Piston and head pitting - Yamaha 2 stroke - Pensacola Fishing Forum

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Old 04-20-2017, 11:01 PM   #1
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Default Piston and head pitting - Yamaha 2 stroke

What do y'all think about these photos? These are from a 40hp Yamaha two stroke. It's really bad on the middle and it as bad on the bottom. It's virtually non existent on the top.
Is it fuel preigniting? Adjust the timing?
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Piston and head pitting - Yamaha 2 stroke-img_0380_1492743474813-jpg   Piston and head pitting - Yamaha 2 stroke-img_0381_1492743499084-jpg   Piston and head pitting - Yamaha 2 stroke-img_0382_1492743522753-jpg  
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Old 04-23-2017, 10:54 PM   #2
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Looks more like a foreign object was bouncing around in the cylinder seen the end of a plug do that


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Old 04-24-2017, 10:46 AM   #3
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I'd check carbs first. That looks like it's lean damage but still has a carbon crust on top of pistons.
I'm sure if the timing was off, all the cyls would have same pitting.
You gonna pull the pistons too? See the scoring in the same two in question?
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Last edited by kanaka; 04-24-2017 at 10:55 AM.
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Old 04-24-2017, 11:20 AM   #4
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I went through the carbs and they are spotless.

I bought the boat used from a guy last year and ran it for several months without any real problems. The previous owner likely ran it real lean. I didn't check compression prior to the head gasket replacement.

I got it put back together and ran it. It ran great.

In hindsight, I wish I would have pulled the pistons and replaced the rings.
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Old 04-24-2017, 01:39 PM   #5
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That looks like it was fun loosening the bolts.

Pre mix engine? Interesting that all the damage is on the outside areas.

I've rebuilt carbs that WERE spotless doing a quick visual but found a small build up in the jet orifices holding them up to a light. Quick pass with torch tip cleaner got the stuff out.
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Last edited by kanaka; 04-24-2017 at 01:46 PM.
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Old 04-24-2017, 01:55 PM   #6
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You were right, preignition.

http://www.boatpokers.com/data/piston.pdf
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Old 04-24-2017, 01:58 PM   #7
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pre ignition detination is when. when the outer rim of the piston is eat away it is most likley from dirty fuel/ water in fuel. or a lean out from a parcel cloged carb.

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Old 04-24-2017, 04:15 PM   #8
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Will all those pits create hot spots?
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Old 04-24-2017, 08:29 PM   #9
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Looks like a part of the intake\scavenge port is missing too
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