68 Vibration

[Blue68Coupe]

My 68 coupe 289 C4 Auto has had a vibration that goes through it like a wave since I had the tranny rebuild about a year and a half ago. It speeds up with the engine and not so much with the speed of the car. I had the rearend rebuild, new motor mounts and new Ujoints but still have the problem.

I noticed that when I replaced the Ujoints that the output shaft on the C4 was movable. Is that right I could grab it and wiggle it around. I was wondering if that might be the problem and if so how do I fix it? The tranny is now out of warranty and I don't think I would take it back there anyway.

My next step is probably the transmission mount. It doesn't look bad but who knows.

Thank for the help.

[bmxmon]

Seems like it would be your transmission. When the yoke is in the tailshaft of the trans you can wiggle it around? I dont know much about C4's, (or any auto ford for that matter, i've always had a manual) but I highly doubt thats right. On the other hand I have a transmission for my buick (A 2spd S/P) that the output on the trans will wiggle around, and to my knowledge it works fine. (though not when the driveshaft is installed) Its probably a good idea to check the mount, but if its tight I wouldnt replace it.

[Bob]

You said that you installed new motor mounts. Were they stock replacements, or were they Polyurethane? If you use a stiff motor mount, it can transfer vibration to the entire car at certain rpm's. I have "Prothane" mounts in my '68 with a 331 stroker. It will send a vibration through the car at about 2,000 rpm. After some investigation, I figured out that it was the new motor mounts.