Ken -
The SU carb needles work to very fixed mixtures at different throttle
settings & loads... what this means is when you tune your SU carb at
idle, it will likely not be in tune as you drive the car at different
speeds.
If you want to tune an SU carb's mixture at all different loads and
RPMs, the only way to do it is to have a test track, a stack of
different needles that run lean, and then drive the car around at
successively different loads and speeds with a mobile gas analyser and
then sand paper the needles down until you get correct mixture at all
speeds and loads. Some people have done this with spectacular results
in terms of mileage and power, but an exceptionally time consuming
process.
Long story short is your mechanic doesn't understand how SU carbs
work. A gas analyser at idle is no more useful then just tuning by
ear and using the dashpot piston lifters.
Alan
'52 A90
'53 BN1
'64 BJ8
On Tue, Jun 10, 2008 at 10:21 PM, Freese, Ken
<Kendall.Freese@aerojet.com> wrote:
> I have recently been conversing with a long time British sports car
> mechanic, but I think he is mistaken in his approach to adjusting SU
> mixtures.
> He is using a gas analyser without a rolling road, adjusting the mixture
> at idle and then complaining when it changes dramatically at off idle to
> 2800 rpm or so. He can't understand it.
>
> I say adjust at idle and don't bother with any other rpm since the
> needles take care of that and if the engine is stock, they are ok. Just
> check the plugs after a drive. If the mechanic had a rolling road then
> he could be concerned with non idle conditions and needle profiles.
>
> Who is right?
> Ken Freese
> 65 BJ8
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