Retarding initial timing

[speED]

The MSD curve kit actually has 6 bushings to choose from. I do not use vacuum advance. When curving the distributor my plug became fouled with the green bushing installed which gave me 13 initial and 36 total. I installed 1heavy silver and 1 light silver spring which is 1 of there slowest options. Before this change I had the blue bushing with 16 initial and 36 total.

Originally I was asking has anyone experienced fouled plugs when retarding timing too much. It seems the discussions have prompted many questions in an effort to help me. Please continue to ask for more info if necessary, I feel I have good knowledge of the topic and my set-up.