Engine Vibration @ 2200-2700 RPM

[LAA66]

 This vib just showed up after the last two drives. It happens in all 4 gears, accel and decel, vehicle speed does not affect it. Not really noticeable in neutral though. I did a quick check and tightened the muffler hangers (which caused a similar problem before) with no changes. Fan is a newer flex unit. Any ideas?   

[MEK-Dangerfield]

So you didn't change anything recently?

It's odd to have the vib at a limited RPM band and not be speed related. My only thought is the driveshaft is unbalanced, but if you've haven't had it off recently...

[PlumCraZRT]

So you didn't change anything recently?

It's odd to have the vib at a limited RPM band and not be speed related. My only thought is the driveshaft is unbalanced, but if you've haven't had it off recently...

If it's only engine speed (not car speed) the driveshaft wouldnt be the issue.

Maybe you lost some mallory metal from the crank (if it was balanced using mallory metal).  Is the engine internally or externally balanced?

[LAA66]

 Replaced the valve springs a few months back, had the plugs out at the time, also did a idle tune with the vacuum gauge, that's about it. The vib is not real bad, but is rpm related. First thru fourth gear at at around 2500 then goes away. Looked over the fan, pulleys, belts, and dampener from what I can see it all looks good.

  The motor was balanced internally, has the forged crank, pistons, and DC rods. The dampener is the original and rebalanced to this rotating assembly along with flywheel and pressure plate.

[Chryco Psycho]

It has to be in front of the trans , so clutch or engine , I cannot think of anything that could change , if the flywheel is balanced it cannot change , so it has to be a problem with the clutch or engine , the clutch could change or something internal like mallory metal , or a slipped damper ring is all I can think of .

[LAA66]

 Here's a few thoughts regarding this vibration problem:

 Water pump/alternator/PS pump going out.... Is it safe to run the engine with the belts removed for a minute or two?
 
 Harmonic Balancer - it's 42 years old.... Would a  timing light show a slipped outer ring?

 Exhaust related resonance..... Everything is tight externally could something inside the muffler cause a vibration?

 Valve train... Bent pushrod or sunk lifter. Vacuum gauge reads fairly consistent with previous tune ups. Some fluctuations but ok considering the cam PS 284 /.484

 Clutch, Zoom diaphragm type... Seems to work fine, what should I look for? 

 Jon
 
     

[Chryco Psycho]

 Yes you can run the engine without belts to see if it is an accessory causing the problem .
The TDC mark on the damper should be off if the outer ring slipped which is not uncommon on 4 spds , a timing light will show this .
 Does the clutch feel weird at all , sometimes they have weights in or on them to balance the unit .
Sometimes a baffle inside the muffler will become loose breaking away from the welds etc but I would think you woul dhave a miss from exhaust restriction if it was bad enough to feel a vibe .
Again with the valve train the engine would have a miss & run poorly & would not smooth out at different RPM