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Hello all
Question about vibration
Have a 235 with an SM420
When driving in third at cruising speed, I notice a vibration that can be felt in the gas pedal
The puzzling part is it doesn’t do this all the time
When it starts “acting up”, if I find the spot that is vibrating, if I let off the gas a bit it stops; if I give it more gas past that spot, sometimes it will go away. Once again, this does not happen all the time

It has a rebuilt tranny, new clutch and surfaced pressure plate

Could this be a harmonic balancer issue?

Thanks in advance

Vibration as opposed to tires/alignment/steering wheel shake? Ujoints?
I gotta think about only feeling it in the gas pedal??? That would be motor hesitation/missing/bogging/timing/fuel pump and all that.
Do some tests when it does it. Push clutch in. Slip in neutral. Then rev up and down. Anyway, if it does it with clutch in or in neutral, it's rolling drive train parts. You can even get it vibrating and shut motor off. I wouldn't suggest all this stuff if I could feel it, but I can't. There are all kinds of what someone may call "vibrations".
Happening in third at cruise sounds like timing/vacuum advance. Fuel or electrical. Third is 1:1 ratio and acceleration is a strain.
That is my best shot at it right now. Not much help.

It may be a motor or transmission mount issue. Is it related to torque on the drivetrain such as, does it occur more often when pulling a hill in third, or when first shifting from second to third? Vibrations can be heard in places that actually have nothing to do with the component. Is the throttle shaft/bushing functioning properly?

The rubber mounts that support the clutch housing attachment to the frame are frequently the cause of odd vibrations.

Ed

Harmonic dampers are so close to the center of rotation, they seldom cause problems unless they come apart. Then its usually just terrible noise..

Check all three motor mounts. Also check the mount at the torque tube ball.
All of them deteriorate from being immerse in oil. New motor mounts would be a good idea even if they aren't totally trashed.
The front motor mount rubber will squeeze down until you have metal to metal contact of the bizarre multiple pieces of metal contraption of which it is.
Someone needs to come up with a better, much simple design. Where's HRL when you need him. Probably working on the next motor sickle to crash, or a 900 pound car with a 500 hp, 7,000 RPM motor in it.
Carl