'95 120hp compression test results

[Dave K.]

I've run this motor for 10 years. Looking for input on these numbers:
Cylinder 1, 2, 3/ 110 to 115
Cylinder 4/ 95
Appreciate any feedback. Ty, Dave

 

[racerone]

Take the cylinder head off.----Blown head gasket or scored piston.

 

[puffitu]

When’s the last time you’ve replaced head gasket?

 

[Dave K.]

When’s the last time you’ve replaced head gasket?

I haven't changed it and dont believe the original owner did either. Is this a tough job? I can turn a wrench but haven't done anything like this

 

[racerone]

The head will be warped.---Refinish it if installing a new head gasket.-----Just a few bolts , nothing special.----Easy job on a fresh water motor !!

 

[Dave K.]

The head will be warped.---Refinish it if installing a new head gasket.-----Just a few bolts , nothing special.----Easy job on a fresh water motor !!

Thank you, 2 questions: does each cylinder have a separate head or does the head cover all 4 cylinders? Do I find and auto shop capable of refinishing the head?
Dave

 

[Dave K.]

Thank you, 2 questions: does each cylinder have a separate head or does the head cover all 4 cylinders? Do I find and auto shop capable of refinishing the head?
Dave

I'll upload a picture of the motor this morning

 

[racerone]

Just one cylinder head.----Easy to refinish yourself if you have a flat surface.----Like a piece of hefty glass and valve grinding compound.-----A bit of elbow grease but good results with minimum material removed.

 

[Dave K.]

I'll upload a pictu

Just one cylinder head.----Easy to refinish yourself if you have a flat surface.----Like a piece of hefty glass and valve grinding compound.-----A bit of elbow grease but good results with minimum material removed.

Big help and really appreciated!

 

[jerryjerry05]

First!!! those numbers across the board are low.
With a good gauge they should be 145-155 maybe even higher, even a new gauge(Harbor Freight Tools) can give bad results.
I think they started in 95 the 120 came from the factory with the top and bottom cyl. lower by 10# than the 2 middle cyl.
Before you take it apart, get a different gauge and d another test.
Then if there's a big difference?? I'd start with a De-carb, Use OMC's Engine Tuner or Mercury's Power Tune, Yamaha makes a de-carb but I don't know the name.
Then if that doesn't bring up the numbers I'd get a small cell phone camera( $11 Ebay)
Then check out the cylinders.
The fuel pump is mounted on #3 port cover. You can remove the covers (4) and inspect the sides of the pistons.
Then I'd pull the head and look.
If the motors ran in salt water it can be tough getting some of the bolts loose.
Do what is suggested and post the results.
As far as the head being warped??? I've never seen it happen?

 

[racerone]

Every outboard I have worked on has had a warp in the cylinder head !!!----Some more than others.----That is the nature of an aluminum casting over time and heating / cooling cycles.---So there.

 

[puffitu]

I had 14 bolts break on me and they are not hard to snap!because I was impatient ! It may be a record. It was a salt water motor though. Use patience and care along with some heat and penetrating oil. It’s not a hard job. Tapping on each bolt doesn’t hurt as you go either-good luck

 

[jerryjerry05]

14 just might be a record??
The exhaust cover wit all the 1/4-20 screws is were I break the most.
That's why I NEVER remove the cover unless it's a complete rebuild.
The heat and the cool down and then heat again tends to eat away
the edges of the aluminum and then the gaskets don't want to seal.
I have a shop that does my boring and he resurfaces the exhaust cover and the block surface.
If you don't then the amount of sealer is doubled and maybe tripled and it still leaks.

 

[puffitu]

I used a mirror edge to check the mating surfaces. Fine sand paper and sealer; lots of cursing at first but turned into a worthwhile learning experience lol

 

[Dave K.]

Question: had the boat out today. It started hard after I ran it for a while and it was warmed up. I dont run the boat hard never have but could this be a sign of a bad head gasket?
Thx Dave

 

[puffitu]

Not necessarily. But…if it were me; I’d check/replace head gasket just to rule that out( they’re relatively cheap and available and not that difficult of job) then as you have the cover off you can visually inspect each cylinder to some degree. Check for gauges and scores turning the motor as it naturally turns. Put the new gasket in place and use correct tourque and sequence then check compression-move on from there. Obviously from reading through your first post your numbers are a bit off; not horrible but enough to say that something is going on. My 2 cents.

 

[Redbarron%%]

There is always the risk of broken head bolts, especially on a salt water motor.
At some point they went with stainless bolts, which when exposed to salt water makes the aluminum corrosion bloom and then you have a problem.

 

[jerryjerry05]

When did they go with stainless bolts?
I've not seen them?
If it started at all I'd say the gasket should be ok???
You do a compression test?
Did it backfire or cough while starting? could be a blown port cover
gasket or 2.
1 backfire can blow the gaskets and hard starting and poor running, idle will result.

 

[Dave K.]

When did they go with stainless bolts?
I've not seen them?
If it started at all I'd say the gasket should be ok???
You do a compression test?
Did it backfire or cough while starting? could be a blown port cover
gasket or 2.
1 backfire can blow the gaskets and hard starting and poor running, idle will result.

 

[Dave K.]

I have to redo the compression test with the throttle wide open. No backfire or coughing. Idle is a little rough. I adjusted the air intake on both carbs, seated then one and a quarter turn out but didn't see a lot of difference. I'll get back with new numbers and a spark plug check for how they're burning.
One more thing I plan to get the OEM dicarb and see if that helps. I think it will.
Thanks Dave