Vibration at 2800rpm

[vintage guy]

......not so much a vibration but an annoying buzziness between 2600 and 3000 rpm in the drivetrain that can be felt through the gearshift lever.

My car is a 1987 Carrera that has had this problem for quite a while. It was present with the clutch in or out, in any gear and moving or not. I did some searching here and found the same issue mentioned a few times but could not find a clear fix.

I recently changed belts and noticed an improvement so I tried removing the compressor belt completely. There was still a hint of the problem but clearly it was further improved.

When I installed belts I had properly set the tension of the fan belt but not the compressor belt. I set it to what I thought was reasonable tension but it turns out that the actual spec. for deflection with finger pressure between pulleys is 2-3 mm (.080 - .120"). This is quite tight and much tighter than what I had set it to.

With the correct tension the vibration was reduced to the level present with no belt at all. I still don't no what the actual cause of the vibration is but it is enhanced by a compressor belt that is too loose. I suggest you check compressor belt tension if you have this problem.