Your vibration may be down to too much clearance in the gears. There should be a maximum of 1mm movement on the tip of an 18" propellor. The age of the motor would determine how you increase the gear mesh. If the crank carrier is square then thereare aluminium shims behind the crank carrier. If you need to increase the gear mesh then simply remove one of the shims and try again. If with all the shims removed there is still too much clearance then you need to machine the face of the crankcase. We strongly recommend that you return the motor for this procedure to be done, it must be done correctly.If it has a gear carrier which is rectangular with four slots the gear mesh is adjusted by swinging the plate clockwise to make the meshtighter. This should solve or help with your vibration problem as it is clatter on the gears which is giving you the vibration. On the older motors there was a different gear form which was not so good.