On a glow engine, ignition timing is controlled by the fuel linearity of the carb. Since the CDI takes over this function of timing, suddenly the fuel/air ratio requirement is no longer determined by the glowplug, but by the fuel's stoichiometry (the "perfect" ratio where very fuel molecule finds exactly the right amoung of O2 molecules to combine with). That is a different ratio.
If you tune very careful, you will find that a glow engine, running Methanol fuel combined with spark ignition, will nopt only need both needles to be set significantly leaner, but also that there is a rich patch in the midrange. Some carbs allow for this to be tuned away, others don't.
That rich spot usually is not that bad that it NEEDS tuning out, but it does cause a bit of a stumbling run at half throttle, what is worse, it contributes significantly to the "slime". The solenoid will clean up the mixture of a sparked methanol burning engine considerably.
I never experimented too much with it after I changed over to gas, but I have flown a sparked methanol burning fourstroke helicopter for a few years, and although the engine was pretty reliable, absolutely more fuel economic than when it still was glow, the carb was pretty hard to adjust (stable once set, but I had to be extremely accurate. I fitted a 3 needle carb, because the original twin needle carb was impossible to adjust after the spark conversion.
It was also extremely loud...