Window Weld motor mounts and vibrations

[Kurt Wubbels]

The car in question is a '91 Classic with about 185k on the clock.

I recently filled the passenger's side and front (in front of the exhaust
manifold, behind the radiator) motor mounts with 3M window weld. I was
expecting more NVH in general but there is a new vibration now that seems to
be related to torque load on the motor (most pronounced in low gears at 3k+
rpms when suddenly letting off throttle) Basically, the whole car vibrates
as if it were directly attached to the motor without any sort of dampening.

I'm trying to imagine what is responsible for this -- I haven't replaced the
driver's side mount or the one by the firewall -- is it possible that the
new mounts are making weaknesses in these more prominent?

Any thoughts?

Thanks,

Kurt.