Well, then, Michael. Let's have a go at SU anatomy. (I can't think
of any on-line pics, so I'll try using words.)
The throttle blades in later SUs have a hole in them which has a small
poppet valve closing it. This popped valve is spring loaded to close
and can be pulled open by intake manifold vacuum when that vacuum gets
great enough. This only happens on overrun conditions (engine braking
in particular or during up-shifts for a short period of time.)
I'm not sure of _all_ the reasons for its existence, but the one I
know of is that it prevents an excessively lean mixture being drawn
into the engine during overrun which increases unburnt hydrocarbon
emissions. (That is the usual reason that you will hear popping
during shifting or engine braking -- the lean mixture fails to be
ignited by the sparking plug, so enters the exhaust system where it
can be ignited by the heat there.) I am quite sure it was an
emissions control device.
As to the possibility of this thread being related to Monty Python's
Flying Circus, that may be possible. The valve assembly is somewhat
bell-shaped, all taken together, but especially if it were covered in
[e.g.] foil -- and the Liberty Bell March was the theme music to MPFC,
FWIW.
And on a less-serious note, I recall the reason the idle speed
behaviour that was mentioned elsewhere in this or a related thread
(where it idles high for a short time after decel, then drops back to
normal) is caused by the ignition advance delay switch. The ignition
is held advanced for a few seconds after the throttle is closed which
causes a slightly higher idle speed. After that, the advance is
dropped back to its greatly-retarded emissions-controlling setting.
And no, sorry, I don't remember which emission the idle ignition
retard helps nor really why the delay was deemed necessary. Though I
suppose I could look it up if someone else doesn't pop up with the
answer and there is an undying need to know.
HTH,
Donald.
> Date: Tue, 21 Aug 2001 07:44:23 +0100
> From: Michael Hargreave Mawson <OC@46thFoot.com>
>
> In article <200108180355.XAA22232@chelseamsl.com>, Donald H Locker
> <dhl@chelseamsl.com> writes
> >Sounds to me like the springs in the overrun valves have relaxed. The
> >throttle blades in later SU carbs have a spring-loaded poppet valve
> >that prevents excessive manifold vac in case of overrunning condition
> >(engine braking.) If the valve gets leaky, or the spring gets weak,
> >the normal manifold vacuum will open the valve sufficiently to keep
> >the engine speed high, until the speed vacuum drops. If you have
> >these overrun valves, you could try shimming the spring a bit to see
> >if it helps. Or do what I did, and solder them up solidly. (It was
> >an emissions control trick and doesn't otherwise affect engine
> >performance.)
> >
> >Oh. You're in the UK. I may be all wet on this then, but it might be
> >worth a look.
>
> Thanks for the suggestion. I'll certainly have a look, but first I'm
> going to have to find a crib-sheet. I wouldn't recognise any of the
> components you mention by name. Is there a labelled exploded diagram
> on the web anywhere?
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