by 1972nail » Sun Nov 11, 2018 4:52 pm
At tick over there will be a lot of vacuum, the throttle is closed but the engine is trying to suck air from wherever it can so yes, the vacuum advance will close up. A slightly wider throttle will give you less vacuum. Once you open the throttle slightly the vacuum decreases and the advance unit opens up. This advance at idle is what gives you a more stable engine at idle speed.
When swapping carbs over for an aftermarket carb with individual jets like the Weber you may need to play a bit with the throttle opening setting and ignition timing. The trick is to get the best compromise between stable sensible speed cold idle, progression between the idle and main jets and a stable sensible speed warm idle. This is usually not necessary with the standard set up ad the manufacturer has done all this work in testing before production and published the recommended settings.
Generally advancing the ignition a couple of degrees at idle speeds the engine up and retarding slows it down.
The ignition timing is set with the vacuum pipe disconnected and the idle speed low enough that the centrifugal advance is not operating.
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