Piston damage diagnosis

[Dubhicks]

So, a few weeks ago I had a #4 cylinder failure on my 98 115 V4 Yamaha. I was driving around normally when I lost power and heard a rattle noise from the engine. Shut it off and restarted and idled fine but took a little longer to plane out and lost 10mph top speed. Long story short, this powerhead was replaced a year and a half ago for the same cylinder failing. I removed the power head and took the unit apart to see what I could find. The piston looks to have some pre ignition or detonation damage, but I'm having a hard time pin pointing what the failure was. According to the invoice I have for the last power head install, the carbs were also rebuilt and the fuel system flushed. I have seen no evidence of an intake leak and the reeds appear to be fine.
To get more to the point, I ordered and installed a remanned power head this weekend. I am just wanting to figure out what caused the failure so I can avoid this scenario repeating a 3rd time. I am running the NGK plugs that the motor sticker says I should, and none of the other cylinders have any damage. I also feel that the oil injection is working, as I do not see really any seizer marks on the cylinder walls. Will upload pictures when I figure out how to. Was told to try this forum by a member on a different forum.

[boscoe99]

Pre-ignition and detonation are the twin red headed ugly orphan step sisters of engine damage. One can cause the other so it is difficult to say exactly which caused the damage and which came first if both are involved.

If it is the same cylinder this time as last time I would lean towards the carburetor being involved but that is just a wild eyed guess. Maybe you want to have it thoroughly cleaned again and maybe you might want to think about increasing the main jet size a number or two. Too late now to think about analyzing the spark plugs although you can certainly do this once the motor has been reassembled to see how the number four plug compares to numbers one through three. If number four runs leaner at that time then maybe think some more about the carburetor being cleaned or the jet being upsized.

By the way, how did the number four cylinder head look from a carbon build up point, if the piston being destroyed did not damage it so that it cannot be known.

What does the thermostat look like? In good condition or not? Fresh water or salt water motor?

If you feel it is detonation related you can also consider running gasoline with a higher octane rating. For insurance purposes if nothing else.

[Dubhicks]

Thanks for the reply. The cylinder head was chewed up, didn't see much carbon anywhere.
Thermostats were operating fine. I'm pretty certain it was definitely detonation damage.
You can go to mbcforum.com under general for the full 4 pages off discussion I have going so far.
Is there a way to attach photos here? If so I can do that.
Long story short, tore down carbs, inspected, cleaned and reassembled. Didn't notice any difference between any particular carb. Oil injection alway sipped oil so I'm fairly confident that isn't to blame. Took a fuel sample, no trash or water noted. Drained fuel tank and added 91 ethonal free for the new powerhead startup. Mixing 50:1 in tank per engine supplier.
I already have the new powerhead installed and everything good to go. Just waiting until this weekend to start the break in procedure.

[Dubhicks]

[Dubhicks]

[Dubhicks]

This is the piston from the other lower cylinder. (#3) I believe it was showing signs of detonation beginning also. Thought?

[boscoe99]

Looks like detonation to me.

Other opinions will vary. Almost impossible to know exactly what happened and even less possible to reach agreement on what that something might have been.

I would have suspected that two or three would have failed before four because they normally run hotter than one and four but anything is possible.

My final thoughts are that if two or more cylinders are damaged about the same time and in the same general manner as your pistons appear to be then it is more than likely due to detonation.

[bajakeith]

Hmmm !!

I would go thru the cooling system on this motor...complete water pump kit and new therms and a PRV..I would also ck all main jet sizes and float level's..I've seen were the carb had a leaner jet in one side and ran decent but very high EGT...BTW the last one I found like this, the "O' ring on the bottom of W/P "cup" was not installed right. I use " Gaska Cinch" on that seal/"O' ring... and dosnt hurt to run a good grade of fuel ether...and nothing else for sealing chems are needed...And Never ever use RTV anywhere on a Boat motor....Tell Ya a little secret,, almost all marine gaskets don't need or want a sealant used on them..its already impregnated..Drive's boat tech nuts when we see RTV !! which BTW stands for "Room Temp Vulcunaising" I would also test run with a vacum guage Teed in on the motor side of fuel line to ck for fuel restriction..Like a antisipion valve or bad fuel line and connect

[rodbolt17]

ahhhh silly cone.
gotta love the stuff. its made me a ton of money since the 70's.

just cause a carb got rebuilt 1 yr ago doesn't mean it gets dirty again.

every time the cyl is bored oversize go up a bit on the main jet size,

[Dubhicks]

Yeah, no silicone anywhere. I have installed a remanned powerhead with a new gasket set. The Pistons say they are .030" over. I know nothing about upping the main jet size, could you lend me a recommendation on where to get bigger jets for this engine? Thanks.

[ausnoelm]

Do the carbs or get ready for another rebuild down the track.