As described, I'm nearly sure (as others contributors) it is a shaft balancing
I posted this some weeks ago :
If you feel that vibration not really 'from the back end' but rather in the
whole rear area of the body, including in your seats, I vote for a propshaft
balancing problem. I think it's a very common problem on Spitfires, specially if
you change Ujoints or take the shaft off the car, then replace it in a different
way. I've seen many Spitfires with the sliding spline at the front, and many
others with sliding spline at the rear.. Workshop manuals and Parts Catalogues
don't always show it the same way..
Balancing the shaft is easy to do by yourself with that empirical method : Use a
clip (radiator hose type, or bigger). You put it on the shaft (near the diff),
in any position, at the beginning. Then go for a try on the road. The vibration
will be worse.. or better. Then turn your clip half a turn, try on the road
again... At the end you will find the exact place where you got less vibration.
When found the best place you can also try a second clip at the same place (more
weight), or a smaller clip at the same place (less weight). It can be boring to
achieve, but it can completely change your car. (My MK3 is two-clips-equiped for
many years, no vibrations).
I'm sure that poor English explanation is not very clear, so if someone
understood it or had the same experience, please repost it in a best american
And I received that from Dave Massey (TR6Massey@aol.com) :
This is the best way to balance the drive shaft (by your self) even though it
is very labourious. I would recomend marking a starting point with nail
polish and trying the clamp there. Next rotate 180 Degrees. Note the
position with the least vibration.
Now rotate the clamp 90 Degrees in either direction from the least vibration
from the previous test. One of these will give less vibration than the
other. With these two "least vibration" points you have narrowed it down to
a quadrant. move the clip around in this quadrant until you get a minimum
level. When you get the minimum you have found the vector and now you can
add other clips as necessary.
One caveate is that the vibration level evaluation is subjective and your
sensitivity will vary with time, temperature, fatigue, beer (or wine), how
many friends are helping, traffic, impatience, whats playing on the radio,
and many other variables. Another posibility is to take the drive shaft off
of the car and take it to an auto parts store to be sent out and balanced
professionally. I did this once (twice actually - the first time a weight
fell off and they had to re-do it) and the car was down for only a day.
Karl, Good luck and give us some news
Francois Demont email@example.com
Spitfire MK3, 1968, valencia blue, every day car