Trying to track down bad vibration

[Jim S]

I still have a bad vibration from about 35mph on up.  I can rev the engine in neutral up to 3,000 rpm and it's smooth as silk. But driving in gear the car vibrates.  I put it on jacks and removed the rear tires and it still vibrates running in drive, so it's not tire imbalance.  I also replaced the ujoints and the motor mounts and rear transmission mount.  I'm baffled!

[Gunslinger]

The driveshaft may need balanced.  

[Jim S]

3 hours ago, Gunslinger said:

The driveshaft may need balanced.  

That's the only thing left I can think of.  There's a shop about 50 miles from my house that can do it. If that doesn't fix it I don't know what will!

 

[64StudebakerAvanti]

How about driveline angles? I had a similar issue with a 64 4spd and it was due to the spacers missing.

here is discussion:

https://4xshaft.com/blogs/general-tech-info-articles/driveshaft-angles?srsltid=AfmBOor0wf1cMqW-8ZB44Bme-ZMX7e3iMTewwuTZlsJauargt1YjTgVj

 

[64StudebakerAvanti]

Not sure for Avanti IIs, but check that engine and transmission mounts are in the correct holes. For Studes, there are 2 sets, 1 for LHD and the other for RHD. 

 

[mfg]

21 hours ago, Jim S said:

I still have a bad vibration from about 35mph on up.  I can rev the engine in neutral up to 3,000 rpm and it's smooth as silk. But driving in gear the car vibrates.  I put it on jacks and removed the rear tires and it still vibrates running in drive, so it's not tire imbalance.  I also replaced the ujoints and the motor mounts and rear transmission mount.  I'm baffled!

Is the vibration a recent development… or has it been there since you purchased your Avanti?

[Stacey]

Had a 1967 Impala SS that developed a vibration, my first car. Thought tires/wheel balance, did not fix it. My dad said he thought a U-joint was binding, removed the drive shaft and sure enough the rear joint was dry and locking up, was not loose, but tight. Back then we were able to buy a new joint for about $5.00 and he and I replaced it, vibration was fixed. Might be worth looking at the u-joints.

[Jim S]

On 11/27/2025 at 9:20 AM, mfg said:

Is the vibration a recent development… or has it been there since you purchased your Avanti?

It has been there since I purchased it.  Previous owners took the body off the frame to paint and never finished the car after they put the body back on the frame.  That's how I got it.  A mystery car.

[Jim S]

On 11/27/2025 at 4:45 AM, 64StudebakerAvanti said:

How about driveline angles? I had a similar issue with a 64 4spd and it was due to the spacers missing.

here is discussion:

https://4xshaft.com/blogs/general-tech-info-articles/driveshaft-angles?srsltid=AfmBOor0wf1cMqW-8ZB44Bme-ZMX7e3iMTewwuTZlsJauargt1YjTgVj

I did notice the vibration increased more at lower speeds than before after I replaced the transmission mount with a new one.

 

[Jim S]

A friend suggested the torque convertor could be out of balance.  But everything is smooth reving the engine up high.  Any thoughts on this?

 

[mfg]

1 hour ago, Jim S said:

A friend suggested the torque convertor could be out of balance.  But everything is smooth reving the engine up high.  Any thoughts on this?

 

Highly unlikely the problem is converter…. If you have the 400 CI engine, the flex plate could have tossed a balance weight, but you would feel that right away just revving the engine in neutral.

Edited 8 hours ago by mfg
Added text

[Zedman]

Here are a couple of photos of my '81's Rear Springs as removed. The general condition of the Rubber pads and Aluminium Wedges is clearly very poor due to age and Corrosion from salty roads.  One wedge was nothing but crumbly muck.  I'd imagine your vibration could be attributed to something like this.  I will be replacing the Wedges with new ones once I have the car assembled again and I can properly measure the Pinion Angle. There was only enough of the 'good' wedge left to measure an approximate 2 degree tangent. The  rubber Insulation  pads are available from SI- Page 261 of the current catalogue ... Part no 554620. Wedges are not listed, but are available elsewhere on the Web.

Edited 3 hours ago by Zedman

[mfg]

On 11/26/2025 at 12:48 PM, Jim S said:

I still have a bad vibration from about 35mph on up.  I can rev the engine in neutral up to 3,000 rpm and it's smooth as silk. But driving in gear the car vibrates.  I put it on jacks and removed the rear tires and it still vibrates running in drive, so it's not tire imbalance.  I also replaced the ujoints and the motor mounts and rear transmission mount.  I'm baffled!

You might try checking for excessive up and down and side to side play at driveshaft slip joint where it enters trans tail housing…. The bushing it rides in could be shot …. Is your Avanti a high mileage car?

[mfg]

Also.. most driveshafts have a weight welded on when factory balanced… Is there any evidence on shaft that welded on weight is missing?