I hadn't ridden my KTM since we moved house two years ago, so I thought I would get it out of the shed, drain the fuel, charge the battery and take it for a spin. When I looked in the tank, the 2 litres of petrol was a brown colour, so I pushed it around to the back of the shed and tipped the whole bike upside down to empty the tank. Once back on it's wheels I looked in to see this mess.
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I think the biggest mistake I made at this point was to push it back into the shed in disgust until the following morning. By then, the brown crap (I will call it varnish as that is what it looked like) had dried and gone hard.
The varnish could have only come from the unleaded, so I naturally filled the tank back up with a fresh batch and left it there for a few hours to emulsify. I then removed the tank and gave it a good shake up before taking the cap off and tipping out what was perfectly good clean fuel. The varnish was still there and still hard.
I removed the fuel pump (it's injected) which was totalled. $1,100 from KTM, but I found an aftermarket one for $400 on Ebay.
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I then had a bit of a google but really couldn't find much. One bloke mentioned that he was going to try CT18, and without any further posts to give an outcome to his experiment I thought I would give it a go as well. After all, CT18 does have a mild acid in the detergent so it should work. I left it in the tank for 24 hours, while also shaking it four or five times. I then removed the cap and tried giving it a light scrub with a bottle brush in the places that I could reach. I tipped out the CT18 and squirted the hose around inside and was left with this.
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The CT18 had hardly touched it.
I was starting to look for second hand tanks online by now, but at that time I also happened to be painting doors for our house renovation and had the metho out to clean brushes. I poured a little bit over the old fuel pump and bugger me if the varnish didn't soften up immediately. I tipped the rest of the bottle into the tank and gave it a shake up only to find that half an hour later it had dissolved 90% of the varnish, and with a quick squirt with the pressure cleaner I ended up with a nice clean tank again. I soaked the fuel pump for a few hours as well, but it is seized solid, so I just have to wait for the new one to come in the mail.
If this was a car fuel tank, it would have been a hell of a job to clean it up, so the moral of the story is to drain the unleaded from anything you aren't going to use for the next year.
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