Forget setting the timing at idle. Set it at the max advance mark "M" at 4000 RPM. Let the idle advance fall where it may.
With that done you should be able to set a smooth idle IF you have the carbs correctly syncronized.
Do you have the factory air box or an aftermarket plate tying the two card together with a support rod to the motor mount? That is absolutely necessary to maintain syncronization.
The engine is set up with trumpets and it does have a metal plate holding both carbs together supported with motor mounts. I have removed the trumpets until i get the carbs in sync.
Having the engine working very rough at idle, it is very hard to get a good reading with the Synchrometer,
I will get the timing on the "M" @ 4000 rpm this weekend.
1971 - 2000 GT Veloce Euro