Visual Low Speed Needle setting:
Open throttle all the way.
Look inside the venturi at the spray bar.
Turn the LSN so that the tip of it is ~1mm out of the spray bar.
(When the throttle is closed the spray bar is twisted onto the LSN effectively closing off all fuel flow. When the throttle is being opened past around half throttle the LSN no longer has much if any affect on fuel flow - it's all HSN.)

For an engine that will not run past half throttle there may be a fuel draw problem. Put a piece of fuel tubing over the needle valve where it meets/enters the carb assembly (getting it under the needle-keeper spring may be test your patience). Works wonders. I won't run an engine without it.

Definitely check the cam timing. It is a likely cause of a poorly running 4S above half throttle. My TT91 came with the cam out of place.

If your cam is set right and there is no fuel leek around the HSN, check the O-ring that fits between the carb and the intake manifold.

A fourstroke plug is mandatory.

Good luck!!