Compression on 1979 Chevy 305

[New_Blue]

Hi all,

Novice here needing some advice(assume I know nothing). I recently purchased a 1979 Sea Ray with a 305 Chevy. I did a compression check to see what I had for an engine today. On the first three cylinders I got readings 65-75 then on the others I got 90 to 110. For grins I went back to the first cylinder and was surprised to get a reading of 90.

Is the fact that I am getting a higher reading on the first cylinder because the motor has had a chance to get some oil on the cylinder walls? My motor won't currently idle(I think it has a tank of old gas). Also, I thought the compression readings should be up near 150. Am I looking at a new motor?

Thank you in advance for the help!

 

[Don S]

Re: Compression on 1979 Chevy 305

You must have a fully charged battery to crank the engine over at the proper speed or the test is worthless. If the engine has been setting for a few years, the test may mean nothing till you run it first and get the surface rust on the cylinder walls knocked off and the valves moving again.
With the numbers you show, even after the second try, you have a junk engine.

Here's a link to how to do a proper compression test.
http://www.boatfix.com/merc/Bullet/97/97_25.pdf

It will mean more if you can start the engine and run it for even 10 ro 15 seconds.

 

[New_Blue]

Re: Compression on 1979 Chevy 305

Thank you Don! As with many things in life there seems to be a little more to it than one would think....now to drain the tank and hope that it's just stale gas that keeps it from running...:redface:

 

[JustJason]

Re: Compression on 1979 Chevy 305

1 more thing I would do before I scrap an engine would be to test again with a different brand compression tester.