I recently adjusted timing on my 73 2.0 with carbs to 27btc,this lowered idle and it runs on for a few seconds when I shut it off,also sometimes hard to start.so should I be setting timing differently. dwell is at 50

50 is the high end of dwell. 47 is perfect. Your timing is off. Do it again and remove and plug the vacuum hoses on distributor and rev it to about 3200 rpm. Are you sure of the timing mark?

Yes I assumed timing is too advanced.pretty sure about timing mark.useing fan blade next to fat blade.tough to get dwell exactly to 47,any tricks

That's the nice thing about injected cars, turn off the key and no more fuel flows to keep the engine running. You don't mention what type of carbs but VW used electric idle jets in their carb cars once the emissions regs came into play and the hotter running engines wanted to keep running on after shut down. If your carbs are capable of being converted to the electric idle jets this might be a way to go, or turn your idle speed down as low as possible this would help also. Of course if resetting the timing fixed the problem your good. I've always shot for 47-48 on dwell but 50 is fine. Just remember to lube the rubbing block and oil the felt washer under the rotor with a drop or 2 of engine oil.