Question about Suzuki GS 400 E Motorcycles
The fuel petcock for your gas tank has a vacuum bladder in it. If there's no engine vacuum, fuel is supposed to stop flowing out of the tank, preventing carb overflow and accidental draining of the tank. However, once the engine starts, fresh gasoline will begin to flow out of the tank into the carbs. I believe but am not certain anymore that the vacuum cutoff mechanism is disabled when you turn your fuel petcock to "prime" or "reserve." If that is correct, you should be able to duplicate the carb overflow with the petcock in one of these positions.
As for the specific cause, it sounds like one or both of your carb floats may be sticking in position, preventing the fuel shut-off needle from seating correctly. This can easily happen if you haven't ridden the bike for a while--the most volatile compounds in the gas in your carbs will evaporate, leaving behind an increasingly gummy and sticky substance that will eventually harden into varnish if it's not cleaned out. That gummy stuff seizes up the pivot points for the carb floats, so they don't move correctly. Gummy gasoline residue can also build up over time, even if you ride the bike often, though this usually happens after the bike has been sitting. The fix for this is to clean out your carbs. On an early (1982) GS450, I was able to do this once by removing the carb bowl without removing the carbs from the motorcycle and spraying the float pivot points with a heavy-duty gum cutter (Berkebile 2+2--it's like carb cleaner on steroids, but it will also damage paint).
By the way, if I remember correctly, your carbs have a crossover tube to ensure that both carbs have the same gasoline level in them. This keeps the fuel mixture even for both cylinders and keeps the engine running smoothly. However, that also means that a single carb with a stuck float will flood both carbs equally.
Posted on May 28, 2011
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Posted on Jan 02, 2017
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