I've noticed the same thing on a buddy's single-speed DB-P212. I think it could be a few things:
Added stroke and bore over the Honda 196 may add some shakes.
Carb not made to pull a load through low-mid rev range.
Balance factor, also not suited for low-revs, high-load.
Light duty engine plate and chassis may be transmitting more, normal movement.
I was discussing the odd light-crank, heavy-flywheel setup with interweb pals recently too. Great for an engine that may strike an object while mowing, so the f/w can maintain momentum by key shearing, but not what the typical high-rev hotrod desires. I'm hoping to adapt a motorcycle-type full-circle crank and run a 3.5" sub-1lb f/w with a curved CDI and lighting coil. Maybe even chain-drive the cam so the engine can run "backward". leaning forward, eliminating the fan and shroud for free-breeze. Honda G400 ran cam by chain, as do a quadrillion other varieties of engine outside the ditch-pump world.