Bob,
I've no experience of this particular carb installation (we had twin
SU on all Spits in the UK), but the 2 main causes of varying idle speeds in
my experience have been:
1 - Sticking throttle cable
2 - Air leak on the induction side, ie., around the carb mounting or the
manifold to head joint. I guess air being drawn through a worn throttle
shaft could give a similar symptom. DPO's have a habit of not tightening the
two almost inaccessible manifold nuts between the Intake and Exhaust
manifolds on the 1500.
Hope this helps,
Bill.
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