2000 Yamaha 50HP bad center cylinder

[Sdominick]

I acquired a 2000 Yamaha 50HP C50TLRY with a bad center cylinder. Any ideas on how a motor that looks very well taken care of can destroy the center cylinder? I have a theory but would like to hear from some folks. Thanks for your time.

 

[racerone]

If you have to pay a shop for repairs you are best to shop for another motor !

 

[boscoe99]

Looks like detonation damage to me. Breaks chunks of the piston off. Piston shrapnel then damaged the top of the piston, the cylinder head, the reed cage, etc..

 

[Sdominick]

Thank you very much for the response, I believe this motor may have been run with straight gas. Is detonation damage a system of something like that do you think? I will be opening the fuel filter to continue my investigation. Thanks again

 

[racerone]

If it was run without oil you may need a new crankshaft assembly at $1200 , 3 new pistons and machine work.-----If you have to pay a shop you are better off to to find another motor.

 

[boscoe99]

Lack of oil in the gasoline would result in a different kind of damage IMO. And to all three cylinders. And to other internal parts as mentioned. Crank shaft being just one. Bearings. Cylinder walls.

From what I can see damage is limited to the number two cylinder. Perhaps to the piston only. Cylinders walls also but I can't see that.

 

[boscoe99]

21 year old motor won't be worth much. Repair costs in parts alone can easily and quickly exceed the value of the motor. If the parts are even available.

 

[99yam40]

what about running too lean do to carb plugging on only #2 ?
that may cause #2 to run too hot and being between the other cylinders it got hotter than the other 2.
do you see any damage to the other cylinders we cannot see?

but as others said, you need to take it apart to see if bearings and crank were affected

 

[boscoe99]

Detonation can result from a cylinder running too hot. From being slightly too lean or for other reasons.

 

[Sdominick]

Number 2 carb running lean from some carb obstruction definitely makes sense. Cylinder walls are scored on #2, All cylinders are sleeved. New sleeve, New piston, new rings all around while open. These Yamaha motors hold their value. This motor running properly is a $3500 motor all day long. The lower is $700 alone. Yamaha can’t even get motors into the country right now. With no end in sight. Crazy. Thank you very much everyone. I appreciate your input. Very helpful.

 

[Sdominick]

what about running too lean do to carb plugging on only #2 ?
that may cause #2 to run too hot and being between the other cylinders it got hotter than the other 2.
do you see any damage to the other cylinders we cannot see?

but as others said, you need to take it apart to see if bearings and crank were affected

That is the same logic that lead me to straight gas theory. Center cylinder will heat quicker being that it has no free wall like the top and bottom. Any water pump issue would not be center. Water cooling the upper and lower while not being able to control center temp. I’m going into the filter and hoses to investigate for straight gas as well. Motor really is barely used.

 

[99yam40]

you did not say if you see any damage to #1 AND #3 we cannot see in pics.
are you going to open it up and look at bearings ?
that will help tell if lack of oil was even part of the problem.

I believe Rodbolt once said lack of oil will get bearings before pistons

 

[boscoe99]

Help me to understand the straight gasoline theory?

 

[boscoe99]

The center cylinder has the same water flow all around it that the top and bottom cylinders do. Should run no hotter.

 

[racerone]

These use a multi piece crankshaft.----Hard to fully inspect the main and big end bearings.-----I say a new or replacement motor is in the future.

 

[99yam40]

These use a multi piece crankshaft.----Hard to fully inspect the main and big end bearings.-----I say a new or replacement motor is in the future.

very true, all you can do is visually inspect for discoloration on rod ends and crank bearings.
maybe feel if they are rolling smooth

I guess if running lean on a C motor would be a lack of enough oil along with not enough fuel since they are premixed

someone forgetting to add oil and running straight gas would hit all cylinder not just #2

 

[robert graham]

Find another motor same H.P., model, year as much as possible, buy it….then use the old motor for parts….you’ll be good to go for years then…

 

[Faztbullet]

That center piston doesnt look OEM...and its not a no oil problem

 

[racerone]

Good eye on that difference of the pistons.----Been apart once before.-----All the more reason to shop for another motor !

 

[Faztbullet]

Most likely center failed before and was repaired but original problem which caused failure was not fixed..see it all the time in customers DIY repairs