Motorcycle Repair: 1972 Honda CB500 Fuel issue, honda cb500, overflow tube

Question
I've got a 1972 Honda CB500.  The issue I am having is that everytime after I go riding and park the bike and let it sit (turned off), there will be a puddle of gasoline under the bike.  It's coming from the "overflow" tube attached to the carb (I believe), so I'm assuming it is an issue with the carb.  Any suggestions?

Answer
Jon-Micheal,

First of all, it is always a good idea to turn off the fuel petcock when you park the bike, especially if you park it on the sidestand. With a leaking float valve, you could potentially lose the whole tank of fuel into the street which is a fire hazard and bad for the environment.
If you have been turning the petcock off and the carbs still leak, then you have more than one problem. Both petcock and carburetors will have to be checked and overhauled.

Disconnect the overflow tubes so you can determine which one is leaking. Unless you get lucky and it is an outside carburetor and you have some good quality, short-length Phillips screwdrivers, you will have to remove the carburetor assembly for service. Not a fun job, all by itself.

The carburetors will overflow the bowls due to several reasons:
1. Float level is set too high. 22mm is the correct setting with float tang sitting lightly on the spring loaded tip of the float needle, but not compressing the spring.
2. The float valve seat is sealed to the carburetor with an O-ring, which has deteriorated and is allowing fuel to enter the float chamber around the float valve.
3. There is debris in the float valve needle/seat interface and/or the float needle tip is damaged/worn and not seating properly.

If the bike has been run up to operating temperature and left sitting hot, the heat from the engine will radiated upwards, heating the fuel in the float bowls and expanding the liquid to overflow, especially when the float level is set too high. Other rather rare similar situations arise when cracks develop in the brass tube of the bowl.

I suggest that you buy a whole set of gaskets/packing kits from Honda to replace all components when carb rack is off and disassembled for cleaning. The float bowl gaskets can be maddening to install and keep in place during reassembly. Lately, I have had to use Superglue to hold them in place. Current fuels with alcohol cause the O-rings to expand beyond specifications after contact. I haven't been impressed with aftermarket carb repair kit components, to date.

Bill Silver