I have converted one of these with a CH ignition and prop hub. It has a mid range, 4000 to about 5500 rpm shutter/vibration with all four 15" props I have tried. The 16" prop does the same at lower speed. It is a very significant and somewhat scarry vibration. The hub dial indicates to about 0.002" all the way out to the end of the prop shaft. I JB welded all thread to be sure it stays true. The props are new and seem to be well balanced. Is this a characteristic of thes engine I need to accept?