While you say that the prop nut is completely fastened, butit is not. The cause of this is simply because you are not getting the nut tight enough when you install the prop. If you are using one of those 4-way wrenches that you use to remove the glow plug with, forget using it to install the prop nut. You can't get enough leverage with it. Get yourself a good box end wrench, and a good leather work glove to protect your hand with as you hold the prop( if you don't have a leather work glove you can hold the prop with a rag). As you hold the prop use the box end wrench to install the prop nut. Don't worry about getting it too tight as both the prop nut and the prop shaft are made out of steel and it's very unlikely that you will strip it out. Tighten is as tight as you can get it. Once you do this you will not have any problems with the prop coming off.

One other possible cause of the prop coming off when using an electric starter is you have primed the engine too much and it's getting a bit of hydraulic lock going on. But more than likely your cause is that the prop nut isn't tight enough.

Ken

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