I like your mount. I almost did something similar. Instead, I cut the entire downtube out of my Schwinn Delmar. I bent up a new downtube out of 1-1/4x.09 tubing and welded it in place. The new downtube runs at 70 degrees to the seat tube where the engine mounts. The motor now seats perfectly into the frame. I’ll post pictures later. There is a little buzzing, mostly through the pedals but nothing bad. I’ve owned a lot of motorcycles, many of them vibrated much worse.
There are two ways to try to handle the engine vibration. The easiest method for an engine this small, is to mount it as rigid as possible. You want the mass of the frame and bike to become a solid part of the engine. A heavy frame mounted solid to the engine will go a long way toward dampening vibration. The front mount is the shortest, most rigid attachment available on these engines. The downtube is the stiffest support on a bike frame. A stiff, solid connection between the engine and front downtube will reduce resonant vibration, and keep the mounting studs from fatiguing. The back mount should also be as solid as possible, though it is not as rigid as the front mount.
I see so many long, spindly, front mounts. Often they are made of relatively thin plates or a long bolt through the frame. Then when the user experiences excessive vibration and resonance, they put rubber between the mounts and frame in an attempt to isolate the engine. A properly designed rubber mount can work well (I had a Norton Commando in college), but it is not as simple as rubber shims. There is a lot to consider. In my opinion, a 66cc engine shouldn’t need rubber mounts.
I like seeing all the creative solutions to engine vibration, but the simplest is mounting it solid. The solution that you came up with takes only two welds and should prevent vibration problems. Good job.
