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Old 06-21-2002, 04:32 PM
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Where Does the Gas Go?

Where Does the Gas Go?
Rick Jemison

If the petcock is leaking the gas will go down to the carb inlet needle/seat and stop. If the needle/seat also leaks then the float bowl will fill up.

From here things start to get interesting. The fuel is now obeying simple Newtonian physics and will try to go down wherever possible. The normal route is up through the main jet / pilot jets and spill into the carburetor throat (where your butterfly and slide are). This is where the condition of the rubber boots, bike's attitude and thoroughness of your installation (or the previous mechanics' thoroughness) starts to be important.

Assuming the bike is level, and the carbs are installed into the boots solidly, the fuel will spill equally over the floor of the carburetor throat. Since the butterfly represents a restriction at the engine side of the throat, the gas will start to spill into the air box rubber boots, and wick up in the air filter. When the filter becomes saturated, the fuel will continue it's journey to the ground spilling first into the bottom of the air box and finally out the air box drain hole onto your engine and eventually the ground.

Real World
The following things (any combination) can allow the gas to spill into the engine:
  • bike on side stand
  • bike on center stand on an uneven surface
  • carbs cocked in the rubber boots
  • carb sync settings resulting in substantial opening at bottom of butterfly
  • incorrect idle setting
In this case, fuel will still spill into the air box but now fuel has a path to the engine. Depending on how good a path you could experience little or no immediate observable fuel leakage from the airbox. The fuel flows into the intake port.

More options now

Valve closed
If the intake valve is close on the cylinder that is leaking the fuel will puddle. If it fills the port, then obviously fuel will now spill into the airbox and will shortly become a visible fuel leak. The problem here is when you try to start the engine a large quantity of fuel will initially foul this plug. Much of the fuel as the engine turns trying to start will be forced out the exhaust valve and into the muffler...

Valve open
This is the worse case as the fuel will have time to sit, wash the cylinder wall oil off and dilute the oil in the pan. It is also hard to detect (or maybe I should say is easily initially overlooked.)

Hope this helps.
 

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