2012-04-05

Oldbike: Fuel Tank Petcock Rebuild


In my previous post I found that a bunch of gas had leaked out of the left carburetor while the bike was on it’s side stand.  Since then it has happened intermittently under the same circumstances though generally only after the bike has sat for a bit and as soon as the bike is brought upright the leaking stops.  After some web research I found that fuel petcock issues are common on the Suzuki GS bikes and that the side stand leaking issue is one of the symptoms of the petcock failing


Another symptom is gas in the crankcase which would make your oil smell like gasoline which is something I identified earlier whenchanging the oil.

The general consensus seems to be that the issue with the fuel tank petcock, which is vacuum actuated (it automatically starts the flow of gas once the bike is running and shuts it off when it is not) is that due to the old age the seal inside the petcock fails and or some of the other rubber parts fail and then you have fuel being fed into the carbs all the time.  Now my situation must be a two stage issue there is the face that the petcock will not shut off and (I believe) the float actuated shutoff in the carb is not working for some reason letting gas flow into the float bowl until the it comes out the overfill tube.  Another part of my hypothesis is that when the bike is upright the flow of fuel does not stop, but the path changes into the crankcase rather than on the ground.  Some people recommend skipping directly to replacing the old vacuum actuated petcock with a manual petcock with a shutoff. I decided to rebuild the petcock on my bike because it was the cheapest option ($24) to start out with.  The manual petcock can cost $79+ and an in-line shut-offs wouldn't keep gas from back-flowing into the carb and proper fitment is not a guarantee so I was leery of that option.  I lucked out and a local shop Motorcycle Performance had the rebuild kit on hand.

After removing the petcock I took some shots of it from different angles which you can see below.







The  first part I removed was the front plate it comes off with the two Phillips head screws you can see in the photo above and below you can see a shot of it removed.




Directly under the plate there is a wave washer, this is meant to put pressure on the switch which in turn keeps the back side of the switch pressed on the gasket that is behind it.




You can see the gasket on the left and the back of the switch on the right.  When set to the “ON” (normal) position the hole at the 12:00 position feeds gas through the center vacuum actuated valve from the from the taller feed tube in the tank.  When set to the “RES” (reserve) position the hole at the 3:00 position is feeds gas through the center vacuum actuated valve from the lower feed tube in the tank.  When set to the “PRI” (prime) position the hole at the 3:00 position feeds gas from the lower feed tube in the tank to a hole at the 6:00 position that bypasses the vacuum actuated valve and dumps fuel directly into the carbs.




This gasket can compact and flatten which can allow gas to the fuel bypass at any petcock position causing overflow problems.




There is also a gasket that goes around the outside of the switch that is included in the rebuild kit that keeps fuel from leaking out around the petcock switch.




Another gasket that comes in the rebuild kit is the one that seals between the petcock and the gas tank.




On the back side of the petcock is the vacuum diaphragm.





Suction comes from the carb via a vacuum line to the smaller of the two fittings on the petcock and pulls the metal plug with the rubber o-ring compressing the spring behind it and opening the valve to let gas flow through the valve and into the carb.  Leaking can happen when the o-ring shrinks and/or the contact point where the o-ring seals to the petcock has inconsistencies in it’s surface.  You can see a demonstration of how this works below.




After you disassemble the petcock you basically just go about replacing the gaskets included in the kit.




Another thing that you should pick up are some washers that have rubber in the center to seal around the bolts that hold the petcock onto the tank lots of times they are shot and are cheap but not included in the kit.  The shop I got my kit from had them in stock so I got them there.


You can see some shots of the different gaskets after replacement below.




I used some grease around the switch’s gasket to make sure that it eased back into position without doing any damage to the gasket. 


After re-assembly I re-attached the petcock to the gas tank and that was as far as I got today.


Stay tuned for the follow-up on whether or not the rebuild fixed my issues.
Thanks (as always) for reading!


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