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.
HTH,
Donald.
> Date: Fri, 17 Aug 2001 20:08:00 +0100
> From: Michael Hargreave Mawson <OC@46thFoot.com>
>
> Dear All,
>
> Carly has started to suffer from a peculiar problem. I'm pretty sure
> that the problem is in one or both of the carbs (SU HS4s), but I can't
> for the life of me work out what is causing it, or how to fix it. Let
> me explain...
>
> Once at operating temperature, taking the car out of gear (or putting my
> foot on the clutch) no longer immediately returns the engine to idle
> speed. Instead, the revs drop as they should until they hit about 1500
> rpm, and then slowly creep down, taking 30 seconds or more from there to
> idle. Putting the car into first gear and letting the clutch out for a
> moment whilst stationary allows the revs to drop swiftly to idle speed,
> except in the hottest weather, when the problem is (a) more frequently-
> encountered and (b) impossible to cure. Mileage has dropped by nearly
> 15%.
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