Bad vibrations

[69340]

Hi Fabo members . I had the motor in the 1969 Dart rebuilt this winter warmed over 340 .040 over kb pistons etc. motor was also balanced. while this was out i found a 391 posi unit and redid the bearings throughout. Had a 323 set before but wanted more for the track. also replaced all bearings and synchros in the A833 tranny. here is were the problem begins. while removing the engine i damaged one of my rubber motor mounts so i used a solid set i had used before. i also replaced my tranny mount with a new poly mount. i noticed during break in of motor that i had some vibration. i assumed it may be the solid mounts. I put the car in the garage lifted the rear wheels off the ground and put it in gear . the pinion was lifting up and down and it made a noise at the rear end. i checked the u joint and changed it with a new one. I removed the rear posi unit with the old one that worked fine still have the pinion lifting up and down. I removed both axles still same problem. front u joint is fine . I have a little play in the rear tail shaft but that is normal. the drive shaft has not lost any weights it appears to be running true. has anybody experienced this similar problem it seems to be pointing to the tranny but all the bearings have been replaced. any suggestions would be appreciated.

 

[70Swinger510]

i had that problem before when i put my motor and trans together, it turned out i had an interanally balanced motor with an externally balanced flywheel. so it threw the whole balance of the engine off. swapped the flywheel and it was fine.