Graham---While the amount of advance or retard you have dialed into the
ignition timing can affect idle speed, it shouldn't upset the idle and
carb adjustment. Consider that the engine should idle well when the
timing is set at 4 deg. ATDC, and also idle well when the retard is
plugged, and the timing is then around 10 deg. BTDC. Just for a trial,
plug the retard line, and set the advance to 10 deg. with an idle speed
of about 900 rpm, or as close as you can get, with the erratic condition
you have. Then see if the timing advances further, as you feed in more
throttle. At 3000 rpm, the advance should be near 28 on the pulley. If
not, the centrifugal weights and springs need to be looked at, for
correct movement. (They may only need cleaning and a little oil)
Can you hear any 'arcing' coming from under the rotor cap during these
misfires?
Also check the carb diaphragms for integrety, if you haven't already
done so.
Dick Taylor
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