OK, I'm crazy rusty on this little bike but here are 3 things I remember:
1. The choke cable was seized solid on mine - it was stuck on a little - enough to look not on but it was. Constantly flooded the bike on starting until I fixed the choke cable (made my own)
2. Seafoam is your best friend for stabilizing. It might also help for un-sticking some things in the carbs. We fought tooth and nail to get and keep poor Dita running before we discovered seafoam. Since 2006, we've used it to stabilize all the bikes for 6 months of the year (yeah, winter sucks here) and never had a fuel related problem with bringing any of the bikes - carbed or not - out of storage. In fact, the dirtbikes have been stored for 2 or 3 years now with stabilized fuel and they started at the end of last summer. If Dita started to burp and fart during the riding season, I'd often throw some SF in the tank and she'd straighten out.
3. The jets on a B4 are pretty much small enough they can't pass a water molecule. The better the fuel, the better your chances.
