[Abchoppers]

1956 Mercury 312, around 61,000 miles. I don’t know if engine has ever been rebuilt. I pulled a plug to see how they look and it had oil on the threads. I pulled all of them and all ended up having oil on the threads. That led me to perform a compression test. All but one cylinder had good compression :
1 155
2 150
3 150
4 145
5 150
6 135
7 90
8 145

I did the test on a cold engine. I removed the valve cover on the drivers side after getting the number 7 cylinder reading of 90. All of the rocker arms were moving and there was plenty of oil.

So what can cause one cylinder to be low? I’m sure there are a few things. Does any of this explain the oil on the plug threads?

The car runs great, plenty of power but does have a slightly rough idle sometimes. It has a 57 intake and distributor, a PCV valve, new Holley 4160 carb.
Thanks all,
Ryan