cause of old gasoline?

gone.

Check/change the fuel filter. Varnish in filter can be released as fuel is passed through it. Could cause more buildup further down the line. Might even build up in new carb. Empty tank. remove and have it cleaned. Kero will do it but it should be done by a pro. better safe than sorry. check for vacum leaks. Remove or disconnect fan(electric) start and listen with a peice of garden hose or rubber tube. dont let it get too hot.

is the new carb fron another 83 or is an after market Webber or something? to adjust the idle on the orioional, there is a knob on the back (passenger side) counter clockwise should turn it up if i remember correctly. all the vaccume lines coming off to the 2 black boxes under the hood are for emmssions and do not affect that much, at least not what you are describing, the high idle and so on is also contriolled by vaccume so there amy be a leak there. i would do what Skootter said and see what the differance is.

Listen for Hissing noises along all of the hoses. Especially where there are clamps and bends. They are the most likely to crack at these places. Also since the cost of tubing is minimal just replace it . Don't try to stretch it to fit. You'll just end up replacing it later. Another tip. After you check a line mark it with chalk. Most cars and trucks have so many it is easy to forget which ones have been checked.

and this one has got a shit load of them, i did a head swap on mine last year and labled and removed up to like 25 or something