Lawnboy Mower Vibration

I have an annoying vibration in a Lawnboy mower that that can be felt in the handle.
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
Thanks,
Bill
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Try a new blade
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I installed the exact blade, flange, washer and nut that ran perfectly smooth on my D mower on the F mower and it still vibrated just as much. I took the exact blade, flange, washer and nut from the F mower and put it on the D mower and there was no vibration, so it is not the blade.
Thanks,
Bill
m Ransley wrote:

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bill wrote:

The blade would make a very small amount of runout very noticeable. That would be my guess.
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Joseph Meehan

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Look for and compare end shaft side movement, just because the new mower does not vibrate does not mean it was built as well as the old unit. A New blade would be the cheap and easy way to confirm a bent shaft or a loose tolerance on the main shaft bearing , that vibrates with a poor blade, plus you only have one blade now. I just threw one away that seemed balanced off the mower, I thought the shaft was bent. It does not take much to ruin a blade.
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bill wrote:

doesn't stop the vibration, then the problem is the motor. Probably a bent shaft even if you can't see any runout.
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The off balance doesn't have to be from the bottom. If you hit anything you could have a bent crank inside, out of sight. The lower crank bearing may be worn and centrifical forces from high RPMs could cause the crank to whip. Something may be loose or whatever on the flywheel. There are some flywheels that have weights added for balance. This is especially critical on 2 stroke engines because of the high RPMs.

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I agree, bearing could be a problem but the op didn't indicate the age. Still most likely cause is hitting something. I've never seen anything that could come loose from the fly wheel and it is usually protected. The only thing that usually happens is shearing the key, but that messes up the spark timing.
John Lawrence wrote:

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The F engine mower that vibrates was just built using mostly unused old stock parts. The short block including crankcase, piston, rod, etc. is all new. It was already assembled in the factory box. The flywheel might be used. I have wondered about the flywheel. Maybe adding mass to the other end of the shaft might give an unbalanced flywheel something to react against.
George E. Cawthon wrote:

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