Mercury 40 HP 4 Cylinder 2-Stroke Compression Question

[moxieman]

Looking at buying a 16' Lund with a Mercury 40 HP, 2-stroke motor. This engine has the 4-cylinders, and appears to be a '91. Engine hasn't been run in two years, and when I ran a compression test, I got the following results: 120, 120, 105, 120. I wanted to get some input regarding those numbers. Some people have told me that it could be just a stuck ring, and will be fine after it is run. Any other input is much appreciated.

Thanks, Pete

 

[ondarvr]

Re: Mercury 40 HP 4 Cylinder 2-Stroke Compression Question

Run it for a while and then check it again.

 

[achris]

Re: Mercury 40 HP 4 Cylinder 2-Stroke Compression Question

If they are readings after not being run for 2 years, I'm impressed.

Chris....

 

[bilge rat jim]

Re: Mercury 40 HP 4 Cylinder 2-Stroke Compression Question

That's good numbers. #3 is a bit low, go to a Yamaha powersports dealer and get a jug of Ring-Free+, put a couple of ounces or so in a spray bottle, remove the plugs, and spray liberally in each bore. Crank the engine for a couple of seconds, wait overnight, and test again. Use the remainder of the Ring-Free+ in the fuel, as recommended on the label.

I've been a Merc guy my whole life, but this product from Yamaha is the best de-carboning agent on the market.

 

[Kinger42]

could someone please tell me what the compression should be on a 2002 Mercury 40hp 2 stroke 2 cylinder outboard?

 

[achris]

It's best to start your own thread for new questions...

The naswer to your questions is about 100-120psi, and both cylinders must be within 10% of each other.