Message text written by "Michael D. Porter"
>Think about this. Owner's manual says 20W in winter, 30W in summer.
Engine is colder in winter and running temps are lower, so the mixture
needs to be richer to keep the engine going in some instances. So,
thicker oil translates to leaner mixtures. Think of it this way, as
well. The needle and jet control the mixture--farther out the needle is
from the jet, the more fuel available from the well, producing richer
mixtures.<
You are absolutely right as far as you go. But all else is not equal!
The reason the dashpot is there is to delay the rise of the piston and
with it the needle. This delay is to cause a temporary enrichment
of the mixture during sudden acceleration (in lieu of an accelerator pump).
In the winter the oil is thicker, so you need a thinner oil to keep the
same (ballpark) viscosity.
The SU carb works on a constant vacuum. With a constant vacuum
you barely need anythng other than a plain old venturi, the needle is there
to fine tune the mixture at various RPMS. When the dashpot is delaying the
rise of
the piston, the engine is running at higher RPMs but the inlet orifice
has not yet opened. This means that the air is running in FASTER
across the jet and at a lower pressure. This sucks in more fuel for the
same
amount of air, causing a rich condition until the dashpot allows the piston
to rise. Thus thicker oil in the dashpot equals more enrichment. The
reason to
go to lower weight oil in the winter is that the 30wt gets TOO thick, even
for the
increased enrichment needs of the winter.
RE: ATF fluid, 5-10wt seems too light. SOmething else is probably better.
-Tony
ARhodes@compuserve.com
-Tony
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