Your basic idea is OK for the starting positions of the jets. You’ll probably get down to an 1/8 on the HI end although leaving it a 1/4 is safe for starters. You may even get down to almost having the HI end closed. It will depend on the length on the straights. The LOW and HI end jets have different profiles. Leave the stock HI end in the carb and replace the LOW end needle valve with the flex-T. You’ll do most of your tuning with the low end. Never crank the needle valves into their seats. Turn them gently into their seats and never force them past that point.
You didn’t say whether you’re running a pipe or can muffler. Either way, suggest that you install an EGT probe in the exhaust. Don’t bother to trying to read the temps as you’re driving as it will be changing faster than you can keep track off. But after a few sessions, you’ll see in Race Studio what your highest temp is and then set an alarm light on your gauge to slightly above that. That will help avoiding a stuck piston, especial with a pipe. The EGT should rise as long as you accelerate. If it drops, you’re too lean. If it rises in corners, the kart is over stuck.
As for pop-off, some racers will say it’s important, others say you need some. 9.5 psi is a good place to start. Spray WD-40 on the needle valve when testing the pop-off. Use a paper towel over the open end of the carb so you don’t get soaked with WD-40. The fulcrum height should be .045″ below the carb body without the gasket.
TIP: Don’t plan on the engine to make you the fastest guy out there. Driving and chassis setup is where the performance is.
Good luck and have fun
LAD Specialties customer / tony kart / rotax / kt100