Low compression on recently rebuilt 1990 90hp

[Double Trouble]

I have 2 1990 model 90hp motors which only have done 80hrs since a rebuild, the motors were sitting around unused for a good 8yrs with regular start ups.

The motors ran fine while on the previous boat, now that i have them running on another boat they idle good and rev up freely but under load one was breaking down allot.
I stripped the carbys and rebuilt them only to run into major drahmas now

I done a compression test today on the port motor, The readings were 125psi, 90psi and 70psi pulled the head off and found slight skuff marks in the bottom cylinder, I can still see the hone marks in the bores as well.

Starboard motor had readings of 30psi, 125psi and 125psi, pulled the head off this motor and found the top cylinder has broken the piston ring and it has smashed into the piston and damaged the head slightly.

The motors were running cool on all cylinders, no hot spots were evident, oil injection pumps worked fine, I am yet to fully dissasemble the motors to further investigate the cause.

What may have caused this as they have done very little hours/work since being completely rebuilt by a marine mechanic
Thanks

[99yam40]

Plugging in carbs would be my thoughts, causing too lean of fuel air mix on some cylinders. Lean burn causes the pistons to run too hot

[Double Trouble]

When i started the motor up after rebuilding the carbys it ran fine and idled properly without the choke lever, revved up freely and ran fine for a good 20minutes,
Once i went to re-check it after 30mins it didnt want to idle at all and thats when my problem started with the starboard engine.
The port motor was still running ok just had poor compression and was breaking down under load.
I set the idle air mixture screws as per the manual reccomendations

[BQUICK]

Clogged fuel filter or maybe fuel line issue causing lean condition.

[Double Trouble]

I cleaned the fuel filters during the process, they were both relativily clean prior to dissasembling. I had drained both fuel tanks so i had new fuel in the tanks, new external fuel filters and water seperators fitted also.

[BQUICK]

I see that you say regular startups over 8 years of sitting. If that means starting say once a week for a few minutes that can cause more harm than good.
Was it ever run hard on old gas?

[Double Trouble]

Here is a fw pics of the internals

[Double Trouble]

The motors were started every 3-4mths while the boat was laid up.
I used fresh fuel from an auxilary tank to run the motors and this was also premixed as the fuel was used in my other boat.
The motors had not been used with old stale fuel and hadnt had a good run in years until last week when i actually got to put a load on them, this is when i noticed the port motor breaking down under load.

Took the powerheads off today, will be stripping the internals tomorrow and try and find the exact cause and damage, will post up what we suspect may have caused my greif

[99yam40]

I have found dried up fuel residue (Thin film)floating around in carb bowl from pump or lines on motor hit freshly cleaned carbs plugging one carb jet before, would idle fine just would not take load on motor. It is a pain to pull carbs on the water.
Good luck with the rebuild

[Double Trouble]

The carbys were the first things i stripped and found the exact same problem, allot of carbon build up in the fuel bowls, here is a pic of the worst one

[rodbolt17]

that same nasty residue,its NOT carbon, will build on the rings and piston skirts and tear things up.
it happens when sitting.

[Double Trouble]

The cylinder that the above pic of the carby was from had the best compression from that motor.
We stripped the motors today and believe that the cause was from getting to hot at some stage.
The motors only had done 30hrs not 80hrs as i thought
A few bores had blow by in the cylinders and a few score marks which would of caused the low compression readings.
Some of the rings were glazed to the piston as rodbolt17 had mentioned
The worst cylinder on 30psi had broken rings and a cracked piston as seen above.
The rest of the internals are good.
I am still puzzled on how the engines overheated the second time round as they were only just run in and didnt overheat at all but from investigating the internals that was deffinantly the cause.
I hope this doesnt happen again.

What would be causing the motor to run hot as the water was always cool in the tell tale and the block temperature was cool every time i checked it? it had new water pump kits installed when rebuilt.

Thats boats and life but it must be repaired regardless of the cost.

Thanks for the replys DT

[rodbolt17]

check the water pumps, pay close attention to the housings.
ANY signs of melt means the housing is junk.
we typically see it on stored engines that get started occasionally on a flusher and once in a while the water is forgotten.
takes about 30 seconds of dry run to destroy the housing/impeller even if new.

[rodbolt17]

another typicall cause of low hour ring failure is inccorect port chamfering after a rebore.
port chamfer is absolutely critical.
not enough and the rings grab the sharp edge.
to much and the rings try to ride in and out.
both cause ring breakage.
thats why I have a trusted nachinist do my blocks.
I am not good with a die grinder.

[Double Trouble]

@rodbolt17
The water pumps were dismantled and replaced with new kits prior to the last trip out.
The housings were in good condition but the s/steel cups and plates for the impellors had a few grooves in them so i replaced them with a full water pump rebuilt kit.
As I mentioned the motors were sitting around for quite some time so i just changed them for a peice of mind regardless of thier condition.


Rodbolt17 wrote: another typicall cause of low hour ring failure is inccorect port chamfering after a rebore.
port chamfer is absolutely critical.
not enough and the rings grab the sharp edge.
to much and the rings try to ride in and out.
both cause ring breakage.
thats why I have a trusted nachinist do my blocks.
I am not good with a die grinder.

My current engine builder said exactly the same, You sound like you know these motors inside out