I have 2 lawnboy mowers. The older one (D engine) runs perfectly smooth. The newer one (F engine) has the vibration. If I run either mower without a blade, the vibration is slight and equal. Neither mower seems to have any runout when I look at the shaft while it is running bladeless or as I pull the starter and look at the shaft (not running). If I put the same blade, flange, washer and nut on each, the D mower runs perfectly smooth and the F mower vibrates.
So, it is not the blade, flange, washer or nut. It doesn't have any obvious shaft runout and the blade runs even on both sides as it is turned (one side is not higher or lower than the other). It doesn't vibrate any more than the good one when the blade is off. What could it be?
Please take the spam-bustin 7 out of my address if responding, so my e-mail address is actually calbert "at sign" and then mchsi.com