IGN advance too slow
Not knowing what type of distributor you are using nor ign. I would simply recommend that you run as much initial timing as you can crank with the lightest springs you can get and honor your total timing limit, what ever it may be. Better yet, locking out the advance in the distributor and set the timing total at idle like a crank trigger does. Some guys use a timing retarder in there ign to crank more easily during start up, but usually are not needed in most cases.
More timing sooner is better and smoother idling and gives a higher vacuum signal which gives better idle performance.
If you have power valves in your carb, you may be getting a over rich condition to boot with the low vacuum signal to the valves at idle in your current timing condition.
Bruce