Compression testing

I am trying to do a compression check on a 304 that I have and are getting low readings on some cylinders and I don't understand it. I did a compression test months ago and all were within specs and the motor basically just sat there in my barn. I have the intake manifold on, but just  bolted to the motor with no gasket/valley pan gasket because I am changing intake manifolds. Do you think the readings would be off because of the intake manifold not being seated/bolted down with gasket on it giving it bad readings? I wouldn't think so. I would assume for compression testing the only places air could escape would be the valves, rings or spark plug holes. I am I correct?

Thanks!

Boris, 

Thanks for replying! The intake manifold is just laying there on a piece of cardboard, for temporary purposes, with bolts in , but with no valley pan gasket at all. So, opening the carb doesn't mean anything, I wouldn't think?

I will re-edit the post to be more clear.

6768rogues wrote: Was it run before the previous test?  Perhaps an oil film on the cylinder walls seeped away leaving them dry and not sealing as well.

That's what I was thinking. Try putting a little bit of oil in the cylinders and rolling the engine over a few times and see what you get. Also if the carb is still on the intake, open up the throttle plates even though the intake is just setting on there.

No, I pulled it out from the vehicle after I did the first compression check to change the top end. This is why I did the initial compression check to see if it was good or not to replace the cam and other accessories. It was within specs, but I can’t find the paperwork yet. The motor sat for 5 months in a air tight plastic crate in the barn because of the wife going into the hospital to deal with Cancer. I bet it the oil did deep out of the cylinder walls. I never thought of that!