How do I stop a gas leak around the bowl on the carb?

Gas leaks out of the TOP of the bowl or at the Jet/bolt?
Some Bowls have a spring loaded Purge, they don't always seal well, is it leaking from that?

Its probably the float sticking in the carb, not shutting off the fuel flow when the bowl fills to the proper level and the leak is really gas flowing out the overflow.

Disassemble the carb and gently clean the needle and seat with Scotch Brite oven pad. Some needles have a rubberized tip and if so, do not attempt to clean with the pad, but inspect it for a worn 'ring' around the perimeter that is failing to seal off the flow of gas into the bowl through the seat. A replacement needle is required if in fact it looks indented.

IF you have already disassembled and cleaned the inside of the carb, you may have jeopardized the intergrity of the bowl gasket. Get some gasket sealant at the local auto parts store and disassemble the carb again. Use a toothpick to run a thin...thin...thin line of Form-a-Gasket sealant around the perimeter of the bowl and GENTLY reassemble the carb. A thin line is a MUST as any excess can be squished inside the carb and possibly block an idle jet of inhibit the action of the float.

Possability the inlet seals on the float bowl are hard or just old and when you ride- the vibration allows too much gas to flow past the inlet seals causing the float bowls to overflow.

How long ago since the Carbs were serviced?.

Is it poss to remove the float bowl while the carbs are fitted to the bike? I am fairly sure back in the eightys the float was addjustable, to controll the fuel level by bending slightly the metal tab on the float, when the bowl is removed check the level of fuel in the bowl and compare it to that in the others, the aim is to make the fuel flow stop at the correct level in the carb(by bending the float tab) while the bowl is off turn on the fuel push the float all the way up check if the fuel flow stops, also check the condition of the sealing washer around the float needle seating.

well it sounds like the float needle is not shutting fuel off whin the bowl is full of fuel you can lightly tap on the bowl of carb an that may make it stop what has happend is the bike has sat you may need to run a little fuel treatment through it or sea foam and that may clean the end of the needle off the way it works when the bowl of the carb fills with gas the float raises a needle valve into a brass seat and stops fuel flow into the carb fuel get old and starts to develop a tarnish on that nice pionty valve an then it won't stop gas the overflows just prevent to much into the motor try some fuel treatment and run it it may fix it self just from fresh gas an being driven worst case is having to clean your carbs still not to bad hope that get you fixed up

Your info is vague; does gasoline leak when engine running or at any time? Does the engine run ok, or is the engine flooding from too rich a gas mixture ?
Most often the leak is the seal on the float bowl; either the top rim gasket or the fiber washer on the center nut/bolt. It also can be from a sticking float that is allowing the gas to continue above the normal level (just like in a toilet tank). This can happen if the float is deteriorated from moisture over the year and gas/water has gotten into the float and making it hevier than normal (thus changing the level at which hte float actually floats and stops the inflow of gas. Also , the needle that the float pushes to stop gas can stick open or closed.
Most of this work can be done without renoving carb from engine but it requires a bit more dexterity to work on it while attached. Goodluck, Macgivor

carburator float is stuck in the open position allowing fuel to continiously run / drain out the fuel tank into the carb and then overflow into the venturi of the carb into the engine and out the air filter. Bottom line the carb needs to be disassembled cleaned and rebuilt

is the leak coming from the overflow or just the seal around the bowls? if you take the bowls off (turn petrol off first) the floats may be getting caught open?