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1971 GTX vibration issue "UPDATED PROBLEM SOLVED"

Posted By: AfterHoursResto

1971 GTX vibration issue "UPDATED PROBLEM SOLVED" - 12/24/12 05:24 AM

I just finished a rotisserie restoration on a 1971 GTX 440 auto 8 ¾ with a 323 posi. I am experiencing a on and off type vibration with begins at 40mph - 65mph. It may continue past 65 but that is as fast as I have driven the car since completed.

I have placed the trans in N while rolling down the road and the vibration continues even though the engine drops down to an idle.

I am running tapered bearings in the rear axels set at .010 .012. The bushing was replaced in the tail shaft of the trans during its overhaul. The drive shaft is brand new and I also tried to but the old shaft in and it did not make a difference.

The center section is a 489 case and was rebuilt using the original ring and pinion.

Everything is brand new or rebuilt on this car. It was restored to factory specs with no performance modifications. I am stumped.

Any input would be appreciated.

Thanks
Posted By: HEMIDOG 70

Re: 1971 GTX vibration issue - 12/24/12 05:27 AM

Tire balance?
Posted By: screamindriver

Re: 1971 GTX vibration issue - 12/24/12 05:33 AM

Also,you'll need to check the driveline angles and see where you're at...From here it's going to be a process of elimination...Start with the easy stuff...You've already taken the first step...No change in the vibration throwing the car in neutral is the first step...And I'm assuming there's no vibration in park... So it's not an engine balance issue..
Posted By: flypaper

Re: 1971 GTX vibration issue - 12/24/12 06:02 AM

on a 71b body
there is a weight that bolts to the floor above the driveshaft.
it is there to help driveshaft vibration.
is that weight missing?

i can get you a pic of it,
if you need to see what it looks like.
Posted By: RSNOMO

Re: 1971 GTX vibration issue - 12/24/12 06:29 AM

Can you load the suspension on jackstands and run the car???
Posted By: AfterHoursResto

Re: 1971 GTX vibration issue - 12/24/12 06:50 AM

I have tried two sets of tires. A brand new set of G 60's and the old set of radios the car came with and there was no change.

The driveline angle is at 2.5.

Thereis no wieght on the tailshaft or floor board. Would like to see a pictures of one!
Posted By: MikeyT

Re: 1971 GTX vibration issue - 12/24/12 07:58 AM

Could it be a bad u joint? I had the very thing happen last spring and it was a bad u joint. Also I thought pinion angle was suppose to range between 2.5-5.5 nose of diff down.

Mike
Posted By: dangina

Re: 1971 GTX vibration issue - 12/24/12 01:50 PM

Quote:

on a 71b body
there is a weight that bolts to the floor above the driveshaft.
it is there to help driveshaft vibration.
is that weight missing?

i can get you a pic of it,
if you need to see what it looks like.



could you take a pic? I'm in the middle of putting my 71 runner back together and I haven't seen or heard of this weight?
Posted By: BigMoneyLewis

Re: 1971 GTX vibration issue - 12/24/12 02:10 PM

My first thought would be a wheel out of balance . I know you said you tried two sets of tires , but are you still using the same wheels, if so have they been balanced on a machine ?

My second guess would be a driveshaft out of balance .

Greg
Posted By: RapidRobert

Re: 1971 GTX vibration issue - 12/24/12 03:34 PM

I know you changed shafts but that'd be my guess, shaft or ujoints. How much freeplay does the shaft have into the trans? Want ~3/4" with the common aftermarket flat extension housing seal. I had too much & had a similar vibration that came & went. The correct slightly longer shaft took care of it. Holler how it goes
Posted By: moparmarks

Re: 1971 GTX vibration issue - 12/24/12 03:36 PM

Quote:

Quote:

on a 71b body
there is a weight that bolts to the floor above the driveshaft.
it is there to help driveshaft vibration.
is that weight missing?

i can get you a pic of it,
if you need to see what it looks like.



could you take a pic? I'm in the middle of putting my 71 runner back together and I haven't seen or heard of this weight?



The weight was not used on 71/2 HP cars. Just LP cars. It mounts in the top of the tunnel right at the seam between the rear footwell and the under rear seat pans. Has a plate with 3 studs welded to the floor and the weight mounts to the suds under the pans.
I guess I don't have pics but can get some.
Posted By: AfterHoursResto

Re: 1971 GTX vibration issue - 12/24/12 03:53 PM

Good suggestions everyone! Thanks.

Yes, the tires are mounted on two sets of rims. One set is about a year old and the other are brand new. So I don’t think it is the rims or tires.

Both shafts I tried have new u joints.

However, I do have more than ¾ sticking out of the tail shaft. Maybe someone swapped drive shafts out before I got my hands on the car.

Does anyone have the correct dimensions for a stock factory driveshaft (71 GTX 440 auto 8 ¾)?

Thanks
Posted By: DANA60

Re: 1971 GTX vibration issue - 12/24/12 04:52 PM

Here's a pic of that weight, not a good one, but all the way at the top of the photo. I don't think it's that.

