Weird Vibration at 2500 RPM's and above

[scott2683]

Ever since i bought this scamp it has a vibration above 2500 RPMs (no matter which gear you're in). I've put new wheels and tires, had the driveshaft balanced, new u joints, and put in a new transmission mount. I still have the same vibration. The guy at the shop that balanced my driveshaft told me the slip yoke looked like it was wore out a little bit and I may need a new tail shaft bearing.

Sometimes if I accelerate rapidly it doesn't vibrate, if I get the car about 3500-4000 RPM's it doesn't vibrate, or at times if I'm cruising down the highway, if I pop it out of 4th gear, and put it back it will stop vibrating. Does this sound like a bad tail shaft bearing?

I know this is one of those issues that you have to keep throwing money at until you get it figured out. But I would appreciate any advice I could get.

The car has a 360 4 speed for reference.

 

[Alaskan_TA]

A pal with a 4-speed had a similar vibration, she also had the drive line balanced & new u-joints put in. The problem got worse.

It turned out that her drive line was bent, I checked it with a straight edge.

Easy to check, just in case.

A new drive line & no more problem in her case.

 

[scott2683]

Were they not able to tell that it was bent when she had it balanced it? I guess I just assumed that was something they checked for. However I will still check mine just to make sure.

 

[Killer6]

1) Will the vibration occur at rest w/the trans in neutral & the RPM's in the offending range?
2) Generally a bent driveshaft won't unbend itself or stop vibrating popping the trans out of & back into gear, nor simply loading it.
3) If You grab the installed prop-shaft at the front, how much can You move it up & down?

 

[scott2683]

1) Will the vibration occur at rest w/the trans in neutral & the RPM's in the offending range?
There is no vibration in neutral at any RPM range.
2) Generally a bent driveshaft won't unbend itself or stop vibrating popping the trans out of & back into gear, nor simply loading it.
When I had my shaft balanced, they told me that it balanced out perfect.
3) If You grab the installed prop-shaft at the front, how much can You move it up & down?
There is some play in it. I can't recall how much, but I was able to move it.

 

[Killer6]

I'm inclined to think the input/output bearings are worn, &/or the pilot bushing. You clearly are certain it is eng. RPM related, "no matter which gear" means it is not the prop-
shaft or output shaft, nor the rearend. A bad clutch disc hub could cause it, but it would still do it in neutral, unlikely it would gyroscopically "center" itself every time it was
engaged in neutral....not impossible but............. I have seen accidental downshifts to too low a gear bend the pressure plate torque straps( they're meant to be pulled on
not pushed on), when they fold up the plate is not centered anymore, but that would not go away as per above. What does simply pushing on the clutch at speed do, &
have you attempted to depress the clutch & maintain the RPM's?

 

[scott2683]

When I push in on the clutch at speed, the vibration seems to go away or at least get better. When I push the clutch and maintain RPM's the same thing happens.

Thanks for the help! Getting pointed in the right direction will surely save me money!

 

[rustycowll69]

are the trans, driveshaft, and rear axle original to car? What I'm getting at, is this a mashup of pieces, and the driveshaft is too short, and not giving sufficient slip spline engagement? Does it have the correct unbalanced flywheel for your 360?

 

[scott2683]

are the trans, driveshaft, and rear axle original to car? What I'm getting at, is this a mashup of pieces, and the driveshaft is too short, and not giving sufficient slip spline engagement? Does it have the correct unbalanced flywheel for your 360?

The engine, transmission, and rear end are not original to the car. I've checked the length of the driveshaft when i had it balanced, and it seemed to have plenty allowance. As far as the fly wheel goes, I have no idea. Unfortunately the guy I bought the car off of wasn't the guy who built it either.