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Why is SU fluid an issue?

Subject: Why is SU fluid an issue?
From: (Russ Wilson)
Date: Tue, 24 Oct 1995 11:24:41 -0800
I'confused.  Why is there so much discussion about damper fluid for SU's?
It's not as if we're inventing something new.  The SU has been around for a
long time and there are well-documented factory recommendations.  The
manuals I have, which cover MGB's through 1974, specify the same
multi-weight oil used in the engine.  Has this NOT worked for someone?

(Having said that, I'll admit to using ATF in my '63 B for 30 years - I
don't remember how I got started but there was never a reason to change.)

>From the variety of responses to this issue, it appears that the type of
oil doesn't matter much. That's reasonable because, assuming it doesn't
corrode things or evaporate (ie, can we drop the water as a candidate?)
viscosity is all that should matter.

Speaking of viscosity, I believe Randy has it backwards and his apparent
satisfaction with his solution is puzzeling.  He originally used ATF but:

> However, maybe because the carbs are getting older and thus more worn, I
>have changed to motor oil (10w-30 or 20w-50..whatever was at hand) because
>the lighter oil allowed the dampers to rise to quickly and caused a rich
>running situation.
>Randy Balogh

A *higher* viscosity oil that *retards* the rise of the piston upon
acceleration is what causes a briefly enriched mixture.  Yeah, it seems
backwards, but no one ever claimed the Bernoulli effect was intutitive.
(It's the increased air velocity through the restricted venturi under the
piston that causes the enrichment, not the increase in the jet aperture.)

BTW, Just as it is impossible to keep the radiator full (as discussed in a
previous thread), I have found is does no good to try to keep the dampers
as full as the manuals recommend.  As long as the oil is over the top of
the plunger when the engine is off, there is enough.  If the resevoir is
filled per specs, some of the oil quickly finds its way to the top of the
air filter cases, etc.  That's one oil leak that is easy to fix!



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