Vibration is an interesting problem. I have two 49cc motor bikes. One is a two cycle on a Schwinn mountain bike, the other is a four cycle on a Schwinn Cruiser. The two cycle is pretty smooth, I have a pull back bar on the bike and don't feel much vibration at any speed in the bars or seat. On the cruiser I get a pretty strong buzz in both seat and bars. I have a 44 tooth on the two stroke and a 50 on the four. I've ordered a 44 for the four stroke which should here Monday. I'm hoping that the lower rpms at cruising speed will reduce the vibes a bit. At least it will slow the engine a bit which sounds like it's really churning. It's hard to be sure since fan cooled engines always sound like they're running faster than non fan equipped ones. So far I have about 100 miles on the two stroke and 20 on the four stroke. Both are running well, I like the centrifugal clutch and pull cord on the four stroke but find the lighter two stroke bike a bit more fun to ride. One change I'm planning on the four stroke is to build a better engine mount I plan on bolting two pieces of aluminum angle (can't weld it the frame is aluminum) side by side with the flanges outward to the bike frame. A couple of holes in the sides of the frame at that point won't affect the strength much since lateral holes in that area are really in a neutral stress point. If that isn't enough my next plan will be rubber shock mounts. One way or another I'm going to make the four stroke smoother and quieter.