Could Running Too Lean Cause Blown Head Gasket?

ERock82

Petty Officer 1st Class
Joined
Aug 14, 2014
Messages
230
I blew a head gasket on my bottom cylinder. I have a 1995 Force 40 hp 2 stroke outboard.

I checked the compression right before I left for the boat ramp. It was 140 on each cylinder. I launched the boat and docked it for the purpose of adjusting my idle mixture on the carb. I turned it all the way lean to all the way rich while in idle just trying to get a feel for things. I am new to boating and had it 1 and 1/4 turn out from full lean. I started her up and then revved it hard to check how things were running. I noticed the water stream was going full to weak intermittently and then eventually spitting. Water started leaking into the cowling (Was coming out gasket) I went home and removed the head gasket and it was blown. The rings and cylinders were all good but a ton of water got in the lower cylinder. My only thought is this happened from revving while set at too lean a mixture. Compression was 85 when I took it home. Still 140 on top. Any thoughts please advise. Thanks.
 

Jiggz

Captain
Joined
Oct 23, 2009
Messages
3,817
I don't believe having it too lean will cause a blown gasket. Instead, it was just a coincidence that the head was not properly torque to start with or worse yet with the new gasket was not re-torque'd after its first full WOT run. Blowing off head gasket is usually caused by improperly torque process or if it was a new gasket was not re-torqued after a full WOT run.
 

MickLovin

Master Chief Petty Officer
Joined
Feb 18, 2013
Messages
822
At 1 1/4 I doubt that is too lean unless your carby was blocked and the motor not running correctly on that cylinder. I would also be checking your exhaust gasket on the side, your impeller for any damage and your thermostat. Don't just replace the head gasket without do some investigation into why it blew in the first place. It may have a blockage somewhere in the water cooling system. Jiggz knows his boating mechanics so check your torque settings as well as sequence of tightening.
Let us know how you go with the repair.
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,923
It probably went at the same time just coincidence!!
I run in salt water and every few years they go bad.
It shows no amount of flushing gets it all.
 
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