It seems you have it sorted out, but thought I'd toss this in just for fun.
The carb was never designed for much pressure on the float valve. If you put too much pressure on it it'll blow it open and flood the carb. Those inline regulators do work, but only when there's flow, in my experience. When the bike is stopped and there's pressure on the high side, it can trickle fuel through and still flood the carb if it doesn't seat properly, like a bit of the crud you're trying to filter out getting in the valve. You're also dependent on, lets face it, a cheaply made aftermarket will-fit accessory.
You just need to increase the flow rate at gravity pressure for that height. I would have suggested a larger filter or multiple small ones in parallel, but you've done that. You also could use a screen rather than a filter- so long as it's equal to the filtration of the carburetor's screen you're set.
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