On some engines, but not all, the idle mxture screw/needle will greatly affect the high-end mixture.
I've gone through this senario:
Top end perfect but bottom too rich-leaned low end <1/8 turn.
Now top end too lean but bottom just right.
Opened top end to 'just right' setting.
Now bottom too rich again.
Yadda, yadda, yadda.

Check all O-rings and take whatever steps necessary to seal them: i.e. slightly larger O-rings, fuel tubing, etc.
Also, make sure the carb body seals well against the crankcase. Sometimes a little blue RTV works wonders.

It may just take some tinkering to find that sweet spot.
Hotter plug? Cooler plug?...