Originally Posted by Karl

12 - 12.5 :1 AFR at warm idle is usually the best AFR to give stable idle when using 403's.

Due to the dreadfull spray pattern of 403's the vast majority of YB engines have a very lumpy idle when trying to idle at anything near lambda 1. (14.7 :1)

A very healthy engine with std cams will usually be ok at around 13.0:1 with 403's, but in general best to run them slightly richer to ensure a good quality idle.

Remember this is only at idle folks and will cause no harm to the engine at all!