I have a theory:
If the distributor advance springs get a bit weak with age, and
the timing is a bit retarded, you can have a situation where
the timing is retarded at idle, and the idle is relatively low
as a result. But as soon as the engine speeds up a bit, the
advance starts kicking in, and the engine power increases enough to
maintain the higher speed even without the throttle being opened.
Scientific types could look at the carb/distributor/engine system
as having a positive feedback loop, that can cause it to have
two stable states.
I had the same symptoms recently when I put a different distributor
in my car (a MK III distributor on a MK IV engine). The idle
seemed to switch between 700 and 1100 RPM. Blipping the gas
would pop it up from 700 to 1100, and lightly letting out
the clutch in gear would snap it down from 1100 to 700.
It was very repeatable. The cure was to advance the timing
a couple of degrees and turn down the idle screws, and then the
idle stayed stable at 800 or so.
Or I could be all wrong. It could be a sticky centrifugal or
vacuum advance mechanism in the distributor, or a sticky
idle bypass thingie on the carbs...
Doug Braun
'72 Spit
At 03:08 PM 8/17/01 , Michael Hargreave Mawson wrote:
>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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