Lastly, have you checked for smooth motor swing with the engine not running? Just move the swing and make sure its not rubbing against anything. Seems silly to ask but diy swinging setups often have a carb or reed block or intake that will bump into the spring during variation. My goal is to get mine to have at least a 4mm gap so nothing gets rubbed or broken off.
Any resistance in the swing (sans spring) can cause weird issues.
You’ve inspired me to go ride a swinging engine bike today. I get so proud in French style bikes when I finally get all the geometry right and everything swings together without eating itself.
But I will say I’m constantly controlling shift on my setups, I could use a much stronger spring and have it all shift automatically but I’ve always felt that moby style bikes are semi automatic, with my preference being acting more like a manual shift (edited)