You know...a two stroke engine is blindingly simple. No pushrods, no valve shims, no camshafts, no complicated chain or gear drives to the camshaft, no valves, no oil pumps, no oilways to get clogged up, etc....You would think that this unit could have been rebuilt with little drama. Unfortunately, in my case that was not to be.
It turns out the the "carburateur diabolique" was not the problem. My first assumption was that the carburetor (which spelling should I use??) was simply not working. I got an overhaul kit from TY Trials and an ultrasonic cleaning unit. The overhaul of the carb was uncomplicated and both purchases made good sense. Unfortunately when the newly overhauled (and very clean) carburetor was installed it made no difference whatsoever. Still...wild revving at almost any throttle setting and complete chaos when the choke was pulled out. However, I did notice a strong blast of air on the left side of the engine case in the vicinity of the cylinder base gasket....which also sent me down the wrong road.
I pulled the cylinder off and installed a new base gasket and sealant and a new head gasket. I assumed that there was an air leak somewhere at the base of the cylinder. Again, frustration. The wild revving continued.
Next, I pulled the cylinder off again expecting to find a cracked cylinder or a crack in the case somewhere. No obvious problems could be seen, but I did notice an excessive amount of dark and heavy oil in the transfer ports. That is when the light finally went on in my head. The seals on the crankshaft had failed. On the drive side, the seal failed and it was pulling in both air and oil. On the magneto side, the failure of that seal was pulling in pure air. The leak on the magneto side explained the air movement on the left side of the case. Air was being drawn in at the opening which was provided for the wires from the magneto.
The fault finding is easy. I am the problem. On the magneto side, it appears that I installed the seal backwards. Hard to believe, but that seems to have been the problem. May be I started drinking too early that day. May be the light was insufficient. May be I was distracted. May be I was in too great of a hurry, whatever the reason the fact is that when when I pulled off the rotor, the little springs which hold the seal tight against the crank were free from the seal and wrapped around the rotor itself. On the drive side, it appears that the seal was damaged during installation. I do remember that both seals were aftermarket pieces that had come with the bearings. They were very difficult to install. It appeared that they may have been slightly oversize. I have ordered new Yamaha OEM seals from John Crane at TY Trials.
Above is a shot of the drive side main bearing seal. This is not the seal that failed. Actually the design of the on this side of the crank employs three different seals which, if properly installed, will provide a comprehensive seal. There is a double lipped seal mounted in the crankcase and that seal rests on a sleeve which fits tightly on the crank and which has a further seal between itself and the crank. I must have made a mess of this seal when I put the cases together. I can see now, working with these seals on the bench that it is not an easy job. The sleeve which goes over the crank case must be installed in a way which will allow the lips of the crankcase seal to fit tightly over the sleeve without folding under. I will be more careful next time.
Stay tuned....new seals will be arriving from England in the near future. I will get this engine together!
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