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Hello all,
My MSD ignition box has started acting up so I decided to buy a new one and replace it. Before changing the box, I ran a compression check and put in new plugs. This is the result:

1 - 140
2 - 140
3 - 140
4 - 140
5 - 140
6 - 140
7 - 140
8 - 115

All of the 140 were in the range of 137 to 141. Really steady.

I rechecked cylinder #8 four times. I put a bit of oil in the cylinder and the compression jumped to 160.

I am thinking possible a head gasket is bad. There is no water in the oil, but when I started her up with the oil still in the cylinder a puff of blue came from the back of the engine on that side.

I have not pulled the head to check it out. That's not an afternoon job for a Saturday afternoon.

The engine is running pretty smooth, starting is good, revs up to 7K easily and smoothly. I have the 7K chip in the MSD box.

So, any ideas on what the problem could be? Is this a problem?

Also, engine specs:
Original 351c rebuilt 3 years ago. About 15k miles on it.
flat top pistons, 169 cam, stock 4V heads, hooker headers, Edelbrock intake and 750cfm carb.
 

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Usually a big jump in compression when oil is added indicates worn out piston rings. Sorry. The blue puff was probably just the oil you added burning off.
 

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Slight possibility that valves on #8 may have carbon buildup on them not allowing to seal properly. Though doubtful, I'd run some Seafoam thru carb and give it a shot before I tore into the engine.
 

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Adding a squirt of oil momentarily seals the piston rings. A significant increase in compression indicates worn piston rings. This is what you have identified. A blown head gasket or brunt valve would not show any increase in compression with the oil added.
 
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