Seems unstable at top speed...

My 1969 GS seems like the front end lifts and it vibrates some under WFO throttle in fourth gear. It has the 3:64 but I'm not sure its safe to red line in fourth right now. Tires have been balanced twice, but they are old original 14" rims and the alignment was done the Riviera specs for some reason. Any help would be great. Thanks, Mike

 

If it were mine, I'd make sure the tires are not old and I'd have the alignment done with as much positive caster as they can safely put in it.

What speed where you going? I've had my car to 140MPH and it definitely gets light above 100MPH. A good set of tires and an excellent balance job are crucial to stable high speed runs.

 

No I'm talking about maybe 70 mph, it seems to maybe lift? Maybe vibration or out of balance.

 

Driveshaft may have a slight bend in it

 

Drive shaft goes in only one of two ways?

 

70 should be smooth as glass. Check out your motor and trans mount. They move around quite a bit under load

 

Maybe driveshaft needs a balance and new u joints.
Do you feel it on the steering wheel

 

Yes, I feel it on the steering wheel. I seem to feel it in second gear also? The driveshaft was balanced years ago and the tires twice in the last year or so.

 

Maybe tie rod ends
Not sure

 

Greg, Read a post of yours from two years ago. I need to check but it will likely vibrate in neutral at second gear speed.

 

have tires balanced on a lug centric machine, most shops now use hub centric and our wheels are not centered by the hub, it's the lugs that center the wheels. if they are balancing them on hub centric they may not be balanced for your application at all