So my vw by great planes has a battery/electronic ignition set forever to 27 degrees btdc, as I recall, and a magneto for the other ignition system, also set to 27 degrees btdc, but the magneto has something I think called an impulse coupling that causes the mag to fire after tdc at cranking rpm speeds, then it internally switches to 27 degrees btdc when the motor is over 300 rpm.
So I always start on the mag at something like 5 degrees after tdc, then after the start I switch in the backup electric. my 4cyl vw motor has 8 spark plugs (2 per cyl). my start switch holds the mag on and battery/electric off during cranking/starting. I have flown home more than once with a dead electrical system, in certified aircraft, over 10 hours one way, so I am a total believer in having at least one mag system. If you hand prop, (or electric start even) you want some retarding of the timing so it kicks forward (fires after tdc) rather than kicking back (fires before top dead center) during starting. once it is running something between 20 and 28 degrees advance will be fine to begin with, you can tweak it from there. The test runs won't care much, but at altitude during climb power it gets more important. In fact for test runs mine doesn't care much at all about timing from 28 to zero before tdc. I just gets kinda weak if it does not have enough advance. Too much advance and it will knock and get hot. Mark can explain how to tweak the timing better than I can. jg > >
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