Ahh it all sounds so familiar heh, I too had some issues re-tuning the low needle after installing the check valve. It takes some time to get it right but once it's there it's golden, my GST is running strong around 230-240s from full tank to empty tank.

Although I don't know if I have my tune perfect, I have to keep it idling a tad high to keep it happy, the clutch bell kind twitches slightly. Is that normal, or is it off a little?