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The scum is from E-10 fuel gone bad, best to use non ethanol. All I use cept for the Jeep, even then I use a conditioner.
I am paranoid about my fuel. The stuff has really gotten nasty. I forgot not everyone can get real gas.My float bowls are clean at least, bikes may sit for few months if I don't keep swapping them around.I wonder what the tank looks like. I just got done doing a tank on a Quota, sat or 17yrs BUT real gas ran in his lawn mower(he says). Lot of junk hard after I got taps off. Haven't gone back to see if pump survived. Waiting on the owner, no hurry I guess it's already been 17yrs.
I hear all that, but I'm saying I've never found gunk in my own stuff with reasonable care or usage. I never add anything unless it's going to be in storage over the entire winter. When I've come across crap in other's stuff it's without fail been in conditions that non-E10 would have gunked too.