Any Ideas what caused this

[stumbler]

View attachment 210753 Motor was running fine for about a mile then bogged down. And would not rev-up. I had a feeling that I lost a cylinder but thought is just going to be a fowled plug or something not this. just the top cylinder and i guess a good thing is it didn't damage the cylinder walls.

 

[JB]

Re: Any Ideas what caused this

Could you tell us the make and model of your outboard?

 

[boobie]

Re: Any Ideas what caused this

Lean cylinder or timing went ape.

 

[F_R]

Re: Any Ideas what caused this

What boobie said. That piston melted from pre-ignition. Lean, timing, low octane gas, or hot spark plug are the chief causes.

 

[stumbler]

Re: Any Ideas what caused this

Its a 1976 25hp evinrude. Any suggestions on where I should start. What controls the timing or what would cause pure ignition

 

[stumbler]

Re: Any Ideas what caused this

I also noticed that when motor was the slow idle rich/lean nob turns by itself. Could this of caused it

 

[V153]

Re: Any Ideas what caused this

What boobie said. That piston melted from pre-ignition. Lean, timing, low octane gas, or hot spark plug are the chief causes.

Yup. Classic case of detonation.

 

[F_R]

Re: Any Ideas what caused this

Its a 1976 25hp evinrude. Any suggestions on where I should start. What controls the timing or what would cause pure ignition

Well, the timing can't be changed on that one, unless somebody has removed the shifter lock or dinked around with the limiter on the armature plate. Gas octane, not much choice unless you were running something less than 87 octane regular. Lean condition? Possibly, if it has the wrong high speed jet, or the right one, but dirty. Could be that somebody tried to soup it up a bit by planeing the head or something. What did it have in it for spark plugs?

The loose needle may have made it idle lousy, but it didn't cause the damage. That happened at full throttle.

 

[stumbler]

Re: Any Ideas what caused this

Champion QL82c. And manual says to run champion j4j Dont know the difference. The guy I bought it from said he got it from a kid that went through the motor while he was going to marine mechanic school. So I guess he could have messed around with timing of some sort.

 

[stumbler]

Re: Any Ideas what caused this

Well after looking at the champion web site i found that the plugs that were in the motor were designed with resistor- CDI. And the plug they recommended is just resistor. Would this have caused this problem? Can I still Get the plugs that my manual said to use champion J4J

 

[steelespike]

Re: Any Ideas what caused this

I believe the J4J is now a J4C.

 

[Bob_VT]

Re: Any Ideas what caused this

You might have sucked up crap into the carb...... there might be a blockage inside. Regardless it is going to have to be taken apart and rebuilt. Did the cylinder walls take a beating too?

 

[stumbler]

Re: Any Ideas what caused this

No cylinder wall is ok from what I can see. I have the engine all apart just waiting on new piston, rings, and gasket kit. Then its time to start trouble shooting. Just curious if was going to be as easy as putting different plugs in but I dont think so. Because both cylinders had same plugs shouldn't I be able to see signs in the bottom cylinder as well if this were the case?

 

[out 2 launch]

Re: Any Ideas what caused this

Both cylinders are fed from a single carb so if a lean mixture caused the top cylinder to implode one would expect the bottom cylinder to show signs of running lean. I can't determine that from the pictures. Another possibility which I have seen is a weak water pump. The pump supplying just enough water to keep the bottom cylinder from getting dangerously hot but not enough to sufficiently keep the top from getting too hot leading to detonation. When water pumps go it's generally the top cylinders that overheat first.

 

[stumbler]

Re: Any Ideas what caused this

Here's a picture of the bad piston. I noticed one part of the piston side looks like this is where it started. Also a picture of the other piston does it look alright. Im going to replace it also but just want to know if it looks ok.

 

[JRegier]

Re: Any Ideas what caused this

Could be too a problem with your reed valves causing lean condition in top piston. Wrong plugs certainly didn't help anything. I've seen some goofy pics on the internets.

 

[jimmbo]

Re: Any Ideas what caused this

Air leak in upper crankcase, Bad upper seal for crankshaft. grossly misadjusted points
What does the cylinder head look like?