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Ok, I was in the garage and I came up with something new, I think. I searched and didn't find it so here is my "so easy a caveman can do it" petcock mod. This will add a "off" position to the petcocks on standards without effecting the vacuum operation of the other positions. If you wanted to you could still remove the vacuum diaphragm and have totally manual petcocks, or just have a handy off position.
First take out the two screws that hold on the front "label" plate to the petcock.
Now remove the cover, careful not to loose the little round spring washer.
You'll have this
See that little nub directly below the "Fuel" marking. This keeps the lever from being able to spin all the way around.
Now take your dremel with a grinding stone
and remove that nub, like this
Now put the cover back on your petcock. You now have the ability to turn the lever to a new position "up". With the lever up, opposite the "on" position your fuel is off. Vacuum or no vacuum the fuel can't flow in the new off position. All other positions work just as they always have. I look at it as a peace of mind thing, when I park for the winter I can turn it "off" and know that gas won't run. And when I pull the tank I can switch the petcock to "off" and let it run for few minutes to drain the gas from the lines so no mess. I just like the idea of having a positive off, instead of having to trust the vacuum deal, and wondering if gas is going to pour out when I pull the lines off the tank.
First take out the two screws that hold on the front "label" plate to the petcock.
Now remove the cover, careful not to loose the little round spring washer.
You'll have this
See that little nub directly below the "Fuel" marking. This keeps the lever from being able to spin all the way around.
Now take your dremel with a grinding stone
and remove that nub, like this
Now put the cover back on your petcock. You now have the ability to turn the lever to a new position "up". With the lever up, opposite the "on" position your fuel is off. Vacuum or no vacuum the fuel can't flow in the new off position. All other positions work just as they always have. I look at it as a peace of mind thing, when I park for the winter I can turn it "off" and know that gas won't run. And when I pull the tank I can switch the petcock to "off" and let it run for few minutes to drain the gas from the lines so no mess. I just like the idea of having a positive off, instead of having to trust the vacuum deal, and wondering if gas is going to pour out when I pull the lines off the tank.
Edit: Note that this will prevent fuel flow if you're having problems with the vacuum diaphram/valve leaking; a possible fix for that is here: petcock spooge hole fix (this one also applies to the octopus on a Special). If you have leaks around the lever, you'll still need to rebuild the petcock.