1995 Yamaha Pro 50 - Piston problems

Neil G.

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Hi, recently purchased a very low hour 95 Yamaha pro 50 at a great price. Initially would not start so I removed the 3 carbs as engine had sat for a year or 2 and gave them a good cleaning. Once cleaned reinstalled them and engine started well but once it warmed up would not idle, but ran fairly well at a higher speed. Brought home from lake and compression test shows #1-135, #2-120 and #3-140, pulled power head and now looking at 2 pistons top and middle with chunks taken out of them but rings still intact! Anyone ever seen anything like this? I believe previous owner mentioned he had a new water pump installed so I wonder if this is the result of an over heated engine. Also would like to confirm cylinder numbering on this engine is top#1, mid#2 and bottom#3? Going in to crank case to pull pistons out, Not a mechanic just a diY'er so any help/comments will be appreciated. Thanks in advance.
 

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mphelle8vld

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Whoa...It may have been running lean from dirty carbs causing pre-ignition at that spot. Amazing that it was running as stated in that condition, it definitely deserves a proper rebuild. I would replace all three pistons, be sure to check the ignition timing after assembly.

Wanted to add that the motor could have been abused in the past by a can of starting fluid, some think that once they get it started, if they can keep it running by spraying it, the carbs will magically clean themselves!
 
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Neil G.

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Thanks for the reply. Your right the plan was to replace all 3 pistons but I may have a bigger problem now.
After pulling crank out I've noticed 1 of the center bearings is shot (making noise) so will need to be replaced.
Problem is from what I've read these are pressed together cranks and if rods or center bearings are no good you could be in trouble.
Doesn't seem right that a $10 bearing could cause me so much grief, hopefully I'm missing something and there is a fix.

Thanks
 

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mphelle8vld

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Working on older motors will always have surprises, the crank will need to be pressed apart to get to that bearing. Check around for a reputable shop and get a price, where are you located?
 

Neil G.

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Yes, thanks everything I'm reading is pointing to having crank pressed apart. So I guess that's the next step.
Located in Winnipeg, Manitoba Canada.
 

99yam40

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which bearing is making noise?
The 2 rods I can see in the pic do not look discolored at the crank so my guess is it is the main bearing that has the problem.
Not much usually causes the bearings to go but corrosion/rust and that is caused by moisture getting to it.

I sent off my C40 crank this year to have all bearings and seals replaced. the 2 center & lower mains were noisy when I rolled them by hand.
Sent to a shop called Carolina Tool and Machine in Wilmington NC.
talk to ken McCall.

I did not find any other shops that did this work
 
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mphelle8vld

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99yam40 What are they charging to rebuild that crank? (I'm old so it will surely sound like too much)
 

99yam40

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I believe it was $435 plus shipping to there, I do not remember about shipping back to me.
if your crank is not a good core I am not sure but probably have to buy a rebuilt from them for more money.

They shipped a crank back to me the same day they received mine, so do not know if I got mine back or not.
may have just pulled mine apart to inspect and shipped out one already rebuilt. It was a quick turn around

Everything is blasted and cleaned up including rods,
miked, polished, with all new bearing and seals between cylinders on crank.
as they told me better than new
 
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mphelle8vld

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Not bad, skilled repair is getting hard to find and I'm sure there's a little black magic involved. OP might consider looking around for a parts motor.
 

Neil G.

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Thanks for the info guys! Yes I rolled the one marked in pic and it was making a lot of noise.
So would you know if crankshaft is pressed into the 2 counterweights on each side of middle piston rod?
Due to the low hours on this motor I'm sure hoping I can find someone locally to do this otherwise I will be calling Ken in NC which is a heck of a long way from here! Thanks again.
 

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99yam40

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Yes the whole crank is pressed together to install bearings, seals and rods
and needs a special jig to hold all the parts in the correct positions when putting together.

All you can do is check locally to see if anyone around there can do it or find a used one and hope it is good
There is always the choice of a new one.

Might ask Ken if he knows of anyone up that way that does it
 
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Neil G.

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Thanks for this, a used one is sounding better all the time and I believe this is compatible with a lot of other models and motor sizes.
 

Neil G.

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Hi, I have a line on a machine shop specializing in crankshaft repairs for outboard engines!
One more question does anyone know what these are marked in red that sit beside the 2 center bearings?
They are loose,spin and move sideways they have what is called a circlip on them but parts diagrams don't name them.
​Thanks
 

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99yam40

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labyrinth seals
Not sure if spelling is correct
I did mention seals before
 
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Neil G.

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Yes, thought you were referring to oil seals on each end of crank.
Will google and figure out what there purpose is, as they have this clip (circlip) that goes half way around and seal sits loose and moves freely on the shaft. Maybe mine are shot and not supposed to move around like that.
Thanks again.
 

99yam40

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they seal the crankcase portions off from each other,
they install new ones when rebuilding the crank
 

Neil G.

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Thanks again for all your great info, off to the machine shop I go and a whole lot wiser on 95 Pro 50 crankshafts! Thanks
 
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