In all the discussion and opinions about what is (or is not) the correct
oil to utilize in SU carbs, there is one point most of us are forgetting.
The grade, or rather the viscosity of the oil in the SU dampers has only one
real purpose: ie. to control the rate at which the vacuum piston moves
upward. It does not control the height the piston reaches at a given throttle
opening (vacuum level). Rather, that is the function of the spring.
Both the spring and the oil viscosity can and should be utilized as tuning
tools. Oil viscosity, by controling the rate of rise in the piston, very much
(in fact totally) controls the "accelerator pump" effect. By lowering the
rate of rise in the piston, the damper forces an increase airspeed across the
jet thus temporarily enrichening the mixture, until such time as the piston
"balances" between the vacuum above it (from the intake) and the incoming air
below.
The point is we should be utilizing this characteristic of SU's as a tuning
option instead of trying to find some "correct" oil for the dampers. It is
very possible to find that of the two carbs on an MG engine, one requires a
slightly lighter oil than the other. Case in point, my Midget needs a bit
thinner oil on the front carb than the rear. Each time I top up the dampers,
I'll put a bit of machine oil (3 n 1) in the front before toping up with
engine oil. On the other hand, my BGT is perfectly happy with 20W-50 in
both. Go figure.
Just thought I'd put in my tuppence worth
Rick Morrison
(" Do you think my mind has matured late, or simply rotted early?" - Ogden
Nash)
74 Midget
72 BGT
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