I'm sorting out the problems on the 74 260Z I bought last month. Here is one that I need some more help in coming up with a definitive diagnosis.

This car has a stock 4 speed and (I assume) a stock differential with stock rear end gears. There are no outward signs that either have ever been replaced.

On accelerating through the gears, if I shift from 3rd to 4th around 50 to 55, I get a strong vibration, almost like the car is going to shake apart. If I shift early, say around 40 mph, and accelerate smoothly, the vibration does not appear until I let off the gas some. I could also get the vibration to appear at 35 mph when I let off the gas to prepare to turn.

I have looked at the trouble-shooting suggestions in a 73 FSM, a Clymers manual, and a Haynes manual, as well as searching online. Aside from changing the differential gear oil (which I did), it almost sounds like a parts swapping festival until you find the culprit. I'd rather avoid that.

I'd appreciate any suggestions to narrow down the trouble area.

Thanks!

First guess - U-joints. Check for play there, probably in the driveshaft u-joints, less likely to be the half-shaft joints. If you find any play in any of them, I'd replace all six.

Edit - I did not mean to imply that the half-shaft u-joints are less likely to be bad, but that they may be less likely to cause the type of vibration you describe. I would recommend that you check all six.

Thanks, Arne. It's one of those things that I'm not going to figure out myself, but as soon as you said to check the driveshaft, I realized how that could be the culprit.