Bruce,
I always thought that SU carbs were a marvel of simplicity:
The linkage only moves the throttle valve. The vacuum causes the
piston to rise. That is all that moves....
the SU has none of the following:
1) a true "choke"
2) accelerator pump.
3) auxilary venturi
Granted, SU does have a linkage to lower the jet so it does have something
moving instead of
a true "choke". But, I still think that the SU has fewer moving parts. The
only variables are the
native size of the main jet, the size of the needle, and the spring on the
piston. When installed,
the only variable really is the needle! SIMPLE, no?
My wife's TR7's webers are vastly more complicated. All sorts of tubes,
venturis, jets and stuff. YUK.
IMHO, a Weber carb is NEVER tuned perfectly, just good enough such that further
effort
is not worth it.
I want to go "back" to (euro spec)SU on the 7!
-Tony
P.S.
re: lift the piston. If you slightly lift the piston you let in more
air for about the same amount of fuel: you make it a little leaner. If you
started rich, the revs go up. If you started lean the revs go down. If
you were about right, then the revs go up momentarily then down a tad.
re: dashpot oil: With thicker the oil, the piston rises more slowly as the
throttle
opens. At a steady state throttle position the piston reaches its desired
position,
and the dashpot oil is not a factor. With brisk throttle openings, you want to
avoid a transient lean condition (a common problem which causes hesitation). A
little rich briefly is much better than a little lean. Use whatever oil seems
to
work. Viscosity and temperature dependence are the issues. In cold weather,
you
might want more richness, so a multiweight oil might be less desirable than
single
weight. Pick whatever floats your boat (piston).
-T
Message text written by INTERNET:owner-triumphs-digest@autox.team.net
>Date: Sun, 18 Feb 2001 09:48:52 EST
From: Eganb@aol.com
Subject: TR7 BFH#56 -- Why are LBC carbs so complicated?
Probably a dumb question on my part, but why are SU and Zenith-Stromberg
carburetors so complicated, compared to something like a Holley or Rochester
carb?
I'm always amazed at all the separate components that make up the carbs you
find on LBCs, whereas a Holley four-barrel proabably has less than half the
moving parts.
Must be a reason for the different technology.
Anyone?
Bruce
1980 Inca Yellow TR7 5-speed convertible
Chapel Hill, NC
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