1968 Charger with a built 440. If I am cruising/rolling at say 60mph 2500rpm and I just bairly crack the throttle (1/8 throttle) I feel a vibration. But if I close the throttle its gone and if I give it more throttle its gone. Only at that one spot does this occur. Anything below 60 and I can't make it do it.

It feels like the transition between loading and unloading the drivetrain. Any ideas where to start...transmission tailshaft bushing, driveshaft??

Trans mount, motor mount. Check them 1st.

Brand new motor mounts, old trans mount. I will change the trans mount first and see what happens.

Why does it occur at only one point in the throttle position and only at speeds above 60mph? Its not a constant vibration....just weird.

How about beyond that 1/8" of throttle?

Try it down one gear. This will let you know if it's engine or driveline related.

No vibration beyond 1/8 throttle or closed and coasting. I have not seen what it will do down a gear. I suspect the extra load and rpms might nulify it at that speed. But its sure something I will try.

It sure does not feel like a motor miss but more like a vibration in the seat. We are pushing 500hp with an old trans mount and a driveshaft that has not been rebuilt.

I just always thought a u-joint issue would be a constant vibration?? It just seems like something gets to shaking just a that throttle position before I load the drivetrain, just not sure what.

Well I went under the car and found the rear u-joint has some play and the trans mount is pretty bad also......

I think I may found the issue. Thanks guys.

Since you say it's 2,500 rpms at 60 mph, try doing 2,500 rpms in second gear and see if it has the same vibration. It could possibly be a lean spot in the carburetor transition when you apply the accelerator. It caused a vibration in my car. Or it could be the distributor advance needs tuning.

That is something else I can try. The RPM can vary, it just seems to be anything above 60mph just when I crack the throttle. I do have a AFR gauge in the car so I can get it to vibrate and see if its really lean at that point.

Standard distributor curve, initial 20 total 35, all in by 2500rpm.

My bet is if you pull the drive shaft and inspect your u-joints you will find the problem. I did drive lines at a dealership for years. Many drive train vibrations varied with throttle load. Case in point. Last year going from Phoenix to Las Vegas during the Mopars on The Strip cruise my buddys 64 Fury picked up a vibration only on light accel. It became worse on the return trip. Took it apart,rear u-joint had no rollers in 1 cup.
Doug

Thanks DVW. I suspect that is it since it has gotten a little worse over the last 12 months, and it acts exactly how you describe. Not to mention I was just under it and there is some play in the rear u-joint.