This is the approach used on my race car, with 1 1/2" SU carbs. No
dampers at all, but still retaining stock springs. I had to move up to
a .100 jet (same as the 2L), and a needle profile that tapers
dramatically when compared to the stock 1600 needle. Last year, I
experimented with the "RF" and "RH" needles, and some hand
modifications, using those as a starting point. This season, I'll be
experimenting with RB and RC needles, both a little richer at top end.
Throttle response is much better than a stock SU, but the idle isn't
quite as smooth. That's partially a result of the "hot" cam I have in
the race motor...trying to make good power at 8000 rpm!
One of the best books I've seen on SU carbs is "How to Build and Power
Tune SU Carburetors" by Des Hamill. Also David Vizard's "Tuning the
A-series Engine" has some good flowbench data on various SU carb
modifications.
Happy roadstering to all-
---
Chris Coker
Royal Oak, MI
1969 1600 Street Car (SPL311-24529)
1966 GP SCCA Race Car www.risensonracing.com
Independent Amsoil Dealer
On Mar 30, 2004, at 10:48 PM, Keith0alan@aol.com wrote:
> Hello all,
>
> A certain amount of head scratching is involved here. The
> slides rise
> has two effects. It makes the space between the needle and the nozzle
> larger
> (more fuel) and slows down the air flow (less signal or air flow over
> the
> nozzle thus less fuel). As it turns out the air flow over the nozzle
> is the bigger
> player. So, slowing the piston rise causing it to be in a lower than
> equilibrium position will enrich the mixture. When tuning the carbs
> you can raise the
> slides to lean the mixture and push it down to richen it. One of the
> racing
> tricks with SUs is to use no oil, a light or no spring and a really
> rich needle.
> It makes the throttle response quicker at the sacrifice of normal
> drivability.
> Roadsters seem to have no problem running fairly rich but hate being
> too
> lean.
>
> keith
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