Mercury 150 low compression

[Apummill]

The oil injection pump failed on my 1990 mercury 150 xr4 and now I have low compression on cylinders #1 and #6. All cylinders are 145-150 psi and cylinder 1 is at 115psi and cylinder 6 is at 125 psi. It's hard to start without a long choking time and runs a little rough. Is it most likely the rings where damaged on those two cylinder or could it be something else? Would a hone and re ring be a sufficient repair or is the powerhead done?

 

[ondarvr]

The only way to know is to take it apart and inspect it, but yes, normally there will be some damage if the compression is low.

 

[Dukedog]

if those are good numbers someone has already been in that motor or at least changed/cut tha heads... its very border line on 92/93 octane rating being enough with ANY timing in it at all...normal good comp on it should be 120 + or - a few.... but like said it needs ta be checked closely for a problem with that much difference..

 

[Apummill]

They are good numbers. I'm a diesel equipment mechanic for a living but have very little expertise with 2 strokes much less outboards. I am sure there is some mechanical damage to the cylinders but being in familiar with 2 strokes and knowing they don't have valves I was unsure if there is any other types of damage caused by oil starvation that could result in low compression.

 

[Dukedog]

Doubt its an oil problem. ...at 115 not hurt to bad long as ya don't run it till its fixed... that comp can be a problem to tha unknowing.... can explain later when I get back to tha house. ... this phone sucks.........

 

[Dukedog]

no oil usually takes down tha motor completely.. usually bearings first.. low oil mixture can cause a heat problem in tha cylinders and take out a piston or two... let us know what ya find when ya take it apart... my concern is tha compression.. with that much it needs ta have tha flywheel "indexed" to see what each cly. is at max timing... they will vary some so max needs ta be set by highest cly instead of "just # 1".... 145 ta 150 @ 25* is what we look for for folks wanting tha "most" outta their junk but it absolutely has ta have good premium fuel and a close watchin' after......

JAN.. when ya get it apart take good cly. measurements checkin' for size, outta round, clearance and etc... still havin' 115 and 125 may be a good sign....