Mariner 100 damaged piston (see pic)

[enn858]

Hi from Denmark

I am helping a friend repairing his 1993 Mariner 100 hp - 4 cyl outboatd

He bourght the engine 2 weeks ago and the seller told him it was runing perfect

However we tested it and it ran wery rough missing etc

We examined the engine and it had a damaged piston (see pic)

Now can you guys help us answering these 2 Q`s

-What causes a damage like that (i have seen many damaged pistons - but this one looks strange)

-Could the engine have been runing good with such a damaged piston?

Thank you for your help in advance

 

[Laddies]

Re: Mariner 100 damaged piston (see pic)

Your picture is difficult to see but I would think a dirty carb may be the problem. I doubt if the engine ran very well in that condition.

 

[UnregisteredUser]

Re: Mariner 100 damaged piston (see pic)

Your picture is difficult to see

Here is a better look at the piston

 

[Silvertip]

Re: Mariner 100 damaged piston (see pic)

There appears to be a very high amount of carbon build-up on this piston. So much so it would appear the rings are stuck in the ring grooves. The burned area of the piston as Laddies indicated, would point to a lean condition, probably caused by a malfunctioning carb. But normally, a burned area of the piston such as this would not be carbon covered so this was apparently an issue that existed for at list a short time. The fact that the rings are not destroyed means the engine could have run fairly well until a compression leak occured between the inside edge of the rings and the piston. In my opinion, this did not just occur. It has been in the making for some time.