Skip - I have an early TR6 that has the two vacuum servo distributor similar
to your 250. I agree with Dick T. that your distributor is likely to be a
unit from a later car with only centrifugal advance and with the vacuum
retard unit. I have disconnected and removed the vacuum switch from my car
as the switch was very worn and loose, and now run with the advance vaccum
unit only. However, I believe that benefit of having the vacuum advance
unit is modest as 1) the carb port that it gets vaccum from delivers no
vaccum to the advance unit at idle, so there is no benefit at all until the
throttle is partly open and 2) the vacuum advance doesn't give any
additional advance at heavy throttle openings. When distributors with
vaccum advance were first introduced on American cars in the very early
1950's, they were advertised as a means to improve fuel economy - not
performance. They gave more advance than just the centrifugal advance alone
could deliver at light loads when the engine could tolerate it without
pinging, but when the throttle is opened, vaccum drops so the vaccum
advance doesn't play any role. As far as the vaccum retard unit, that is
installed to lessen exhaust emissions when the car is coasting, i.e. high
vaccum & no load. It doesn't play any role when the throttle is open.
Conclusion - a good distributor with a properly working centrifugal advance
is about as good as you can get. Going back to the original two vacuum
servo distributor would only be for originality purposes.
Rex Townsend
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