This one has me pulling my hair out as multiple tests cancel out the solutions.

About 2 years ago I started to notice a slight vibration on my '75. It was really minor and with the tops off I didn't notice it at all. As time has gone by it's gotten worse.

When the car is cold and I first leave the garage I don't notice any vibration. After about 2 miles it starts to appear.

Cruising between 45 and 65 mph it comes and goes with perfect timing sort of like a harmonic vibration:
.....rrrrrrRRRRRRrrrrr...........rrrrrrR RRRRRRrrrrrr........rrrrrRRRRRRrrrrrr... ..

When I let off the gas it happens faster but not as pronounced:
...rrrrrr...rrrrr...rrrrr...rrrr...

If I put the trans in neutral (TH400) it's smooth as silk, which rules out the tires/wheels.

If I'm driving along in 3rd at 2500 rpm, the vibes are strong. If I come to a stop and put the car in 2nd and bring it up to 2500 rpm I don't notice the vibration.

If I'm in 3rd gear and drop into 2nd raisng the rpm but maintaining the same speed I no longer get the vibration.

If I'm driving along at 2500 rpm, drop it in neutral and then raise the rpm back up to 2500 rpm while in neutral I still get NO vibration.

I honestly can't tell where the vibration is coming from. I don't think it's the diff. or wheel bearings. Drive shaft should be ruled out. If it was a bad torque converter I think it would vibrate in any gear?

I thought for sure it was going to be an out of balance fan blade as I accidently bent one blade and straightend it several years ago. Sort of made sense with it not vibrating when cold. Last fall when I replaced the waterpump and radiator, I installed a new blade (but not the clutch) and the vibration was still there.

I'm now starting to lean towards the possibility of it being engine related and the vibration being transmitted through the exhaust?
Can this be fuel system or ignition related? I know that if the carbs are out of sync on my Corvair it'll cause a vibration and the same even goes for the throttle bodies on my fuel injected bikes.

Fwiw the car is mostly original and just turned over 80K miles. Only changes have been the removal of the emissions system, installation of Headman headers and an Allens dual exhaust system.

Any thoughts on this would be greatly appreciated!

Thanks for reading,
George

 

I wouldn't completely rule out the drive shaft. I bought my '80 in Jan. and it had a pretty wicked vibration under torque. Let off on the gas or throw it in neutral and nothing. The u-joints were spent. The front one especially. I changed them out and problem solved. I hope it's something that simple for you!

 

Sounds like it's in the transmission.Bad third gear from what your describing.