
I thought you already addressed the throttle travel including the throttle stop? The throttle stop screw is a big one and the screws are almost always “throttled down” on these engines as they come from the factory. It is good to remove the screw entirely on all engines for karting and minibiking use.
You have a Subaru Robin engine, right?
Anyway, 30 mph is probably the max speed that the governed engine plus the kart’s gearing can provide at this time so don’t be sad, because “uncorking” the engine as you’ve done is a great success. That speed is plenty enough for some people.

A tachometer is a
good great idea that will let you see the max revs your engine is (currently) capable of. Then you can plug that info into a speed calculator. Have you used a speed calculator?
About the throttle connection, make sure you take as much slack out of the throttle cable as possible because I see some slack where it comes out of the sheath/conduit in the photos from your post #35. You don’t want the pedal moving and nothing is happening on the engine until the pedal is pushed haflway down or something like that. With the amount of coils visible on the throttle return spring, I don’t think you will need to alter the throttle pedal range but it could be all jacked up. Show us pics of the pedal when you can.
By the way, I agree with everything WTR wrote, could have written same but didn’t want to get too wordy.