IMHO, the vent is the main problem.

When you go up a steep hill the fuel goes to the back of the bowl and flows out the vent straight into the throat of the carby. The vent extension is to ensure that fuel cannot get out. You could DIY with a bit of plastic tube.

I had this problem with my Vitara with standard carb.

It sound to me that maybe you did something to the float level when you added the doodad. It helps also to lower the float level ,and to get a needle and seat with a spring assist. (This is probably the doodad)
Regards Philip A