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
> Keith,
>
> Since I've been running Solexs for 37 years, my SU experience comes from
> working with MGs and Triumphs in the 60s, but if remember correctly, thinner
> oil will give you a richer mixture more quickly, not the other way around.
> The thinner the oil the quicker the piston moves up which takes the needle
> with it, thus moving the lower thinner part of the needle to the fuel
> dispersal area of the jet providing a larger orifice area. The choke works
> on the same principle by pulling the jet down to a thinner cross section
> area of the needle with the piston still down all the way at idle. The
> acceleration stumble is caused by an overly rich mixture, not lean.
>
> Jerry Krakauer
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