I think that you should thin the fuel (add methanol) and keep the spark
point at 5 degrees after TDC for starting.

As you have found out backblows are nasty. I don't know what you mean by
'blowing air out of the carb'. If you have a proper fuel system and the
throttle is open that should be a short, solid column of fire. It is
called a carb fire. I was not joking whan I suggested that you fuel the
engine with a pipette while trying this out, and do not let it run until
you are sure it won't bang on you.

You need to find out why the timing is unstable. Revert to glow, and fit a
neon bulb to the HV coil instead of the spark. Use a bright one. I found
some bright green fluorescent ones. They have phosphor on the glass and
something that makes uv inside. Shine bright green when lit.  Make a mark
on the crankshaft and on the body at TDC, start the engine, illuminate the
mark with the neon bulb, and see if it moves around when you use the
throttle. If it does you need to fix it until it stays put (the bulb
should light exactly when the mark is 5 degrees beyond the mark on the
casing if you set 5 degrees after TDC, and it should stay there no matter
what the RPM is if you have no timing advance correction). On a normal
engine you would do this with a strobe.

Imho do not fit the spark plug again until you get this sorted out
permanently. You are playing with fire and you were very lucky.

Peter

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