Just fired up my motorcycle after 3 years in storage. Changed the oil and replaced the fuel in the tank. It starts easily. It is leaking fuel from somewhere under the gas tank. The engine has a caburetor (not fuel injected). Some reading tells me common problems are dry o-rings or gummed up carbs causing them to overflow.

Is there anything I should try before starting the daunting task of dissassembling the carbs?

(replace fuel filter, fuel additives, run the bike for a while?)

3 Answers 3


You said that it is leaking from somewhere "under the tank" and then jump directly to the carbs. If it is the carbs, you'll probably have to tear into them.

If it is from under the tank, there are a few things you can check:

  1. Is the tank intact? You may have rust burrowing through the tank and causing leaks. This is bad. Should be visible in the fuel coming out of the petcock.
  2. Are the fuel lines intact? The rubber fuel lines can dry-out and crack pretty quick.
  3. Is there any leaking at the petcock? Depending on the design of your CBR, there may be a vacuum-activated petcock, inside of which the membrane can dry out and crack, causing leaks. Or the seal between the petcock and the tank has cracked.

That should give you somewhere to start.


A stuck carburetor float-bowl valve (commonly referred to as a 'stuck float') is a frequent cause of overflow problems. Each carburetor has a fuel reservoir on the bottom containing a floating shut-off valve inside that stops fuel from flowing to the carb when the fuel level rises high enough in the bowl.

When there is a stuck float, fuel keeps flowing even when the bowl is full. This causes gas to back up into the carburetor and eventually out a fuel overflow tube. This tube is sometimes on the top of the carbs, sometimes on the bottom. All carbs typically share the same overflow tube. Note: This overflow tube is different than a drain tube.

If you are lucky, a stuck float can sometimes be jostled free by rapping sharply on the float bowls with the wooden handle of a screwdriver. If you are not lucky, you will have to remove and clean the carburetors.


I would get the tank up and take a very good look around for the leak. Does it only leak when running? That may give you a clue as to whether it's tank or carb related. The chances are that it'll have a vacuum activated petcock as Jack M. mentions above. They can stick open and allow fuel to flow all the time into the carbs, whcih then overflow into the crankcase. You can open up the oil check port and sniff to see if that smells heavily of petrol, that would indicate that the carbs are leaking the fuel.

Another trick to try it to get the tank off and wrap a little rag or paper towel around each joint in the fuel system and then leave it for a bit. Then go back and inspect the towel for petrol. the towel should be soaked if there's petrol leaking. (I also do the same trick when working on any home plumbing to make vey sure I've not left a slow leak in any joints)

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