Need some help with crud identification

[bentwrench]

I'm pretty sure I know what this is, but I need the opinion of the masses.

I went out to drain carbs and put the battery on a charger for the winter, and I noticed two things: 1, nothing came out of the drain plugs on the carbs, even though they sloshed, and 2, there was a growing brown stain around my gas cap. I opened up the tank, and there was a brown crumbly dirty residue around the gas cap. Not rust, this stuff is soft, like fine dirt.

Of course, it's everywhere in my fuel system.

Could this have been caused by still having a little bit of water, or possibly vinegar in the tank from when I cleaned it out? The bike has been sitting only 3 months and ran before it sat.

 

[16VGTIDave]

1) Sounds like you will be cleaning your carbs. Don't forget to spray them with oil or such to prevent corrosion before next season.

2) iron oxide aka rust. Bare steel, air, and probably some moisture in your tank. Clean, clean and clean some more! Then seal and protect the inside of that tank or it will rust again in no time!

Dave

 

[bentwrench]

That's just it - it doesn't feel like rust. Very soft, and collected near the gas cap. Almost like dirt, but I can't imagine dirt collecting like that...

 

[drewpy]

have you got tank liner in there?

ethanol may be affecting something in there.

 

[bentwrench]

No tank liner. I wonder if there was a thin layer of rust that I removed, exposing the bare metal that's caused this? But then, rust is pretty porous...

I keep thinking this must be a reaction to something I put in the tank, and that's water or vinegar. I guess in either case, it's time to remove, clean and try again.

 

[Lou Ranger]

... there was a growing brown stain around my gas cap. I opened up the tank, and there was a brown crumbly dirty residue around the gas cap. Not rust, this stuff is soft, like fine dirt...

The brown stain was on the outside of the tank around the cap? In the recessed area around the cap only or on the higher painted surface too?
And the crumbly residue (detritus) was inside the tank concentrated just below the cap?

 

[bentwrench]

The stain was in the recessed area, and all over the outside in an expanding radius. It wipes off with my finger or a rag, revealing my pristine steel underneath. And yes, detritus just inside the tank, well away from the gas, just below the cap.

 

[Lou Ranger]

The stain was in the recessed area, and all over the outside in an expanding radius. It wipes off with my finger or a rag, revealing my pristine steel underneath. And yes, detritus just inside the tank, well away from the gas, just below the cap.

It sounds like the source could be the cap itself. I wonder what quality materials it is made of. Chromed steel or chromed aluminum, rubber gasket, steel spring.
If one of those materials was substandard (compared to the steel in the tank itself) or if there were 2 types of metal or alloys contacting each other (or both) maybe galvanic corrosion is taking place. Is the cap an original Yamaha cap or a repaired or aftermarket one?

 

[bentwrench]

I'm pretty sure it's the original Yamaha cap, but only because the key matches the ignition (who knows after 30 years what was replaced...)