1985 mercury 90hp inline 6

[markodecember]

Hello there,

I was wondering if anyone can help me with this particular engine please. I went to buy a boat with this outboard on the back and done the sensible thing and ran a compression test first.
#1 133
#2 130
#3 000!

I stopped at number 3 and retested several times and it was knackered. The piston was loose inside and completely shot. Wouldn't even give me a reading on the gauge!!. I might still get the boat and replace the block on it because I have read up on these particular ones and I think it is worth doing. My question is what could have possibly caused this to happen as I don't want this to happen to the replacement block.
Any replies would be most gratefull.
Thanks.

 

[emckelvy]

Re: 1985 mercury 90hp inline 6

Burnt-up piston can be caused by overpropping, overheating, too much spark advance, poor quality fuel, use of poor quality oil leading to carbon buildup and ring sticking, lean condition (carb problems etc). Or even mechanical failure.

Best thing to do (if you get it) is to pull the powerhead down for a careful inspection/fault analysis. You may find the reason for the problem is obvious.

If not, addressing all the 'normal' Inline Six maintenance and engine setup routines will likely ensure you don't have another repeat performance of Piston Meltdown following rebuild.

Good News is, even if the block is shot there are a lot of Inline Six parts floating about and that 90 HP, rated at the prop, runs out as strong as an older, crankshaft-rated 115HP.

BTW be sure to use the broken motor as a bargaining tool and for those purposes don't give the motor any more value than it would have as "Just Parts".

HTH & let us know how it turns out...........ed

 

[markodecember]

Re: 1985 mercury 90hp inline 6

Thanks for the reply ed.

I did think about it and came to the conclusion it must be spark or carb problem of some sort.
But it just seemed wierd how just #3 was completly shot to pieces and the others didn't seem that bad?! (Couldn't check #6 as below cowling)

Suprisingly enough the engine did actually run o.k considering it was knackered. Just thought if coil was faulty and there was no spark on #3 could something as simple as this alone be enough to do this kind of damage to the engine ?!

Thanks again
Mark.

 

[Chris1956]

Re: 1985 mercury 90hp inline 6

Mark, #3 cylinder is the first to go, followed by #1 cylinder, on those old inlines. The cause can be what was specified by the prevoius poster. You can fix it up, or buy a remanufactured powerhead. iBoats sells them.

If you want to fix it up, it will cost about $150 for a new oversize piston and gaskets and about $50 for a boring of the cylinder. You will need to do all the work yourself, for this cost. After you have repaired it, replace the waterpump impeller, clean the carbs, and do a link and sync, or it will fail again.