77 35hp compression loss

[77 johnson 35]

when I do a compression test it goes up to 80 or so while im turning it over and then suddenly goes to zero when i let off the key?
there seems to be a bit of gas and oil in the cooling water discharge,
is the headgasket serviceable without removing the power head, it looks so
but the book seems to suggest taking the whole thing apart

[77 johnson 35]

so whats my next move?
any advice

[boats.net]

That would be your compression gauge, check valve in gauge is leaking.

[77 johnson 35]

thanks alot , i was just getting ready to tear it apart you saved me, for now

[77 johnson 35]

it has been setting 12 years , only starts for a short while if i prime it , its not the carb i have even tried a second carb, could it be a leaf valve?

[boats.net]

Sounds like it's not getting gas which is usually a plugged up carb.

[77 johnson 35]

you were right about the compression gauge , i rented a new one and it read 125 on top and 115 on bottom cyl.
go a rebuild kit for the carb today will tie into that next