Can a bad gasket cause low compression on bottom cylinder?

[OCboat]

My 1968 Merc 650 has the following compression on the cylinders from top to bottom, 120 , 125 , 120 , 105. Could there be extra air getting into the bottom cylinder from a bad gasket between the powerhead and exhaust plate? There does not appear to be any water in that cylinder. Thanks for any help.

 

[Droll]

Re: Can a bad gasket cause low compression on bottom cylinder?

No, a bad gasket would not cause lower comp. but a bad lower seal ( crankshaft seal ) could cause lower crankcase pressure, and that could cause bit lower cylinder compression.

But you should try to decarb the engine before starting to dismantle the engine.

Arne Kjetil

 

[OCboat]

Re: Can a bad gasket cause low compression on bottom cylinder?

Thanks. I have decarbed the engine using a can of Seafoam in 3/4 of a gallon of gas as described on this site but will pick up some Mercury engine tune and spray directly into the carbs to see if that helps.