ouch . Maybe some winter metal damage pics in a few months ? Sounding like some bad ring mojo with 20% and fouled plug

Will find out in the spring:drink:

might have a bad #4 valve seal, leaking oil in and fouling that one cylinder.

starter strength, cam, battery strength, all play a role in compression test #'s.

Can you post pictures of all the plugs?

How many revolutions are you turning the motors over for your compression test?

That's why I posted the Mercruiser Service Bulletin in a past post - per what OEM Mercruiser lists and how they want a compression test done on their marine engines.

1. Is mercruiser wants the engine at room temp not warmed up.

2. They want at least 100 psi in all cylinders,

3. They want any min. compression test # in any one cyclinder should not be less than 70 % of the highest recorded cylinder compression tested #.

4. Also they want 4 compression strokes min to record the final compression #'s.

I was trying to help the OP out with OEM info but I guess not many read it. Here is the SB and it still holds current today as if things would change Mercruiser will send out a revision of the SB like they did in 1999 to replace the 1997 SB to all Merc dealers and class attendees. Link below of the actual Merc SB and they use it for training as well.

www.boatfix.com/merc/bullet/97/97_25.PDF

I tried to do each cylinder five turns new battery, only one was fouled all the rest looked good. But the boat is put away for the winter, tommy if you now what that is:drink:

Sail, that is what I am hoping for, but we will see.

One spark plug fouled - was that fuel fouled or oil fouled ? Also fuel foul and or a lot of excess fuel entering in the cylinder can cause wash down of the cylinder - losing possible ring seal hence lower compression # and high % of leak down test # as air passing by the rings. And of course oil fouled spark plug(s) worn - or scored cylinder walls and or worn rings. Or worn valve guides.

Bup yes I did see what you posted and thank you, the only difference was I did warm the engine a little I did follow the rest.

My compression on the cylinder that fouled was 125-130

And the rest of the #s are ?

All numbers were between 120-130. I also know that my intake had a little water coming out the front, no water I'm the oil.