Could be carb float valve has a spec of dust and is weeping into the throat. Gurtner carbs have brass on brass seat arrangement so it is very sensitive to dirt. Check that the point is facing UP on the float valve (often it is assembled pointing down, almost everyone has done that - OOPS)
Also make sure the blade is fully retracted for the choke when it is released, partial covering the bore wil give a richer mixture.
Verify your timing in case you are creating too much parasitic heat in the engine by too much advance timing.
The condenser can be troubled by heat (sworn enemy of all condensers) and the spark you see might not be strong enough to fire across the gap when it is in the compressed environment of the combustion chamber. It would not hurt anything if you did the external condenser modification anyhow.