Shawn:
Have you tried going to thinner dashpot oil? It seems like it
is possible that the slides are taking too long to respond to the
increased vacuum as the RPMs climb.
Usually the dyno readings are taken with the pedal mashed to the
floor, and are derived by how fast a calibrated mass is accelerated.
During the measurements, the engine is accelerating rapidly, and the
carburetors must respond rapidly.
The dip in the curve implies that the slides are failing to rise
initially as the engine accelerates. At first this is correct, since
you need a richer mixture to correct for the sudden opening of the
throttle butterfly. But then as the engine accelerates and manifold vacuum
rises, you want the slides to rapidly rise in order to follow the
engine speed.
Try it with a thin oil in the dashpots (3 in 1 or ??) rather
than motor oil.
Just a thought.
Vance
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net]On
Behalf Of Shawn J. Loseke
Sent: Tuesday, December 23, 2003 10:50 AM
To: 6pack@autox.team.net
Subject: New topic follow up from CR discussion...
Hey all this is a little long...
Based on the CR topic discussion I have some other power related issues that
I am dealing with. Regardless of my now uncertain CR, I have a GP3 cam with a
Goodparts Tri-carb set up that is having some mixture issues. I believe that it
is
because of the needle profile. This belief comes from soem chassis dyno time
with an A/F
meter reading the exhaust. The mixture and the power start off well but then
there is a
sudden and huge rich mixture problem that kills the power for about 1000rpm and
then stays
rich all the way to redline (@5,700rpm was as high as we went on the dyno).
The A/F was
right between 13 and 14 starting off then drops below 10 (don't know how far
below
because the scale stopped at 10) then climbs back to 11 and stays there.
<snip>
So now the long awaited question. Does anybody have any idea of what other
needle profiles might work better for my application? I have some ideas of how
to go
about comparing profiles but don't have the time to model them right now. I
have
the Haynes ZS book that has a selection of biased needles and the measurments
for them. I
am also ordering Kas' new book to see what he says about modifying SU needles
etc...
any help would be appreciated.
shawn
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