Force Compression Question:

[Bigprairie1]

Ok guys, I would like to get some quick opinions on the following:
-Force engine 1990/90hp with compression readings as follows:
100 psi/ 60psi/ 100psi (dry test...done quickly by the seller)
This is a local 'parts engine' for sale that is the same as my '90/90hpmotor and I was loosely considering picking it up as a 'spare' for everything else on it (lower unit, coils, flywheel, stator, etc, etc, etc).
My current motor (same '90/90hp) is humming great and always has and to be honest....I have yet another 91'/90hp waiting in the basement...just in case. (what is about outboard motors???)
Anyhow....I remember seeing posts here talking about bringing up the compression in cylinder readings of 60-70 psi...with some serious decarbing...or a new head gasket.
Another question: what kind of compression readings are associated with a broken ring? 10psi? 20psi?....5 psi?
Now, I know nothing else about the motor...but given the dry compression tests he is selling it as a parts motor....fair enough.
So, is there any hope for these compression readings rising with readings like that?
Thanks for the input!!
BP

 

[BuzzStPoint]

Re: Force Compression Question:

That's a lot of compression loss.

Now you have to determine if it's valves, head gasket or piston/rings.
I'm not sure on outboards, (using my automotive background here)
In the affected cylinder, put a few ounces of oil in the spark plug hole and do another test. If the compression reading increased. Then you are dealing with piston rings.

I would do a leak down test. (apply an air compressor to the cylinder). If it's the same as an automotive engine, when the piston is at "top dead center" the combustion chamber should be sealed an you wont hear any air. If you do hear air leakage. Listen to where it's coming from. This should narrow down to where the compression loss is from.

That being said.
Depending on how much the outboard is, and how many good usable parts are on it. You may want to pick it up for spare parts.

 

[moparman]

Re: Force Compression Question:

These motors don't have valves in the sense like a 4 stroke,just reed valves[ will do somewhat the same thing , but will not cause a loss of compression if they leak]. yes a good decarb might bring the compression back up if there is no mechanical damage[burned piston,broke ring,blown head gasket, etc]and it's just stuck rings on that cylinder , but as mentioned before,you need to find out whats causing the low compression[do a leak down test,and that will show where the problem is by the air passing past rings piston or where ever].You might be able to look in the spark plug hole to see if any damage has happened to the piston,or cylinder wall. just my two cents worth

 

[BuzzStPoint]

Re: Force Compression Question:

These motors don't have valves in the sense like a 4 stroke,just reed valves[ will do somewhat the same thing , but will not cause a loss of compression if they leak].

Good to know...

 

[john from md]

Re: Force Compression Question:

Anytime you buy an engine as is, you take a chance. Why worry about your current engine failing? I run engines 25 and 30 years old into the Chesapeake and the Atlantic and don't worry about them coming apart.

Maintain and feed them properly and you don't need a spare.

John

 

[Bigprairie1]

Re: Force Compression Question:

Anytime you buy an engine as is, you take a chance. Why worry about your current engine failing? I run engines 25 and 30 years old into the Chesapeake and the Atlantic and don't worry about them coming apart.

Maintain and feed them properly and you don't need a spare.

John

Yeah, John....I get this too and agree. I have a great faith in my 'ol 90hp. Most of this is rooting around mostly in putzing with these and looking for another hull with a buddy of mine to put one or the other 'spare' on.

Mostly with this motor, I thought if I could get the compression up closer to even it might be a good bet for a cheap runner on another hull.

I will try to get a reasonable thread posted regarding my current motor...which, as usual, is running great...after 20 years. Seeminly defying the Force legend of endless and constant failure (which I think is a bit of bs).
All Good
BP

 

[JB]

Re: Force Compression Question:

That much compression loss on #2 calls for probable rebuild. More bux than it is worth.

 

[jerryjerry05]

Re: Force Compression Question:

It COULD be????? You need to take off the head and check the gasket and piston.It might be a $30 or $300 fix.
You didn't say how much you were gonna pay for the powerhead.If it's real cheap the electronics on it are worth a lot of$$$.Trigger and stator bolt list over $300.J

 

[Bigprairie1]

Re: Force Compression Question:

It COULD be????? You need to take off the head and check the gasket and piston.It might be a $30 or $300 fix.
You didn't say how much you were gonna pay for the powerhead.If it's real cheap the electronics on it are worth a lot of$$$.Trigger and stator bolt list over $300.J

Yeah, Jerry...the guy is asking $250 complete for it. It's mostly a curiousity not a need at this point.
Most of this curiousity is in the 60psi cylinder and what kind of problems lead to that range of low psi numbers.
I remember something posted about broken rings showing lower numbers than that maybe????...perhaps this looks more like a head gasket issue or psi range?
As for the over value vs. the price??? It does have working tilt and a working lower unit.
BP

 

[jerryjerry05]

Re: Force Compression Question:

Just for parts or a back-up(possible easy fix)It's worth that.Sell the parts you don't need or want.J