Thanks for the words of caution Mark and next time I will be prepared to get it to stop. I will probably do the crank seals before another test but am having trouble finding a genuine Yamaha replacement for the right side.

these 360s are quite the abomination, lean it out enough and they become a low-compression diesel!

DaveinSC, I found that replacing the main seals was a snap so long as you have an impact driver to get the 3 magneto screws out. I used two small holes and drywall screws to "pull" the seals, which came out easy, but took quite the whacking to install and is made more difficult because your pressing on an edge vs surface since they go in "backwards". I created a driver using some PVC fittings and even with oil it took some solid hammer belts to drive them home. Once this was done I confirmed that I had no additional air leaks 1. because even the smallest change to the idle mix screw impacted idle and 2. because spraying WD40 all around cylinder base and intake, etc. caused no change in idle.

A few genuine right side crank seals on ebay, here's one:


Good luck!

Lessons learned from other people’s glitches are sure helpful, thanks for sharing.

Thanks for the link to the seal, just ordered up that one and one for the left side. I got the throttle straightened out today and got a kill switch ran so those 2 things are taken care of. I did fire it back up for a minute or so and it didn’t rev out again but not taking any chances of riding it until the new seals are in. I think it does have bad seals, it’s not responding to carburetor adjustment. I took the cover off and saw a little oil under the magneto and it seems to be smoking more than it should so probably sucking transmission oil in on the right side.

2 of my RT3 regular riders were restored by someone else before I bought them. He used aftermarket cylinder base gaskets that were thicker than Yamaha OEM. After I had them for awhile one of them went into a screaming high RPM where it would not shut off with the key turned off and the throttle closed. Had to put it in first gear and let out the clutch with the front brake applied. Turns out the base gasket would allow the head bolts to loosen with the engine expanding and contracting from heating up and cooling down. The 360 has 4 more bolts and studs that hold the head to the cylinder so it maintained compression. It was able to suck unlimited air through the gap at the base of the cylinder and draw fuel through the carb resulting in a runaway lean condition. Any source of air after the carb body will get into the combustion chamber. I checked the other RT3 and 2 of its head bolts were loose. Proper gaskets and seals are vital.

I also had a stump grinder with a Deutz diesel engine flip over and go into diesel runaway burning its crankcase oil. This was also scary.

This has been an interesting thread. I’ve personally witnessed a runaway diesel but never knew a 2 cycle could do that. Is this more of an RT3 phenomena? Or in theory could it happen with any sized engine?

This happened with a 250 so I guess in this case size doesn’t matter

I've had it happen with 125's and 175's too. I think it can happen with any of the Enduros.

Scary. I hate it.

One thing... if you have a later CDI bike do not pull the spark plug cap with it running/ignition on. Could fry the CDI. Points are must more robust in this respect.

Here is a video I was taking when the engine went crazy on me. Pardon the rag for the gas cap, there was only a cup or two in the tank though.