High RPM vibration

[73challenger_TN]

Hey guys I haven't been around in awhile. I lost some interest in hot rods but this warm weather has sparked my interest again. Anyways I have had a vibration in my car for awhile. When I installed my new engine I had a vibration that was from a 360 flex plate(balanced) with a torque converter that had weights. So anyways my vibration now is really noticeable if you get up to about 70 and let of the gas. It's a very rough vibration that seems to be in the drivetrain. I'm thinking it is either my driveshaft needs balancing or my chain torque strap I have installed. I do not feel the vibration cruising at 70. Thanks for your advice!

[femtnmax]

Does the car vibrate as you accelerate up to and beyond 70mph, or only as you decelerate from above 70mph??
If only on decel, this could be from a bad u-joint cocking to the 'decel' position and throwing the driveshaft out of balance.
If only on decel, accelerate the car to above 70mph, and put the trany in neutral as you decel.  Is the vibration still there?   Also try with engine held steady at mid-rpm, trany in neutral, and decel.  Again, is the vibration still there?

I always balance driveshafts as a matter of course.  Sounds like this has not been done...Take the driveshaft out, check both u-joints for play.  If age of parts is unkown and there is the slightest question on the u-joints then I'd replace them both and have the complete driveshaft rebalanced.

[73challenger_TN]

I wish it wasn't raining so I could try a few different things. I have not noticed any vibrations while accelerating only on decel. I have not balanced my driveshaft...in fact it was pretty rough when I bought it for my 727 swap. I do not know the history of the ujoints. Sounds like this is my problem! Thanks

[femtnmax]

If the driveshaft tube is true and round all over, and the end yokes are good, then the factory balance should still be ok.  New u-joints should not change the balance; very little if anything.
Clean the entire driveshaft good and clean, a bench grinder with wire wheel will do a great job.  Then install new u-joints, and run it in the car and see if the vibration goes away.  If it works out, you saved the cost of rebalance.   

[IMNCARN82]

Check the rear yolk on the insides.Where the clips wear small grooves.It may only move just a tiny bit in one position.Or only on one side too.  Pinion angle?  I went through the whole rear end to find an out of balance engine. Front yolk play.  Random thoughts...       I've been fighting a vibe. R/T

[73challenger_TN]

Well good news I think. I pulled the driveshaft and it has a dent in it. I am going to take it Firday to have it balanced and have my Spicer ujoints installed.

[femtnmax]

Well good news I think. I pulled the driveshaft and it has a dent in it. I am going to take it Firday to have it balanced and have my Spicer ujoints installed.

If the dent is bad enough, I would consider having a new tube welded to your old ends if they are still good.  Glad to hear your making progress.   
Street racing downtown Missoula, I saw a Camaro drop a driveshaft while moving, just like in the movies it was not a pretty picture, tore the car up pretty bad as the car went sideways off a tall curb.  Kinda made me think about u-joints a little more.

[73challenger_TN]

  It fixed it! It's really smooth now. I had new ujoints installed and balanced. I ordered the wrong dang ujoints though. My car has the small ones. I am just going to put them on the classifieds later.

[IMNCARN82]

I always like to hear about someone fixing something. Glad it was relatively minor.   I bet some
would spark your interest some more too!