Steve,
I've had the same battle with my Alpine running the twin downdraught
Zeniths. Mine is definitely related to heat exapnsion rates of the throttle
plate and butterfly. I still haven't solved it completely. Next step it to
recon another throttle plate to eliminate the troublesome unit. Could your
problem be related to heat rather than engine on/off?
On the Zeniths the extra return spring worked for a while as the car warmed
up, but it still stuck sometimes when the car was at full operating temp.
No vac secondaries on the Zeniths ;-) so I can't help on the second point.
Cheers,
Paul.
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