Re: Woodruff key
Regardless if the wobble on the rear sprocket is causing the problem with your woodruff key, you need to fix that wobble. You will have nothing but trouble if you ride with that wobble. I had to put the rag joint on a 32 spoke wheel (its made for 36 spokes), three of the holes interfered with spokes. I had to make some new holes through the sprocket, rubber and brackets and put two bolts equally spaced between the three that interfered. After much tweaking I got mine dead center. So either spend some time and work out the wobble, get some kind of sprocket adapter that fits, or change your rear wheel. You need that sprocket to be wobble free....
On the woodruff key, this is not a a super common problem. Perhaps your motor sprocket isn't tightened down properly? If its a little loose it will jerk one way when you drop the clutch to start the motor, and then jerk the other way when the motor starts to pull the chain. Doing this time and again could cause that key to sheer. This is a hard nut to tighten properly, the spark plug tool they give you that tightens this nut is hard to use with good results, and the motor likes to rotate when you crank down on it. I recommend you go to the hardware store and buy a real socket that is the size of that spark plug tool (I forget how many mm it is off hand), use the real socket with a real wrench and you will get that nut tightened properly. To prevent the motor from turning over while tightening, remove the gear side cover, take a shop rag and double over the end a few times so its thick and put it between the big and small bevel gear so that it gets dragged into the gears when you tighten the nut. If you have a thick enough lump of rag there the gears will seize immediately and allow you to torque that nut, and the rag will come right out easily when done. (if your rag is too thin it can get dragged in and stuck in the gears, I like to use the doubled over sewn edge of a rag) Other people use other things, wood dowel, penny, one manual shows a screwdriver jamming those gears. The rag works well for me, I don't want metallic items jamming my gears. Also, I like to use some BLUE Loctite on that nut on the motor sprocket.