Anytime you see an engine liking, wanting, or responding well to running a butt-load of timing at idle it simply doesn't have enough compression for the cam being used, or not nearly enough idle fuel for the amount of vacuum being produced, or a little of both.
A basic "rule of thumb" I use here is to set initial timing around 10-12 degrees, get the carbs idle system up to par for the application, and see how well it will idle, vacuum produced and control with the idle mixture screws without nozzle drip. If I'm not seeing at least 10-12" vacuum the engine simply has more overlap in the cam that what's it's happy with. At that point you have options to increase initial timing to save the day, or add more compression, or change the cam for a better choice.
In almost all cases I'll take steps to simply add more timing to make things happy, and may even come in with a little more idle fuel and bypass air to compliment that deal. It's certainly a LOT easier than going back into the engine again and in almost all cases I'm able to reach an acceptable end result.
I've said it many times on here and other Forums. Cam companies "push" and cater to the "bling" crowd by grinding cams on tight LSA's. This provides the end user with that nice "chop" at idle that everyone loves and folks that hear it assume that the engine is now going to run like John Force's Pro Stock engine and make butt-loads more power than a similar build with a wide LSA cam and smooth idle quality.
Nothing could be further from the truth, but for sure for most of the customer base out there they don't want their new engine to idle like a stock 1987 305 Chevy pick-up truck!
I've actually helped customers with compression ratio increases and cam swaps making HUGE improvements in engine power and vehicle performance and the FIRST comment they make is that "it's idling too smooth"!..........FWIW......
