Steve - Your bike year/model added to your signature would make any provided advice a lot more specific...
The way a float works is that it is bouyant and rides on the top of the gas in the bowl. It has a needle that rides in a seat and when the bowl fills, the needle is pushed up into the top of the seat and shuts off the hole that gas comes through.
When the valve doesn't work as it should and gas continues to flow, it will first overflow through tubes located in the bowl on VM models. BS/CVK models don't have this tube and gas just continues to rise till it eventually fills the bowl area totally and comes out the needle jet located in the bottom of the venturi area (the thing your jet needle goes into).
If the overflow tube on VM carbs is overwhelmed, these carbs will also eventually overflow out the needle jet.
When you have gas flowing out the needle jet it can ONLY go two places. 1. AIRBOX 2. COMBUSTION CHAMBER
After you fix the problem, DO CHANGE THE OIL!!! The oil is fouled with gas and dry out your air filter so you are not a fire hazard....
The MOST COMMON cause of flooding is that OLD bikes (like yours may be) rust in the tank. The rust and other sediment makes its way through the gas hose and into the carbs where it is drawn into the float seat. The debris gets between the needle and seat and BINGO... you are flooded. To check/fix:
1. Drain tank, remove petcock and clean screen. Flush tank if necessary or if rusted, line the tank (POR15 or KREEM) or boil it with phosphoric acid. Watch as this stuff eats paint.
2. Start using inline gas filter(s)
3. Open up carbs, remove seats and clean debris. If using BS style carbs, clean screen.
The other common causes of overflow are leaving your gas on when parked. The use of an inline fuel shut off indicates problems in this area. BTW: The lawnmower filter is NOT allowing sufficient gas though for top performance, no matter what model you have. Get a Pingel inline fuel cut off valve if you need this. If you have a gravity petcock, it will say Res/On/Off. If you have vacuum, it will say On/Res/Prime. A gravity petcock must be shut off when bike isn't operating. A vacuum is supposed to shut off. If yours is vacuum and may not shut off, test by pulling off gas hose while bike is off and petcock is in "ON" position. If it dribbles, the petcock diaphragm is bad OR you have a leaking vacuum hose which goes between carbs and petcock.
hwms covered all the other possible causes of leaking... CHANGE YOUR OIL BEFORE RUNNING THE BIKE AS IT HAS BEEN DILUTED WITH GAS!!!