1989 Yamaha PRO 50. Crankshafts prone to failure?

[James R]

Before I took this motor it had been bored .040 over and a new crankshaft fitted. Investigating loss of comp on #3 showed the top of the piston damaged. Fortunately the bores are still good. I did find however that the crankcase is scored in the #1. Looks like the bob weight has made contact due to the previous crank or bearing failure. No biggie. The crank shaft looks good but noticed one circlip is broken. No pieces anywhere and not embedded in the piston.
I have been told that this model is prone to crankshaft failure.
Does anyone have any input on this.
One thing I have noticed is there does not appear to be much oil residue in the crankcase.

 

[mphelle8vld]

Re: 1989 Yamaha PRO 50. Crankshafts prone to failure?

I'm unaware of any chronic problems with the cranks of this motor. The bearings survive on a thin sheen of oil, any excess or residue would quickly be swept away in the tornado of mixture in the crankcase.

Did the motor ever run right or did the performance fall off causing you to discover the low compression?

Are you referring to the wire clips that wiseco uses to retain the wristpin?, if so, were the missing pieces on the same cylinder with piston damage?

 

[James R]

Re: 1989 Yamaha PRO 50. Crankshafts prone to failure?

Thanks for the response mph'.
Whilst testing for other issues a loud tapping emanated from the motor at idle. Quick check of compressions revealed a loss of 15lbs on #3. Removal of the cylinder head revealed the piston damage. The circlip is one of two used to locate aluminum rings in the case which are fitted on the crankshaft. All parts are on their way for rebuild. I have pulled the oil pump drive and will run on 50:1 as a precaution. I see that the oiling system is almost identical to that of Suzuki and I have never had problems with those. But just to be on the safe side. Carbs appeared to be clean but you know how that goes.
Anyone else have a take on this crankshaft issue?

 

[mphelle8vld]

Re: 1989 Yamaha PRO 50. Crankshafts prone to failure?

From your description so far it certainly seems like your motor swallowed some metal, I just can't see how a piece of the clip that locates the labyrinth seal could find its way into the crankcase. Was there any damage to the groove in the aluminum ring or the machined recess in the block that the seal mounts in? Anyway, I'm always looking to learn something new, is it possible that the person that last rebuilt it used a partial broken circlip, and something else, a piece of bearing separator or reed valve was swallowed?

I have two of these motors so keep me/us up to date on your progress.

 

[James R]

Re: 1989 Yamaha PRO 50. Crankshafts prone to failure?

Thanks mph'.
I did find a small piece of the clip in the case. It could have been in the groove along with the large piece and fell out when I took the CS out. A piece or pieces of the clip could have dropped into the lower cylinder cavity and been pumped up above the piston. There are no pieces in the piston or the head but I am betting that it was this clip failure. The top ring groove is damaged jamming the ring in the piston so something passed through. The reeds all look ok and the bearings are like new.

 

[mphelle8vld]

Re: 1989 Yamaha PRO 50. Crankshafts prone to failure?

Thanks for the update, hopefully your shop can get you back on the water soon.

 

[James R]

Re: 1989 Yamaha PRO 50. Crankshafts prone to failure?

Honed and ready to rebuild.