You cannot do the same thing on a Magnum 120. Another word, you cannot install two T's in the fuel lines. The Magnum 120 does not have built in fuel pressure regulator like the YS, so trying to fill up the fuel tank through a T will flood your engine. On a Magnum, install a third line to the tank with its own clunk just for fueling and draining. Make the fuel line attaching to this second clunk inside the tank slightly shorter than the line attaching to the main clunk so the two clunks don't interfere with each other. Also, you don't need to install a T in your vent line to the muffler. Just hook up the vent line directly to the muffler pressure fitting. When you fill up the tank, excess fuel can just run out of the muffler.

As for your low end problem, either your engine is still not fully broken in or your low end is too rich/lean. What does the engine do when it's idling? When you chop the throttle, does the engine drop down to idle, then speeds up a little and quit or does the engine slowly quit?