UPDATE:
Well, I'll tell you what, this little tank-cleaning project has been kicking my butt...
I realized that, indeed, the root cause was just still unclean sections of tank. Either the new gasoline or the ethanol IN the new gasoline simply started dissolving what was left behind and sent it on downstream. I guess I should've left the vinegar in there longer the first time around... In the past, I've been able to get away with 'good enough' on tank cleaning and just using an in-line filter to catch anything left. Not this time. That varnish went right on past the filters and didn't plate out until in the intake tract.
So, I decided to use something a little more aggressive than the vinegar next, thinking that the deposits left behind are going to need it. So, I used Muriatic acid (garden-variety swimming pool acid). I diluted what comes from the pool store 3:1. That is, I made 4 gallons of mix from 1 gallon of acid.
All safety gear on, etc. etc. and poured it into the tank. Within
10 MINUTES that diluted acid had eaten right thru the aluminum petcock and was gushing out into the plastic catch pan. :shock: :shock: Fortunately, I had decided to use the plastic tub rather than the galvanized metal tub this time around. Sigh. It ate right thru the aluminum portions of the petcock. The petcock lever got completely dissolved - no evidence left whatsoever. The sediment bowl is 70% dissolved. The copper tubes that poke up into the tank - very clean and shiny, but still OK. Rubber 4-hole gasket? Steel wavy lever spring? Steel bolt? All untouched...
Well, I managed to not harm anyone nor damage the tank any further, but that was the end of the acid experiment.
I wish I had read others' experiences a bit more thoroughly first, because two or three other threads on the subject clearly warn not to mix acid with aluminum...
Bike Bandit to the rescue with a new petcock, but still, a $48 'lesson-learned'! :shock:
So, back to vinegar... I fabricated a steel blank-off plate with gaskets for over the petcock & bolt hole. This time, I let it sit for 48 hours! By the time I dumped it out, I had liberated a whole new handful of gravel from my dad's previous attempts at tank cleaning!!

I hadn't even realized it was IN there!! This time, the tank was very nearly perfect! Using a bright light and a dental mirror, I could get a line-of-sight on the entire inside. It was very nearly perfect. Still a few stubborn deposits along the seams.
So, a THIRD batch of clean fresh vinegar went in, and this evening will mark the 48 hour mark for THAT batch. I'm thinking the tank will be very clean this time around. I checked on it yesterday, but other than a slight layer of scum on top of the vineger, I couldn't tell much. Fortunately, nothing has started leaking - no seams nor rust holes.
What a learning experience. That's the only thing that makes it all OK.
I'll re-do the Ospho treatment when it's all over, because the Ospho does/did a great job on stopping the flash rusting that occurs after rinsing out the vinegar.
