I have played with two M43's in my life, a Hornet and a Bee. In both I got best accuracy with plain vanilla Sierra 40 & 45 grain bullets driven with modest charges of 2400. L'il Gun wasn't around back then. Best I could make either of them do was an occasional MOA group, not often enough to call either one a MOA gun. Any bullet heavier than 45 grains was an invitation for disappointment. Actually, I got my best accuracy with a 35 grain cast bullet, Lyman 225107- consistent 10 shot ½" groups at 50 yards. (I like discussions like this as they give me an excuse to look back through my notebooks!)
The thing about .22CF's built in the 1930-1960 time period is we tend to analyze them with 21st century eyes. Bullets and powders today are way better than what the original owners had, and accuracy is generally better with them as a result. But we're still talking 70-80 year old gun making technology and as such we can't rightfully expect them to shoot as well as new stuff that's "hot off the press".
To have your eyes opened and to gain a perspective for expectations of this old stuff, dig up copies of C.S.Landis and F.C.Ness treatises on .22 centerfire rifles, loads, and history. (Ness, "Practical Dope on the .22", and Landis, ".22 Caliber Varmint Rifles"- both published immediately post-war) Therein you'll find that guys back then were happy to achieve 1½" groups with their Hornets and Bees, and were cock-a-hoop to have a .22CF rifle of any flavor that gave consistent MOA accuracy. Even at that, they managed to decimate local varmint populations and had as much fun working up loads as anybody drawing breath today. It's all a matter of perspective.
To the OP: keep experimenting. Try other light-ish bullets and other powder/primer combos. It's a neat old gun that deserves to be used, but don't expect it to perform like a new CZ. 1½" groups may not be worth writing home to Mom about, but trust me, you will still kill stuff within it's range limitations if you do your part.
