Hi Guys
not withstanding what you have said, which is all good stuff To me the fact it does the same thing on each ignition says it is probably NOT the ingnitin system but something more basic / mechanical.
The start should be back to basics
cylinder leakage test ( check valves rings etc)
take the valve covers off and check the valve lift is the same on all valves. No need for dial gauges and fancy stuff just visual with a 6 inch ruler
A very thougher visual inspection particuarly for vacuume leaks around the carby stubs and balance tube
drop the float bowl and check for contamination like wise the fuel filter for water /dirt
you may well find a problem, some years ago I had a similar problem with a 912 it turned out to be a split carby stub causing a vacuume leak, and lean mixture when the manifold vacuume was high closed throttle. At wide open throttle it made little difference.
Just something to consider
cheers