Tank Cleaning and Petcock

The tank was rusty inside when I got the bike, a pretty common problem when a motorcycle's been sitting around for as long as this one had been. It wasn't rusted through or anything, which is good, and the pits didn't look too deep.

The fuel gauge appeared to work when I first turned it on, but the petcock leaked badly. With the tank on the bench and drained of old nasty fuel, I removed the petcock for cleaning. Then I pulled the fuel gauge to make cleaning the inside of the tank easier.

The biggest problem with a rusty tank is the little rust particles that get knocked loose and get into the carburetor. There's a couple of ways to deal with the problem, including some involving expensive liner kits that tend to be finicky and sometimes cause more problems than they solve. The tank has a dent, which is another problem for a liner kit - removing a dent will usually chip the brittle liner and cause it to flake off. Since I'm not planning on having it painted right away I decided to just clean out the rust as best I could and add a fuel filter after the petcock.

First I rinsed the tank with solvent, in order to get rid of any old fuel deposits inside. This loosened quite a bit of rusty flakes, but there were plenty more inside. To remove them and clean the rusty surface of the tank, I filled it with a gallon of phosphoric acid. You can get it at Home Depot, and while it is nasty stuff it's not as bad as it sounds. Really - it's used as a preservative in cola, among other things.

I taped up the petcock and fuel gauge holes with duct tape and rotated the tank slowly, over about 24 hours. Phosphoric acid eats rust but attacks the good metal only slowly. At the end of the 24 hours, I drained off the phosphoric acid and returned it to the container. It should be good for a few more uses, at least. I then rinsed the tank with the hose outside.

To get rid of the water and prevent flash rusting, I threw in a quart of denatured alcohol. Alcohol is hygroscopic (will mix with any remaining water) if I remember my high school chemistry lessons well enough. After swishing the alcohol around for a bit, I drained it off too.

Finally, I mixed a couple of teaspooonfuls of motor oil with a couple of ounces of solvent. I dumped this mixture into the tank and swished it around to give the inside of the tank an oily coating that would keep out moisture and prevent rusting until I can get some gas in it.

The petcock ("Fuel Cock" in Suzuki speak) leaked when I first tried to start the bike, so I pulled it apart for inspection. The main inside seal was rotted and the outer lip seal was no better. The molded o-ring between the petcock and tank was intact but hard as a rock. The metal parts of the petcock were fine, so I ordered new rubber to rebuild it. Like all the other OEM parts I've bought, they were fairly inexpensive - under $10 for all three.

With the petcock and fuel gauge sender back in the tank, it was ready to go back on the bike with some new fuel line and a fuel filter. I reinstalled the seat, too. Suddenly the GS is starting to look like a bike again, but without the forks, of course...