Vibration question

I have a 70 GT6+ and I noticed that when putting power to the rear end I get a vibration. At 60 the vibration is worse then at 30. It also depends on the gear that I am in. If I am doing 50 in third it is worse then if I am doing 50 in forth. If I let off the gas the vibration goes away. We just put a new clutch in the car and also the car sat for 3 years.

The 4 items I can think of are:
1. It is the drive shaft is out of balance

2. The rubber donuts are going bad in the rear

3. Tires are out of alignment (but wouldn't this still cause the shake with the power off)

4. Did something wrong when putting the drive shaft to the gear box. I marked the fittings before taking it apart and put them back together in the same holes.

What are your guys thoughts on this? What else should I be looking at.

I had that once and it was U joints. Drove me nuts for a long time before I found it.

I tend to agree with Adrio here: probably propeller shaft u-joint(s). A slim possibility is that the propeller shaft was disassembled at some point and then reassembled with the u-joints "out of phase" (possible to do with the sliding joint on these). That's worth checking. Slimmer possibility, but still possible, is a front wheel bearing problem.

Oh, and I'd also take a look at the gearbox rubber mounts!

Car sat for 3 years, so were you able to drive it before the clutch replacement in order to know that it didn't vibrate? Sitting on tires for that length of time could cause flat spot vibration, but intensity wouldn't change relative to gear selection or power on/off.

Might be any of the 4 you listed, OR, the new clutch may be out of balance. Do you get vibration sitting still, out of gear, with engine reved?

Did you replace the trans mount or replace any of the u-joints? U-joints assembled 'out of phase' will cause a vibration. Bad trans mount or loose mount bolts could be the cause.

If all that was changed was the clutch and all else was reinstalled as it came apart, IMO the clutch is the culprit.

Go back through every thing and make sure all bolts are tight first. No fun snatching the clutch again only to find out a bolt is loose.

I will be intrested to learn what you find as the cause.

I am sure the tires don't have a flat spot because the car was on jack stands off the ground. As for did it vibrate before it is hard to say because it didn't get driven more then a couple miles home and then the clutch got put in. I didn't replace the trans mounts, they seemed to be ok.

I will go back through the mount bolts and make sure they are tight.

If it changes with engine rpm at a given road speed it seems it would be coming from the transmission forward. Which includes the clutch and engine. Could be as simple as a worn or missing pilot shaft bushing, or like I had recently with a broken part in the pressure plate.

Well, I got a bad feeling it is in the clutch area. When I take my foot of the gas or press in on the clutch the vibration goes away. Also it happens at higher RPMs no matter what gear. So this tells me it is before the transmission. So it has to be clutch. Most likely out of balance or the pilot is worn more then I thought.