I had the same set of symptoms with my 4G kit. I'm a heavy guy about 280. I'm running the gear that came with the kit, 48t sprocket, 12t freewheel, 100 driven pulley. My clutch would strain and bounce trying to get engaged because it was had to get me moving. I actually thought my engine mount was coming undone it was so bad.
I found possibly three things to do to get rid of this clutch chatter. I've tried two of these with success.
1. I mounted a 56t rear sprocket which made the gearing very low. I lost speed for sure, but I gained smoothness. Take off from a dead stop without peddling was no problem.
2. I accidentally got oil on my clutch shoes. Take off was a little slower but the engine assembly was very smooth as the shoes were slipping on the bell inside surface. Eventually, the oil will burn off and the clutch will go back to how it was. The fact is, I like the oil on the clutch shoes solution so much, I drilled a small hole in the clutch bell so I could oil it without dissassembly required. I expect to get lots of negative feedback about deliberately oiling the clutch shoes. I don't care because it works well for me.
3. I haven't tried this, just read about it. Shim the springs in your clutch so the clutch won't engage until higher RPMs. That could work well, I don't know for sure.
Whatever you do to solve the vibration problem, if it's coming from your clutch it's very important you get it stopped. Clutch vibration puts great strain on the clutch key and keyway walls. It is possible to damage the keyway to the point where you will need to replace the crankshaft.
Oh, one other way to eliminate clutch chatter would be to peddle like a son of a gun to get speed and then turn the throttle on. I don't care for that idea however. I mean why have a motorbike if you're going to peddle all the time?
Hope some of this prattle helps you.