My 68 RR had sat dormant for about 2.5 years due to medical issues. I'm back in business and took the car out . It ran fine except at speeds in excess of 65-70 MPH there is a substantial vibration.
I took my tires/wheels in for balancing. Still had the vibration
I took the car in for a front end alignment yet I still had the vibration.
Could the fact the the car sat so long that there are flat spots on the tires?

Unfortunately, it was not stored on jack-stands.

The car was in a heated garage out of the weather.

The tires are B.F. Goodrich Red-Lines from Coker - 215/70R/15
The tires have only about 10,000 miles but are about 15 years old.

My next look will be at the u-joints.

I can't think of anything else.

Any thoughts?

same deal here but I have changed EVERYTHING. I'm thinking on transmission tailshaft bushings. Vibration comes straight down the console. My transmision doesn't have the harmonic balance dampener/weight either and will be added too... if that helps

will be hearing ideas too

Not familiar with "harmonic balance dampener/weight" for a transmission?

Mine is a 4-speed. I rebuilt it about 2,000 miles ago and seems to be functioning just fine.

Thanks for the idea. I'll check my trans mount to make sure it's solid.

Can you swap the tires front to back or at least side to side to see if anything changes?
Although 10k isn’t many miles, 15 years is beyond the recommend service life time for tires.

Check the road force/uniformity of each mounted tire and wheel assembly. You can balance an out-of-round assembly but it'll still vibrate.
If the vibration comes and goes with varying speed (harmonic-related), it is almost certainly related to uniformity/out-of-round problems. Any decent tire shop
should have the right equipment to do a road force check although it is probably related to flat spotting, as you mentioned, if you had no
problems before it sat.

I had a part time vibration & it was not enough yoke engagement into the tailhousing. the right driveshaft fixed it.

15 year old tires that have sat in the same place for the last 2 years could have a problem caused by flat spotting, unfortunately they probably won't be cost effective to try to have the tires shaved to make them round again. The 1st warning sign when old tires go bad is a new vibration, usually the belts in the tires are loosing the bond with the tire plies. The next notification is a sudden worsening of the vibration, just before tire failure. Often times the time frame between the 2nd warning and failure isn't very far apart, often measured in feet travelled. Been there, had the destroyed fender to prove it.

If it were me, I'd put new tires on the car. Gene

U joint.

If you do decide to throw tires with 10,000 miles on them away, send 'em to Me: I'll put 'em to good use.

This pic was taken from the automatic transmission section on parts catalog. I guess manuals got the same

21-04-19 on diagram

in all my years, i have seen those damper weights only on automatics.
both 727's and 904's [plus their variations].

Of all things that will go bad with time, I'd say tires are number one.

Have you popped it in Neutral at speed to make sure vibration still exists, just to be sure?

Not OPs issue but I’ll throw this out for others following,
I had a nagging vibration at speed, tried everything under the sun, twice.
This was after swapping in a 518, turned out there are slight pitch differences in the 727 and 518 yoke splines.
I swapped in the 518 yoke and it cured my issue.

So for others fighting a nagging vibration, worth looking at the transmission yoke.
I had the DS balanced at speed twice and it never occurred to me to change the yoke until another member here PM’d about his experience.

That’s why they recommend checking runout at the front and rear of the driveshaft before doing anything.

RR, you, others - been a consistent issue with many posts here, excessive runout.

The other question that should first be asked - when did this all start? If it didn’t vibrate when parked then it did on first drive after hibernation, pretty much leaves out u joint phasing, driveshaft angles, bent shafts, etc.

I had the same after new tires. My trans guy said if good or new bushing in tail shaft with seal and new u joints and balanced prop shaft. It all went away.
He said the trans dampener keeps the bushing in good shape as,well as helps the rear engine mount (trans mount) in good shape.
I have run one ever since.
They must have spent the money on.them for a reason. So input tgem on now.
I did have one that was the harmonic balancer on the 340 (ie) no weights like,72 up that went bad. A,simple swap fixed it. It had also moved off zero 1st as well as the ring not being stright that should have been a clue to my cheap butt that they do get OLD and antifreeze can als o ruin tges rubber as,well.
They are very old and may look nice with New Paint but may be bad.
So check everything.

Unfortunately this part is no longer available.
I bet you’d have a hard time finding in JY, as stated, most people took these off and tossed them.

i'm sure something could be easily fabbed up and bolted to the tailshaft if a fellow knew the weight of that item.
of course it wouldn't look factory, but would serve the purpose intended.

I say your repro tires are the issue. Granted I've only been involved on two sets from a previous workplace but both sets of tires, even though new never mounted previously, were not round. And also of both vehicles, they were using stock steel wheels that did not spin true. Way out of tolerance. They would still balance though. Once they were loaded and at speed they vibrated. Since both customers supplied their own stuff, they had to live with it.

Even most tire shops can not get this through their head. It doesn't have to be round to be balanced.




I would try to borrow another set of wheels or 30 minutes on a wheel dyno can tell if the rear ones are round. Then swap front to back and repeat.

Well, Here's what I done so far,

First, for my own peace of mind, I put in a new U-joint at the rear end. I moved the tires front to back (not changing rotational direction.) Had an old time wheel alignment guy do a front end alignment (it was out of spec's but not terrible). I checked the rear trans mount to make sure that was not loose, It was OK.

I'm headed off to a car show tomorrow (about 65 miles - highway).

That will be the test. If that doesn't do it, I have to figure it's the tires.

Remember, the car didn't display this prior to the inactivity.

Just a suggestion, but I wouldn't risk a trip that long and on the highway to test the tires.

Here's an example of someone who learned the hard way about a tire coming apart. Fender and front valance were bent pretty good.

Might be time to try another set of tires.

Mike.