Geoff---In the order of probability on why your idle is not returning to
the same position, I would offer this:
1) Throttle linkage sticking. Hook your toe under the gas pedal when
this happens and gently lift. If this helps, you then need to disconnect
the linkage at various points and try to duplicate this by hand, to see
which section needs attention.
2) With the engine not running, try lifting up on each air piston with a
finger to see if it returns to the bridge, quickly. If one does not,
loosen the four top screws and try again, as the machining here is very
exact, and will not tolerate mis-alignment or dirt. Lightly oil the air
piston guide rod(s), as Brooks suggested. The internal spring should
return the air piston to the bottom.
3) You probably are not running with a vacuum retard, but just in
case...Don't.
On the engine vibration problem now solved with the changing to the SU
carbs, I guess we'll never know why, will we?!
Dick T.
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