merc 650 - melted top piston rings! what could have caused this?

[casi77]

motor: merc 65 1975 model, 3cyclinder

just looking for opinions on what could have caused this?

i took the powerhead apart last night, and found out that the top piston rings were melted down, (flush with the piston outer body).. cyclinder walls were not that damaged, a nice light honing will do the job.

what i have noticed was on the reed cage, the spacing between the reed and the reed block is visible .. unlike the bottom reed cage block.. could this have caused it ?? too much air!? or most like the previous owner ran it with straight gas?? if he did lets say, why isn't there damage to the other two pistons??

i did notice some rust on top of the other two pistons.. thats another ????

thanks...

 

[WillyBWright]

Re: merc 650 - melted top piston rings! what could have caused this?

Full timing advance too advanced, poor fuel quality, lean burning from any number of sources. Those are extremely susceptible to all of those. Worst of any motor around for that!

 

[DHPMARINE]

Re: merc 650 - melted top piston rings! what could have caused this?

Actually rust on the other two pistons might be question one.

DHP

 

[jebeebe]

Re: merc 650 - melted top piston rings! what could have caused this?

Actually rust on the other two pistons might be question one.

DHP

my 60 did the same thing. I wonder about bugs building a mud nest in the discharge tube and causing a steam buildup in the top of the powerhead where the water exits? Just a thought. If they sit for an extended length of time expecialy in the fall when the bugs go to sleep.

 

[AMD Rules]

Re: merc 650 - melted top piston rings! what could have caused this?

Likely candidate for the top rings going bad is lean fuel mixture, or overheating, or a combination of the two. If it were a timing advance issue, you should see damage on the top of the piston.

These motors did have a tendancy to eat #1 pistons due to overheating. Might be a good idea to check your water pump, and cooling pasages for blockage.

As for the rust, I suspect you lower crank seal might be bad, or more likely you are ingesting moisture from your exhaust cooling jacket (cracked)?

Upper reeds need to be replaced, but may not have been related to the damage you've noted.

Was the motor showing good water flow out the telltale when last operated? Have you inspected the water pump yet?

 

[casi77]

Re: merc 650 - melted top piston rings! what could have caused this?

i have no history of the cooling as i bought this as parts motor and did not come with the lower unit. Last heard was they ran it without oil-mix (ie. straight gas) ... owner had lended the boat to his friend and his friend didnt know about the mixture...

there is damage on top of the piston, also the rings are melted down flushed with the outer piston wall.

another question about the reeds, can they be flipped over and reused or get new set?!

Likely candidate for the top rings going bad is lean fuel mixture, or overheating, or a combination of the two. If it were a timing advance issue, you should see damage on the top of the piston.

These motors did have a tendancy to eat #1 pistons due to overheating. Might be a good idea to check your water pump, and cooling pasages for blockage.

As for the rust, I suspect you lower crank seal might be bad, or more likely you are ingesting moisture from your exhaust cooling jacket (cracked)?

Upper reeds need to be replaced, but may not have been related to the damage you've noted.

Was the motor showing good water flow out the telltale when last operated? Have you inspected the water pump yet?

 

[AMD Rules]

Re: merc 650 - melted top piston rings! what could have caused this?

Reeds cannot be flipped over for re-use. Flipping them over increases the probability of fatigue fracture. Same principle as bending a paperclip back and forth to bust it.

I'd pursue the fuel system (carbs/fuel pump), the water cooling (exhaust baffels, water cavities and pump), and double check the timing before running it again with a rebuild.

Good luck.

 

[WillyBWright]

Re: merc 650 - melted top piston rings! what could have caused this?

When I tune these up, I retard the max advance by 2 degrees of spec. Nobody has ever complained about loss of power or speed, and that 2 degrees can save it from detonation destruction. Gas these days ain't what it used to be and it ain't gonna get any better.