The contributions of Randy Wilson and A.B. Bonds have given me much
enlightment on a subject I felt I understood well.Of course, I did not
figure with the mad efforts of the British manufacturers to comply with the
emissions regulations in USA. Thus, whatever I stated in my short paragraph
about "ported vacuum" refers to that idyllic "pre-emission control" era,
not unlike the Garden of Eden. Then all the lbc's had the system I referred
to, a ported vacuum controlled by the carburetor butterfly, so that at
idling and low rpm's the vacuum was very small and increased with the speed
of the engine.No retard mechanism on the distributor either. Vacuum and
centrifugal effects added all the way to a maximum of about 30-35 degrees
at 2500 rpm or so.
It is remarkable, as Chip Weems points out, that this universal system in
the years of innocence has ended as an almost unnoticeable exception in the
US-delivery British cars.Without wanting to make a case of it, I must
confess that I have never seen in Australia an lbc fitted with manifold
vacuum advance. Checking among the many (old) car handbooks I have, I found
that all show the ported vacuum connections,Minis to Hillmans to Jaguars,ZS
and SU's.
A.B. Bond's advice of keeping together carburetor and distributor is most
correct, and it makes the fitting of aftermarket manifolds doubly
difficult, as probably a new distributor will be needed too.One may
speculate that the Weber replacements for SU's will have much the same
problem: an incorrect advance curve.
Sergio
Sergio Montes Department of Civil and Mechanical Engineering
University of Tasmania
Box 252C,Hobart 7000,Tasmania,Australia
Ph. 56-02-202113 (Int) 002-202113 (Australia)
Fax 56-02-234611 e-mail Sergio.Montes@cmech.utas.edu.au
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