Cause of Piston Scoring ?

[greasemonkeyjm]

I have a 48 hp evinrude that would not idle, i reied several things and ended up tearing into the internals anf found both pistons had severe scoring on the exhaust port side. what is the normal cause of the scoring on that side ? I am going to get it bored and rebuild it, but I dont want it to happen again.

 

[F_R]

Overheating. In combination with lousy oil that can't stand the heat.

 

[greasemonkeyjm]

Thanks for the respnse F_R, I just bought the rig so I am not sure of its past, it pumps a good stream from the telltale, and on muffs, it was seeming to run really cool, but when I tore it down i noticed the pressure relief valve has been melted, there is no discoloration on the heads or any melted plastic that would indicate a ran extremely hot past, it has been running on a 100:1 mix, but I added enought oil to bring to a 50:1 mix, using pennzoil synthetic blend outboard oil, carbs looked clean inside, so i dont think it a lean fuel condition.

 

[johnson89]

100 to 1 is way lite on the oil.....that would be my guess.....damage might have already been done when you got it....if it was just 1 cyl. then a carb. issue also but both cyl. not enough oil.....

 

[nwcove]

can only assume that motor is an early 80's? with the 100:1 decal . if so, 100:1 was fine for running if the motor was in proper tune and healthy. the service bulletin that switched the mix back to 50:1 had to do with off season storage corrosion issues.

 

[greasemonkeyjm]

the engine is an 87 model 48 HP evinrude, when i bought it, it would idle on the hose, but idled rough, when in the water, it would not idle at all, and would go from almost dieing to raceing up to around 3k RPM's i thought it was a carb issue so I started there, and thats how i ended up here

 

[metriccrescentwrench]

Jeez those are ugly!!!!

 

[F_R]

Well there ya go...A melted check valve says it got hot. That, combined with borderline oil mix ratio and who-knows-what kind of oil makes for nasty results. What you are looking at there is where the aluminum pistons' surfaces actually got to the melting point. Combustion heat + friction.

 

[greasemonkeyjm]

Yea they are definately ugly, I was wanting a diagnoses of why it happened, I am going to rebuild it, but I didnt want it to happen again so i was wanting to right what was wrong.

thanks

 

[racerone]

Lack of lubrication is a major cause of this since the block was not discoloured from overheat.----Running at 100:1 is a major mistake. ----Running with too big a prop is also a mistake ( lugging ) and can lead to high temperatures.---It has been made clear to me that most folks are not well versed with what happens to the oil once the fuel goes through the carburetor !!!!

 

[greasemonkeyjm]

I have got to get a tach on it before i get my engine ready, so i can keep an eye on things, i am running a 13 3/4 x 15 3 blade on a 24 foot pontoon, its what was on it two weeks ago when i bought it.

 

[Silvertip]

Too much prop. That engine used a 12-1/4 inch diameter prop. For pontoon service a large blade 11 or 13p is in order.