SM needles

I recently rebuilt my SU carburetors and went with SM needles-i have a stock 240 engine (71) with headers and the plugs are black after less than 1000 miles-1 of them actually quit firing which made me pull them in the first place-anybody successfully running SM needles on a stock 240 engine? I've got Scott's video, and the SU's are tuned properly-as a matter of fact the car runs great until the plugs get fouled up from the rich mixture...just wondering if perhaps the SM needles dump too much fuel into a stock engine so that's why i wanted to know if anyone has a stock 240 with SM needles without mixture problems. Thanks for any help you can provide.