Vibration - There seems to be a relation to the engines and who manufactures them. I take all my engines apart before I install them to see if the parts are in the correct order. I have one bike that is as smooth as glass, one that is a little worse and one that is horrible. The one that is the worst is the one that has no balancing holes in the flywheels. The engine came from Kings, they just turned the flywheels and assembled them. The engines from Dax have removable outer rings that are hollow in the correct places, it is just done in the castings but at least it is done. I have found that there are some frames that lend themselves to engines, one being the green Hawaiian bike from Wall mart, my Green Hornet. My pacific old bike is the one in the middle and the Schwinn is the worst. One thing I have recently found is that they need a hanger for the muffler I added one to the Pacific and it did a lot of good. I have tried to put rubber in the mounts and found that did nothing. Good tight mounts work. Have fun, Dave
PS: There is one fellow that added a mount to the bottom of the engine, a plate welded into the frame and picks up the bottom engine bolt, just an idea.