Mystery bucking problem

[KTPhil]

I have an old fastback with original mount rubber and a worn trannie. When I let up or hit the throttle in low gear, I get quote a bit of bucking. It's not the motor. I have a few theories, but can't distinguish the problem, or know what the next diagnostic step is. It feels like there is play somewhere, taken up on initial throttle application, but present again, then taken up... it "oscillates" like this for 4-5 cycles before smoothing out. Does it in first and second only.
1) Front trannie mount? My bug needed one to stop a similar problem.
2) Play in final drive pinion?
3) Play in geartrain?

I am pulling the trannie to have it rebuilt (shot synchros and grinding second gear), but if any diagnostic work can be done while in the car then I'd like to try it before removal.

Any suggestions?

[Tram]

Sounds a lot like a trashed trans mount to me, as long as the engine is ruled out. Does the gearshift buck or move when this happens?
_________________
Немає виправдання для війни! Я з Україною.

[KTPhil]

Good question! My bug did, and that clued me to the tranny mount. But the fastback doesn't (much). I guess I'll discover if I have a bad mount when I pull it.

I was intrigued years ago to hear many shops don't preload the final drive/diff gearing correctly, leading to this problem. I was hoping a good tranny shop could tell by driving it, but I don't have a shop in mind yet (north LA County area).

I guess I can instruct the shop to check for this in particular. I just don't want to do any of this twice!

[Russ Wolfe]

You dont say what year you have. If it is early with swing axle, get out the manual and adjust the rear hanger preload. If it is a late model with IRS, make sure all of the rear cross member bolts are tight, and the engine is hanging at the proper height.
_________________
Society is like stew. If you don't keep it stirred up, you end up with a lot of scum on the top!--Edward Abbey

Gary: OK. Ima poop.

[KTPhil]

Sorry, '71 with double-jointed axles. Rear mounts seem good. I'm leaning toward front mount and final drive wear, but I'm open to other ideas before I pull it out in a few months.