JR3 engine diagnosis help please

[1arunem]

Ok, per my other post regarding an engine swap, I pulled a supposed bad JR3 engine out of a G16 and swapped in a known good JN6.

When I began, the JR3 would barely turn over with the starter.. Very slow. Well after installing the JN6, the starter wouldn't even turn it.. I swapped starters and all is well with it. Makes me wonder if anything was really all that bad with the JR3 to begin with, however...

Couple things of note, the JR3 did have a bit of oil accumulation in the air filter housing.. Seemed to turn over by hand a little easier than the JN6 leading me to think maybe low(er) compression.

I can hear compression coming out of the spark plug hole when I turn it by hand but it doesn't come close to blowing my finger off the hole.

Long story short, any first things first to check before I go to tearing into it?

Thanks!