Fuel Leak

Hi Ken,
Assuming it's a 900 thunderbird (?), as chiari1 said, switch the fuel off and let it run for a bit to drop the levels in the carb bowls.
If they're full when put on the side stand they flood to the left carb and overflow into the air box.
Its probably the first characteristic I learnt of these bikes when I bought mine.
Note characteristic, they're full of it, I wouldn't worry about the starting issue, it'll soon sort itself out and a other problem will crop up

 

It's not sticking floats, necessarily. Just the lean when on the side stand makes the fuel in all 3 float bowls flow to the left, then flood into the air box.
I find it happens more often (when I forget to drop the levels) with a full tank.

 

Ken Burns
Pretty much the best answers you are going to get here. But you could also throw a carb cleaner into the fuel and let it flush the carbs.
Regards
Joe.

 

Ken Burns
I use the Lucas. But I think they are all pretty much the same potion.
Regards
Joe.

 

I use Redex in my Trident 900 which works very well. Availability, recommendation, personal preference and price will all have a bearing on your final choice.