Charred inlet rubbers

Hi all
i just removed my carbs to cure a petrol leak and found the two right hand rubbers on nos 3and 4 cylinder are burned badly, they are only a few months old, replacement parts from zpower, timing is spot on and bike runs very well, any ideas? I’ve never seen this before(, I did re set timing a couple of weeks ago as it was running a bit rough ]

I would say that the fuel that was leaking was flooding the airbox which has subsequently ignited (lucky the bike didn't catch fire) ignition blowback in the airbox would normally be a "timing" issue or an inlet valve not sealing (tight valve clearance etc)