As Bob mentioned, Seafoam is a pretty standard use item on the Ventures. Start with a full can to a half tank of gas, and ride it to get it completely through the carbs. Then every day take it out and ride it like ya stole it till the gas is about gone. this will clear out a lot of carb issues.
As Bob mentioned the oil leak is most likely the grommet around the stator wires, very popular leak point. The wires are covered in a cloth sheath and the oil can and will wick up the cloth right thru the grommet and leak. You will need a gasket set for the left side cover and the middle gear cover. Many of us use a new crush washer on the bottom bolt of the middle gear cover as that is the drain for the oil in the transmission. Once you get the side cover off clean it up real good. use brake cleaner to clean all of the oil off the grommet and out of the cloth insulation of the stator wires. Then slide the grommet up the wires a bit and clean the oil out of the newly exposed cloth insulation. I use the black Permatex RTV to rub into the cloth where the grommet will sit and then slid the grommet back into place. Clean the cloth above the grommet again to get the oil that was dragged out of the holes and apply a seal of the black RTV on both sides of the grommet as well as to the sealing surfaces between the cover and the grommet and put it all back together.
When doing normal oil changes you do not need to pull the trans drain plug, there is not much oil actually trapped in there. The only time you need to drain there is if you are cleaning out contaminated oil from a coolant or gas issue and you need to get out as much as possible.