I'm lead to believe from the Sharp preparation manual and old Solex
tuning manuals that the length of the accelerator pump linkage can be an
issue. Sharp recommends using the longest linkage (third stop), to
limit bogging due to momentary over-richness below 3000-3500 RPM.
- Bob Mann
Marc Sayer wrote:
>
> Gordon Glasgow wrote:
>
> > > 3) I'm slowly on getting them tuned (they sat for a long time without
>being
> > > used). I assume the initial hesitation on hard acceleration calls for new
> > > accelerator pump diaphragm?
> >
> > Not necessarily. That's pretty normal behavior for Solexes. There's no choke
> > valve, so when you put the pedal down, the intake ports see atmospheric
>pressure.
> > Roll on the power, don't just hammer it, unless you're above 3000 rpm.
>
> I am going to add here that this is seldom a pump problem (it can be but
> its unusual). Here are some possible causes:
>
> fuel level is wrong (on the older Solexes this is not adjustable and you
> must adjust float level and fuel pressure but on the newer
> Mikuni/Solexes this can be adjusted externally with the engine running)
> idle jet is too small (while you can get a proper idle mixture with an
> undersized idle jet by altering the idle mixture adjustment screw, the
> transition ports are also fed by this jet and the transition mixture can
> not be adjusted except by changing these jets-if the transition mixture
> is too lean then the "hole" between idle and main jet operations will be
> more severe and can cause a stumble)
> throttles are open too far (it is possible-especially if the timing is
> too far retarded-to adjust the idle speed screw too far so that you are
> idling on the transition ports. If this is the case you will not have an
> off idle and transition circuit to help cover the transition from idle
> to main jet operation)
> ignition advance curve is wrong (not only can this cause problems as
> described above, but too little advance-especially initial advance-can
> cause symptoms similar to what you are describing-Mikuni/Solex/Weber
> type carbs like much more initial advance than the SUs to work properly.
> 20 degrees at idle is often required to get optimal performance and
> drivability.
>
> There are certainly other possible causes but these would be my first
> three choices barring other info.
>
> >
> > > 4) What is the likely cause for backfiring with hard deceleration (car was
> > > already doing some of this with SU's).
> >
> > Deceleration? good question.
>
> An overly retarded ignition will cause backfiring on decel.
>
> So based on the two symptoms listed I'd look at the ignition before I'd
> look at the carbs. It may not be the ignition, but that's where I'd start.
>
> --
> Marc Sayer
> Editor/Publisher
> Z Car & Classic Datsun Magazine
> http://zcarmag.com
> Voice 541-726-6001
> Fax 541-746-0863/726-6001
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