Test for the first kind of leak, the one at a fuel passage or hose, by cleaning the carbs then powdering them all over. Use Tinactin athlete's foot powder aerosol. It goes on like flat white paint and when done testing, it washes off easily. The powder will plainly show where the leak originates. Again, many times it's just a poorly fitting fuel hose, whether at the carb or at the petcock. So even if you see fuel all over the side of the carburetor, realize it could have originated at a completely different place.

The other kind of leak, that related to the float bowl, is a bit more work, mainly because this kind of leak has more causes even though all are related to the float bowl. To check the gasket, do the powder test. To test the drain screw, put in one from another carburetor. Test the float by checking for distortion or with the hollow floats by holding them under water. The float setting rarely causes a fuel leak but it's a quick check. Don't alter the float height from the factory spec. The float valve is best tested with a Mityvac, but you can also hold the carb upright with the bowl off and some fuel feeding in, and raise and lower the float while eyeballing the valve. Most float valve leaks, when the valve parts are OEM, are caused by contaminants in the fuel, so be sure and drain some fuel into a clear container to examine it. Aftermarket float valves are famous for not sealing well, if not at first, then in a very short time.

I see this a lot. There really is no excuse for it. If one or two raps on the bowl don't stop the overflow, quit!

The float bowl vent, essentially a part of the float bowl, is a special case and fairly common on certain carb models such as the PD series Keihins. The 77 model CB750 is practically the "poster boy" for bowl venting issues. Enlarging the vent slightly with a number size drill is advantageous on this carb. For all carbs I spray brake aerosol through the external bowl vents, watching for cleaner to exit inside the carburetor's bowl area. When rebuilding your own carburetors, it's important not only to check the float bowl vents but also to not make any modifications that interfere with that venting. I see this a lot. An unvented float bowl will push its fuel back out even as it enters the bowl.

Lastly, the float bowl overflow standpipe. Some Honda carbs don't have standpipes. For example the carbureted Gold Wing never had them. The V4s neither. Many of the VD and VP series Keihins lack them also. However, where they do exist they can be troublesome. These brass tubes develop cracks along their lengths that allow fuel to overflow constantly. The VB series Keihins are the worst in this regard. Most are easily repaired by soldering. The really large cracks however require replacement of the tubes, which you can get on the aftermarket.

And let's not forget the occasional overflow that appears out of the carburetors' overflow hoses. Very occasional overflow here is not really a problem. Carbs are pretty fundamental devices subject to contamination and not especially equipped to deal very well with it. Inline fuel filters help a lot however. Experienced riders of vintage motorcycles know that in the event of an occasional overflow, the first tactic is to gently rap on the offending carburetor using the plastic handle end of a screwdriver. Nothing harder, if you please! And, not repeatedly and persistently! Instead of knocking on the carbs non-stop, realize you have more than just a tiny bit of debris in that float bowl. The next level of troubleshooting a recurring overflow is to open the carb's drain screw and the petcock and flush out that carb's float bowl.

Now notice I say, "the" and "that" carburetor. It won't be easy to tell which carburetor on a multicylindef bike is overflowing unless you remove the overflow hoses and keep them off. I advocate running without those hoses partly for this reason. Another reason is, not having hoses makes regularly draining the bowls when the riding season either begins or ends far easier.

One last thing. When you get carbs back from a rebuilder they have traveled to you in a box on a truck and have been bounced, maybe even dropped, and certainly knocked around a bit. It is not unusual therefore for the float valve to receive some shocks that will cause it to not function the first time you gas them up. Be aware of this. Use the screwdriver handle method if needed to straighten them out.

Make understanding, expecting, and resolving fuel leaks a part of your maintenance mindset. When the very best OEM float valves are used in the rebuild, they will prevent almost all overflow issues. But even then leaks can hapoen, whether of the first kind or the second, and you must know how to deal with them.