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This little mod can either give a functional but slightly leaky vacuum petcock a positive shut off, or do the whole thing and make it a completely manual petcock. This works on any GS prior to 1980 as far as I know, doesn't work on anything 1980 or later as they use completely different petcocks. Any petcock that looks like this:
My 550/675 I did the whole thing, there's no vacuum function at all anymore. The 750 I'm doing today I think I will leave the vacuum system in place. It mostly works, but doesn't shut off completely anymore. I have been disconnecting the fuel line when I park it more than a day or so, and putting on a long hose, looping it up over the gas tank so no fuel can come out. Pain in the butt so I'm doing the mod to add a real off position.
On the front there is a steel plate held on by two screws, the one labeled On, RES and PRI. This one:
Take it off, there's a little tab at the top of the hole which stops the travel of the lever at either PRI or RES.
Grind off the tab, so the lever can go all the way around, you now have an off position. The lever straight up is off.
It works because the curved slot in the inside of the lever merely joins the two inlet pipes together, from the two pipes in the tank, the ones at 12 o'clock and three o'clock in this pic. It does not join them to either the outlet to the carbs on the bottom or to the vacuum valve section in the center. There is no path from either inlet pipe to the rest of the petcock.
Depending on your tank and the exact petcock position, you may have to shorten the lever to be able to put it straight up. Some tanks the lever hits the welded edge of the tank, some tanks it hits the bottom of the tank, some tanks it doesn't hit at all. Grinding the lever end off to the edge of the circle has been enough on all of them I have done. My 550 didn't need any grinding, although the lever hits the top of the carb if you turn it in that direction. Going the other way works it fine as is. With the stock VM carbs it wouldn't hit at all.
On the 750, the lever hits the tank, but it goes far enough around to shut off the fuel first, so I didn't grind it off. This is off:
That's it, you're done if adding the positive OFF is all you want to do.
To disable the vacuum function completely, take a piece of inner tube rubber or similar and make a gasket for the cover of the vacuum side.
Pull that vacuum stuff out and toss it. Or sell it on Ebay.
There's a tiny hole that must be blocked, about a 1mm hole which vents the back of the diaphragm. The hole in the 4 o'clock position in this pic:
If this vent is left open fuel will just run out all over your engine. Block it somehow, tap it and put in a tiny screw, or fill it with JB Weld, use your imagination. The one I did yesterday on the 550 I hammered a round toothpick into the hole and broke it off. You could also make a flat metal plate to cover the vacuum area, use that instead of the old cover and the rubber gasket would block the tiny vent hole completely. You will see what I mean when you take it apart.
Put it back together, try it out. If the off position still doesn't shut it completely off your rubber piece with the five holes that makes up the valve is probably wasted, or the little wire spring is flattened and not putting enough tension on the lever. Could be the lever needs to be shortened to go far enough around to shut it off. If it leaks out the front around the lever, you need a new O ring on it. These parts are still available from Suzuki. No need to buy the expensive rebuild kits that don't work. Don't forget to block off the vacuum port on the carburetor!!
These part numbers are from a '79 550, but the others probably use the same parts.
44353-33154 GASKET, LEVER $5.85 This is the one that seals around the lever as it turns.
44341-33150 GASKET $2.09 This is the one with the five holes in it that makes up the valve itself.
44348-31051 O RING $5.85 $4.74 This is the one that seals the whole petcock to the tank.
