mrvette

Threads on driveshaft angles and vibrations symptoms and driveability problems got me to thinking, so took the '72 out for a spin around the burb just now....what happens is I get some very bad vibrations from the gear shift **** at ~2500 engine rpms, this is true in any gear 1,2,3, but on 4th/ overdrive, at 80 mph on the freeway, the engine turning 2400-2500 rpm and in lock up the vibrations seem much less......

I have been noticing that in traffic the vibes come and go with acceleration, engine drops speed say from 1-2 shift, and the vibes go away, 2-3 same thing.....it seems to be engine speed related.....

but the mystery is, the engine was balanced when first installed....trans is a 200 4r and so I thought it was drive shaft angles and crap, but from what I reading, the car seems to tell me it's engine speed.....

So I rev engine in neutral in the garage, and it feels silky smooth all through the RPM range......

74modified

Gene
It kinda sounds like you are having convertor vibration that is reduced when locked up.

mrvette

Except when I hold it back into first gear, the converter has a light on the dash in parallel with the solenoid, and the light functions, and remains OFF......

I thinking on the driveshaft and reversing the phase by 180* on one end, and give it a shot.....

noonie

Old wives' tale.
The driveshaft is a separately balanced assembly.
Remove it and check the joints first, then do the placebo.
Check the tranny tailshaft bushing too

doorgunner

Rookie suggestion.......also check for damaged/broken mounts on the engine and transmission....my engine had a worn mount that was causing rpm-related vibrations.

mrvette

I dunno man, I revved the engine in the garage and felt no vibration issues on the intake top.....smooth until screaming.....

put it in gear, brakes locked and rev fore/aft and the thing does not move hardly at all......mounts solid to the frame, damn little to no tilt, and I seen bad mounts too over the years....

I wondering about the shifter itself at this point, but why so much smoother at ~2500 rpm and ~80 mph??? drives me nucking futz if it was the same, it would have to be a driveshaft related issue,

dochorsepower

What about sticky advance weights in the distributor? That's about the speed where they would be changing advance. Or, a plug wire problem.

Barry's70LT1

I had a similar problem a few years ago.
Vibration was "only" under acceleration. The shifter would really vibrate, between 2K to 3K.
Dead smooth when not under acceleration.

I had the flywheel/ pressure plate balanced and also changed the motor mounts, no improvement.

I replaced the harmonic balancer and it completely cured the problem.

The balancer (8") had no visible signs of damage or wobble. The balancer was over 40 years old and the rubber was dried hard and the balancer was not able to dampen the vibrations.

Seems to be more of an issue with a forged crank.

I replaced the balancer with a GM one from a ZZ4. Same dimensions.