low compression

[alan62]

i have a 1978 johnson 85 hp 3 of the cylinders have good compression 4th has compression at 90 also started a knocking noise thats when i found the compression problem not sure where to start thought maybe a gasket but then theres the knocking any help would be much appreciated

 

[Bob_VT]

Re: low compression

Welcome to iboats

What is the compression of the other 3 cylinders?

 

[alan62]

Re: low compression

119 120 120

 

[Daviet]

Re: low compression

Maybe pull the cylinder head on the low cylinder and see if you can see any damage.

 

[alan62]

Re: low compression

alright i will be able to start that in the morning thank you

 

[alan62]

Re: low compression

is there any thing i should look out for

 

[Daviet]

Re: low compression

Just remove the cylinder head and inspect the piston and cylinder wall.

 

[Bob_VT]

Re: low compression

It is a bit low since they should be within 10% of each other.

Before you pull the head and replace gaskets (i.e. spend any money) I think you should de-carb and continue to run the motor. The knocking does disturb me but, I would suspect it is from a low cylinder.

Just doing re-builds in mid season is not enjoyable.

If you decide to pull the head (or both) I would inspect the water deflectors and probably change them too.

 

[alan62]

Re: low compression

how bad would it be to the motor if i left it alone till winter

 

[jimdd810]

Re: low compression

It sounds as if you got some piston slap going on. do a cylinder leak down test. I would definatley pull the head and take a look. Outboards most of them are rattle traps and vibrate like a son of a gun, but never have i heard one with aknocking noise and have it turn out ok. I usually agree with bobvt, just not on this one. You might have a major problem that is repairable and turn it into a catstrophic failure that is not repairable.

 

[alan62]

Re: low compression

well i have a friend coming over tonight to help disassemble the motor get back with you soon thanks for the help

 

[emdsapmgr]

Re: low compression

Might be just a blown head gasket.

 

[bigpoppakdog]

Re: low compression

that's what mine was this summer. Had a pretty big hole in the gasket.

 

[kenmyfam]

Re: low compression

I'm with giving it a good decarb and checking the compression again.