scored piston

[sldunker]

Picked up a 1989 60 hp tiller for a really good price. It was all in a box as it had been torn down do to cylinder number two having scoring on exhaust side only. I have found that the water pump impeller which was a 6 blade had one blade that was broken off and just hanging there. I have taken carb apart for #2 and found nothing out of ordinary. The tell tale has not been relocated and I plan on relocating this along with a new omc water pump kit. Were else should I look for possible causes of this scoring. Thanks

 

[rickdb1boat]

Re: scored piston

Make sure to clean and rebuild all the carbs and verify the timing is correct, too. Most likely you found the cause of the scoring due to overheat, but best to be safe...

 

[sldunker]

Re: scored piston

The tops of all the pistons are fine with good color so I would assume timming was correct.

 

[clint]

Scored piston

Scored piston

Your first cause was water pump. I would also think the carb wasnt delivering fuel oil mix to that scored cylinder. Do a complete rebuild on the carb and a new water pump. The previous owner might have scimped on the oil gas mix and failed to change water pump. Hee have had to have signs of a frying engine. Over time it will gradually destroy the engine. Good Luck!
Clint :$

 

[sldunker]

Re: scored piston

I'm inclined to think that the oil to gas mixture was OK being that there was no scoring on the intake side of the piston. Could be wrong though.

 

[Dhadley]

Re: scored piston

Cleaning the carbs is a standard part of the rebuild. Don't skip it, especially on a motor that you're not completely sure of the background.

Obviously a new pump kit (not just the impeller) is called for also.

Now, those motors tend to run hot, as far as combustion temp, on the middle cylinder. So, we look for anything that might have, or will, aggrivate that situation.

The wrong spark plugs, the wrong octane or old fuel, lugging etc will add to the cause. Make sure to use Champion plugs, fresh 87 octane with good oil and set it up at 5800-6000 with an average load. If it were mine, I'd richen up the mid range by about 3 numbers on at least #2.

What you're seeing is not unusual.

 

[sldunker]

Re: scored piston

Dhadley, when you say richen up the mid range are you talking about going up on the high speed jets or are you talking about increasing the size of the air jets( thats what the service manual calls them).

 

[Dhadley]

Re: scored piston

Depending on which carbs you have the mid range jets will be either air bleed jets or fuel jets. The carbs that you should have are the ones that have air bleeds in the front for the mid range. If they are air bleeds (not covered by a screw) then you'll go smaller to go richer.

We've found that those motors run lean in the mid range and when run in the mid range for an extended period may lean out the middle piston and score the exhaust side. They'll run forever at wide open with no problem.

 

[sldunker]

Re: scored piston

Thanks. The ones I have do have the air bleeds in the front of the carb. This is very good info. Thanks again for the help.