drive shaft unbalanced?

I am trying to fix a vibration my car has at the top of its shifts and at high rpm. I have tried the engine in neutral and it doesn't seem to do it. I replaced my u joints but its still there. Is it possible that my drive shaft is unbalanced. If so how does the effect it when it is at high rpm in second or first?

 

Sounds like a tranny problem to me David. Is it an auto or manual?

 

Its a c-4 automatic. I had a rebuilt one put in about 2 years ago so I was hoping it was not it. Would it be a major problem then?

 

Some cars have a vibration, most don't.
In the Comets, the factory had a small flywheel on the front 'U' joint to help. It could be the drive shaft. Raise the rear and hold a chaulk nearly touching the shaft in the center third while turning the shaft to see if you can detect any runout.
Could be trans mount.

 

I suppose since it is a 3-speed you might simply not be reaching the RPMs in 3rd to make it vibrate. In either case, if it were the driveshaft there's no way it would be RPM related. It would be vehicle speed related in that case.

So you are looking at the engine or torque converter most likely. Take a look at your front dampner.

 

well i guess i fail to mention it also does it at about 80mph thats important but i forgot to tell you

 

Balance: I'd visually look at the harmonic balancer while the engine is being revved to the rpm you're experiencing the vibration at. See if there is any 'wobble' in the balancer. Check to ensure the balancer retainer nut is tight.

I agree with others regarding the imbalanced drive-shaft. Since the drive shaft turns at a different speed than the crank, it shouldn't have any corelation to the rpms of the engine . However, it's not a bad idea to check that anyway

Also, I can't imagine why this would change, but the flex-plate may be out of balance (as it has the offset weights for crank blance - like the harmonic balancer in front)

Elctrical: I'd also check to make sure all the plug wires are good (yes, an intermittent miss can happen at higher rpms with a faulty wire).

Fuel: Carb providing correct amount of fuel at that rpm?.

Tranny: If it's in the tranny - good luck (I know VERY little about those. I depend on my tranny man for that. All those valleys and springs and bands and gears and seals and etc scare hell outta me).

Good luck and keep us posted on the culprit.

 

I will try all the tips u guys gave me, and I appreciate the help very much. I will keep you posted on what has been done.

 

Torque converter. mine did the same thing. I really thought it was a drive shaft problem. i did the whole U-joint and balancing thing. didnt help. I was going to live with it wasnt that bad i thought. then one day my tranny puked all of its fluid out and a great smoking glory on the way to work. when i tore it down to rebuild, it had tore the pump all to hell