You mentioned "my old carbed gm 350 Volvo I/O".
May we assume that this engine is equipped with a mechanically advancing ignition system?
If not a carburetor issue, please read on.
(disclaimer.... this is a long shot!)
.... at idle speed, there is no mechanical advance being added to BASE advance.
.... BASE advance should be constant at either 8 or 10 degrees BTDC.
.... your system uses a flyweight/cam affair (2 of these).
.... the flyweights are controlled by two small springs that pull them both back into (and keep them in) BASE mode at idle speed.
.... the spring value allows the flyweights to progressively advance the spark event as RPM increase above idle speed.
.... when/if the flyweight return springs have become rust compromised, they loose their spring value.
.... when/if they have lost their spring value, they will not consistently pull and hold the flyweights back into BASE ADVANCE mode.
.... if this was to occur, the spark "lead" at idle may be somewhat erratic or pulsating, so to speak.
.... even with a good fuel delivery system, a pulsating BASE advance spark event (of let's say 2 degrees or so) will present itself as engine RPM "surging".
Connect up your standard mode timing light.
With the engine at idle speed, see if the BASE advance is steady or if it is erratic.
If the BASE advance is erratic, your flyweight system is not holding BASE advance at idle speed, of which may indicate weak springs.
CAUTION:
Do not haphazardly replace the flyweight return springs (no matter what the supplier says) without the aid of a shop who owns/operates a Sun, Allen or King distributor machine.
These little springs not only hold BASE advance while at idle speed, their spring value also controls the very important progressive advance.
Again, this would only apply if all else is OK.
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