vibration while driving

[kingfish318]

I have a 73-318 Cuda. I had a 3.91 posi unit installed and now that it does turn more rpms, I have noticed that there is a vibration in the car. I have check the driveshaft for balance, angle, new U joints, etc....When going down a hill and thrown into neutral the car is smooth as silk, which eliminates all the above. When under power it vibrates. Its not excessive but enough to make you not want to drive it at 55 mph. It has a newer rear tranny mount installed. Back when the tranny was rebuilt I had a re-manufactured torque converter installed. (No weights) That was before changing gears. Could that be the culprit? Only 2500 miles on converter. Any other ideas what this may or could be? Almost feels like motor mounts touching frame, but is not. Thanks in advance for the help. Jeff.

[70chall440]

A 318 is a internally balanced engine so there should not be any weights, however this does not mean the TC is correctly balanced itself. Check your front balance to make sure it is correct, also make sure the flex plate is the correct one. Check the tail shaft of the trans to see if there is any play/movement.

[kingfish318]

The front harmonic balancer is new and is for a 318 internally balanced crank. The flex plate has been the same since the I have owned the car. I will check the tail shaft and report back. How do you check the tail shaft? Just grab and see if there is movement by hand?? Won't be till tomorrow or the next day, but I will let you know. Thanks for the fast reply!

[kingfish318]

Well I managed to sneak out and check. With a dial indicator I managed to get a reading just under .100". So when driving I would assume its got to be approx. 1/8". Slop was checked with the drive shaft installed. Seal is still good but is the longer nosed seal if that make any difference.

[70chall440]

I am not a transmission guy per se but that seems like a lot. How did you check the driveshaft for balance?

[kingfish318]

I had a shop do the balancing. After questioning other shops in the area recently I have found that in some instances their work is not up to par. So tomorrow its going to a reputable place for another check. I would feel better about that anyways. I am calling an old timer guru tranny guy and see if he can tell me anything about the .100" clearance. Otherwise, I will pull the tail shaft and just go ahead and replace that too. Thank you for the replies!

[70chall440]

Very welcome and sorry we could not get it sorted quickly. I think you have a good plan, let us know how it works out.

[kingfish318]

Well, turns out the drive shaft on the transmission end is .012" out of. It also had a dent in it approx. .050" deep. I missed that. He is making another as some part is not available any longer. I should have it back this week. Will post when I get it back. Is .012" out (wobble)and a.050" dent gonna make that much difference? He claims it will.

[70chall440]

It certainly can especially with any other play in the system. Think about this way, that drive shaft is very long and spinning very fast, if there is the slightest wobble it will exasperate any other issue that might be present such as play in the pinion bearing or transmission output shaft.

['Cuda Hunter]

It certainly can especially with any other play in the system. Think about this way, that drive shaft is very long and spinning very fast, if there is the slightest wobble it will exasperate any other issue that might be present such as play in the pinion bearing or transmission output shaft.

[kingfish318]

OK, after installing the new drive shaft it does run smoother. So,...apparently it needed one. Nice to be able to drive it again!! However it still vibrates a little. While putting the shaft back in the car I noticed the rear leafs (which are 47 years old) are almost straight across and I have coil over springs around the rear shocks. (Tried to avoid air shocks because they always leak) So, after a little research I see that is likely the other problem. I think the springs are weak and allowing the rear end to wrap up a little. Checked the pinion angle at the back and its 8* negative and the driveshaft is about 2*. When I looked at the pinion snubber I see it has the floor polished nicely. lol. Must be time for new springs. Anyone go through this or experience that?

[70chall440]

Yep sounds like the springs are bad, pinion angle should be more like 3* negative.

['Cuda Hunter]

Sounds like the true root of the problem.  Glad you have found it. 

[kingfish318]

While I was at it, I bought a new vibration damper. I got a rebuilt one with new rubber poured into it from Damper Doctor. I had a newer one from summit racing and after reading some articles on the net, I decided to buy one. I can honestly tell ya, that too made a difference. Smoothed things right out! Still need the springs though. I will post back when I get the ching and the results of the new springs. Thanks for the help.

[70chall440]

Great news, glad it is working out.