If you have eliminated tires and wheels, how about a bad axle, disc brake (rotor)? Can you put it on a lift and run it? This may show if it's front or rear. You can even test it without tires and wheels. How was it before resto? The 3/4 inch probably is not a problem (keep that in mind till later).

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Posted By: fourgearsavoy

Re: 1971 GTX vibration issue - 12/24/12 05:02 PM

Quote:

Here's a pic of that weight, not a good one, but all the way at the top of the photo. I don't think it's that.

If you have eliminated tires and wheels, how about a bad axle, disc brake (rotor)? Can you put it on a lift and run it? This may show if it's front or rear. You can even test it without tires and wheels. How was it before resto? The 3/4 inch probably is not a problem (keep that in mind till later).


I have seen brake drums out of balance causing your exact problem
Gus

Attached picture 7516704-savoyburnout.jpg
Posted By: AfterHoursResto

Re: 1971 GTX vibration issue - 12/24/12 05:20 PM

I am getting ready to lift the rear, take the tires off, then the drums and see what happens. I will let you guys know. Thanks
Posted By: fourgearsavoy

Re: 1971 GTX vibration issue - 12/24/12 09:31 PM

Quote:

I am getting ready to lift the rear, take the tires off, then the drums and see what happens. I will let you guys know. Thanks


Just remember don't step on the brake pedal with the drums off
Gus

Attached picture 7517035-savoyburnout.jpg
Posted By: DANA60

Re: 1971 GTX vibration issue - 12/25/12 03:44 PM

Yes, and be carefull not to put the car in reverse or park right away after run up. There is no way that the drivetrain will stop on it's own. It's important to make sure the drivshaft stops turning before dropping out of drive. Maybe even turn the car off before that

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Posted By: RapidRobert

Re: 1971 GTX vibration issue - 12/25/12 04:03 PM

Quote:

Maybe even turn the car off before that

With the drums off that's what I'd do, as you know it can still be turning a bit in neutral. Easy to get fooled since the car ain't moving your brain automatically thinks it's OK to move the selector to a different position. Yes you'd have to have alot more than 3/4" end play for that to be the problem like it was in my case & in diagnosing that problem I noticed that the yoke had to be engaged quite aways in on the output shaft spline till it felt solidly engaged which I thought was strange. I think that was because not all of the inside length of the yoke is splined & that distance varied from yoke to yoke. Holler how things go.
Posted By: moparmarks

Re: 1971 GTX vibration issue - 12/26/12 10:28 PM

71 GTX 440/727/8.75 drive shaft is 50.75" center between joints.

Here is the non-HP weight and mounting bracket.
I've had/have upwards of 20 71/2B HP cars and none had this weight. I did just partout a 73 Charger 400HP car that has this weight as well as all my 71/3 non-HP cars.

Attached picture 7519451-DCP_0057.JPG
Posted By: moparmarks

Re: 1971 GTX vibration issue - 12/26/12 10:29 PM

Mounting bracket welded onto the pans.

Attached picture 7519452-DCP_0056.JPG
Posted By: 318 Stroker

Re: 1971 GTX vibration issue - 12/26/12 11:08 PM

I had the same problem. You may want to have the driveshaft re-checked. I had 2 new d-shafts from local d-shaft companies. Both fresh and "balanced".

Guess what...? They weren't...
Posted By: AfterHoursResto

Re: 1971 GTX vibration issue - 12/29/12 04:06 AM

Thanks for all the suggestions and pictures. I had to step away from this issue because of the holidays but will get back on it tomorrow. I will keep you posted with what I come up with! Thanks.
Posted By: gdonovan

Re: 1971 GTX vibration issue - 12/29/12 12:34 PM

Quote:

I had the same problem. You may want to have the driveshaft re-checked. I had 2 new d-shafts from local d-shaft companies. Both fresh and "balanced".

Guess what...? They weren't...



They can't spin them fast enough, I ran into this as well. Most truck shops only spin 2000-2500 rpm since the big diesels don't go any higher than that. Their eyes just about popped out of their head when I informed them that would not even cover my cruise speed and driveshaft revolutions over 6000 rpm were common.
Posted By: rth

Re: 1971 GTX vibration issue - 12/29/12 10:34 PM

Quote:

Quote:

I had the same problem. You may want to have the driveshaft re-checked. I had 2 new d-shafts from local d-shaft companies. Both fresh and "balanced".

Guess what...? They weren't...



They can't spin them fast enough, I ran into this as well. Most truck shops only spin 2000-2500 rpm since the big diesels don't go any higher than that. Their eyes just about popped out of their head when I informed them that would not even cover my cruise speed and driveshaft revolutions over 6000 rpm were common.




There is a difference between eng rpm's and d/shaft revolutions. d/s is measured by mph. 2500 rpm's could be 60 mph depending on gears and tire size.
Posted By: AfterHoursResto

Re: 1971 GTX vibration issue - 02/11/13 02:28 AM

Hey Guys, I shelved this issue for a while but finally got back to it. So the issue was the big ball bearing that rides on the output shaft in the tail shaft housing. I did get lucky as far as it being an easy fix. It really sucks to tear apart a freshly restored restoration.

Thanks for all the suggestions and pictures. The vibration is gone!

Roman



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