finally did a compression test to find the number 1 and 7 cylinders only producing 75 and 90 while the others being from 140-155 in a mostly stock '71 360 I rebuilt years ago. I don't have a leak down tester but did put air to the cylinder by using a connecter used to hold the valves up for service, it is apparently the rings as air escapes through the valve cover breather. It puzzles me why two would go to hell, Broken ring? IDK just needed to vent. However in all the cylinders the compression was higher 15-20psi oiled vs dry.

^^
With Humble indulgences,
Pictures of #1 & #7 spark plugs ?

 

[krazykuda]

Try squirting 2-3 squirts of oil into the cylinders with an oil can before testing each cylinder again. This is called a wet test. If your readings increase, then your rings are the issue. If there is no difference between the "wet" and "dry" compression test, then you most likely have a valve sealing issue.

If you have to pull the pistons out, check the location of the two compression ring gaps. They should be at least 90° apart or more. Also make sure that the two thin oil ring gaps are also 90° or more apart from each other. The ring end gaps should not be close together. Also, make sure that the rings can spin freely by hand.

When you put the pistons back in, make sure that the rings spin freely by hand before installing them. Also make sure that the ring end gaps for the two compression rings and the two thin oil ring gaps are 90° or more apart from each other.