The first thing I would check is the floats. Make sure they are not taking on fuel which makes them sink. An easy way to tell is remove them and give them a gentle shake. If you hear fuel sloshing inside one or both, there's your problem. Another thing that happens is sometimes the needle valve may hang up in the seat bore and do the same thing as a sinking float would do, and that fill the fuel bowls up to the top. When I rebuild Carburetors for my customer's, I always take a piece of super fine steel wool and burnish the needle valve. If there's a little nick or spot on the needle, that could hang the needle up and allow the bowls to overflow. Another issue is dirt in the tank.Always make sure the tank is super clean. The Viton tip on those needle valves work great when they seat, but if a minute piece of dirt gets stuck to that Viton, its like glue. The needle will not seat and fuel will creep through. Another problem with a fresh rebuild, is when opening up the petcock on the sediment bowl, only open it up half way or less. Sometimes what may happen if the float bowls are empty, and fuel comes rushing in, the floats could be slammed up and stick. This will also overflow the fuel bowls causing the fuel to run out. If you open the valve up just a little, get the engine running and then open the petcock the rest of the way. This gives the floats a chance to level out.  HTH

Steve@B&B

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