Sounds like a sticking float valve, but you say it happens with the fuel tap off, so it must be both crap in the tap and a sticking float valve, probably caused by the bike standing for a long time.

Pre-mix is much more likely to cause problems when left in a tank than straight fuel.

Pull the carbs off and thoroughly clean them again. Maybe a soak overnight in carb cleaner or even Coca-Cola! (You'd be amazed what that stuff shifts... and just think what it does to your insides!!) Then unscrew the fuel tap and clean it best you can. The easiest way to test the tap is to hold the fuel tank off the bike and turn the tap on/off - whilst hoding it over a bucket, not the gutter, of course!! ;)

Andy.
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