For the tank, clean out the flash rust with a quick treatment of vinegar again, then drain it, spray some WD40 around inside, blow it out with compressed air or just drain out the excess WD40 and then immediately FILL IT with gas. Keep the tank topped off as others have mentioned. You let it sit for weeks or months half empty or less and you will get rust. But otherwise don't worry about it. Don't bother sealing or coating it, i know others will disagree but don't bother... That crap sucks... I have had so many tanks that were sealed and they flake off and plug the filter or fill the petcock with crap. I have an ATC90 that was rusty inside when i got it, i soaked with apple cider vinegar for 3 days, dumped it, it was nice bare metal, sprayed with WD40 to help get the water out, filled with fresh gas, waited a day, turned it on to reserve and drained a quart or so out to get any water that settled to the bottom, had a few drops that i missed, then tossed it back on the trike. It's been fine for years now, i forget what year i rebuilt that, the thread was on here, i'm pretty sure its been longer then five years now... That tank is still completely rust free, never been sealed or coated or any of that BS.
As for the rear end lube on the 200ES, i just did mine this spring, i generally change the rear end every year or so, depends on how much i have been using it. I do as the manual suggests, drain it, fill it until it dribbles out the fill hole, and call it good, just like a rear end fill on car.
coolpool if your getting oil puking out the breather tube my suggestion would be take the breather tube off, run a piece of wire through it and some hot water or just replace it. Mine did that when i first got it. The breather tube had some dirt in it and was partially plugged, causing it to kinda BURP when things heated up and it couldn't breath freely, it would suck in some air and puke out some oil with the pressure change. I cleaned out the breather tube and it stopped doing that. Only a few ounces of oil in the rear end isn't really enough to lubricate it properly. It does take almost 12oz like the manual suggests.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....