1993 Nissan Serena FX eating fuel and stalling in idle

Hi there, I have a 1993 Nissan Serena FX 2L that has developed some issues just recently. Car starts easy but then idles badly but does not stall, but if I decelerate to idle (junction) than it often stalls. It also uses heaps of fuel and exauhst fumes are quite black. BUT, when driven on highway for a while all appears to be fine, idles great and fuel consumption is normal-ish. Problems persist if car is restarted soon after.

Any ideas? The speedometer also reads super high until driven fast for a while.