Thanks for trying it without the anti-oscillation spring Mark, I've never done that and consequently didn't know how much effect it would have - I'd expect it to do what it did, but I did not know how severely.
The set-up process for positioning the governor arm on its shaft is fairly standard for various types of engine, and needs to be done routinely when you are checking over an engine. Bad things can happen when you let one run uncontrolled, especially if it is someone else's engine and you don't want to be embarrassed by the results. I don't think you need a new rod and spring - just pull the governor arm fully to the right, hold it there, and adjust the central bend in the rod until the throttle is firmly wide open. The issue with the governor arm needing to be bent just slightly away from the crankcase so it doesn't rub, and arranging the breather hose so it doesn't touch the control rod, is necessary if there has been some ham-fisted handling of the engine before you get it, but not otherwise. However you always check of course.
With regard to the broken idle mixture screw, if you can unscrew it with vicegrips or whatever, it is not expensive to replace. I suggest you look on ebay, or send a PM to roebuck.
