Is there a way to tell if the compression is good on an outboard motor by just running it in a barrel?

Question

Or, do I need some sort of guage?

Answer 1

Yes, the lower it will idle smoothly, the better the compression.

But if it doesn't idle smoothly, you still won't know if it's the compression or a tune-up component that needs attention without a compression gage.

Answer 2

Just remember that just cause you have good compression in a 2 stroke doesnt mean the motor is good. The rings make compression and the piston skirt makes vacumm. Thats just in a 2-stroke though.  

Answer 3

The safest way is to buy or borrow a compression tester.  They cost less than $20.00.  You perform a compression test by removing all of the spark plugs and then insert the compression tester into each spark plug hole and have someone crank the motor several times.  Read the result and write it on a paper. Release the pressure in the tester and repeat the test for each of the cylinders.  Then check the results that you wrote down.  If any cylinder's compression is 15 lbs or more LOWER than the highest cylinder there is a problem in that cylinder.  Here is a site that gives you more info on compression testing. Although it describes testing a 4 stroke car engine, most of the concepts are the same for a 2 stroke outboard.  Good luck.

http://www.automedia.com/Engine_Compress...