you have 3 circuits in a conventional carb
idle circuit
low rpm circuit
high rpm circuit
idle circuit is effected by the idle jet and air mixture screw.
turn it in (clock wise) you have less air so more % fuel in the mixture.
the low Rpm circuit is effected by the slide and needle height.
you have different slide nr with a different size of cut away which effect the speed of the air and thus effects the % mixture between gas and air the needle does the same thing as it is a tapered rod in a hollow tube so it controls the restriction factor for the gas to be sucked out.
the high RPM circuit is needle and main jet.
the needle can have different shapes and different angles which effect again the restriction in the gas supply.
the bigger a main jet the more fuel can pass in a shorter time span through the jet.
the pull factor is created by the air forced through the carb as a result that the motor is sucking it through.
few things that have to be in place.
make sure your float level is set right.
to high and the fuel might be pushed in the venturi shaft (needle shaft) and results in not idle ling and poor low rpm performance.
to low and you get to less fuel in the % mix which mainly affects high rpm.
make sure you have a clean air filter.
blocked air filters result in to rich on the plug (less airflow is more % gas in the total mix)
make sure you don"t have air leaks between the carb and the reeds.
that would make the motor hard to control and air would lean the gas mix in the cylinder.
check your plug at moderate loaded bike at low rpm and at high rpm.
as the engine can be to rich in one circuit and to lean in another one, and yes all the separate circuits do have some affect on the other circuit.
then we have environmental influences like temperature, humidity and elevation.
a hot dry summer requires a different setting as humid damp conditions either hot or cold.
chasing a perfect setting can be time consuming and with jets, needles, slides also costly.
I am in the process to set a lectron on my SM (have it for a long time on the WR) and that is a totally different carb.
by far superior then the conventional carb if you ask me.
the biggest issue i had with the mikuni carb is the sensitivity on the environmental conditions.
living in the tropics if a storm was brewing during a trip I had to change my carb settings on the trail (not the kind of thing i want to do on a ride)
Hope this will help you with your bike.
check the plug at several RPM ranges

I ve seen a few time a complete engine failure due to a low circuit to lean yet high circuit was fine.
Robert-Jan