I posted a couple of months ago about vibration I had in my '63 Sprite.
It has a 948 with a ribcase. The vibration is strongest around 3K
RPM. Several people suggested I replace my U-joints which I did
but unfortunately it didn't help. I've had an opportunity to
road test the car some since then and I have discovered some things
of note. You can really feel the vibration through the shift lever.
Most curious though is if you "bump" the clutch slightly so that the
disc slips a little you can make the vibration go away. If you keep
doing this you can cause the vibration to fade in and out. As if some
harmonic vibration is passing in and out of phase. The RPM range which
leads to the vibration is very localized, about 400 RPM. I am not
certain about 1st or 2nd, but the vibration is there in both 3rd and
4th gear. I am thinking this may be caused by the pilot bearing or
maybe bearings on the transmission input shaft. Or maybe even a bent
shaft. I suppose another possibility would be the clutch. Anybody
tackled a similar problem, or have any suggestions?