Flywheel out of balance?

[olympiab]

1949 Dodge begins an intermittant but regular vibration at 35mph. I can feel the vibration if I rev up engine with car in neutral. Flywheel? Engine was rebuilt completely and did not have vibration before rebuild. Car actually has a Chrysler 251 engine that appears to have been a factory installation. No vibration damper. Does it need one?

Edited August 5, 2010 by olympiab
Better information

[Don Coatney]

If your flywheel were out of balance it would vibrate if you race the engine with the transmission in neutral as soon as you hit a specific RPM. From your description it appears the vibration is possibly drive shaft related. Have you swapped ends on your driveshaft? Have you tried bolting it in 180 degrees off?

[greg g]

What type of u joints is your car equiped with. the old cross and trunion type are prone to vibration as if they run lightly loaded on smooth roads, they don't get a lot of change in lenght due to up and down travel, and will work a grove in one place. As Don sugested if both ujoints are the same try swapping the shaft end for end. If the vibration is thee unbolt it from the diff, and with the trans in neutral rotate it 180 degrees and reattach. You might want to make some chalk marks so you can be sure but the bolt hole will only allign in two phases.