Black smoke on start-up
 

I recently changed my plugs and cords (NGK Z06 cords, and NGK BUR9EQPs in both leading and trailing positions). I read on another thread that the only reason Mazda originally used 2 7s and 2 9s was related to emissions, so thats why I thought it would be OK to run 9s all round. It has made a huge difference in power (esp above 5500-6000rpm, but thats probably due to the old plugs being completely fried), but I now notice a thick puff of black smoke with a warm start-up. If I give the accelerator a good poke it seems to clear it out fairly quickly. Any ideas?? Should I go back to stock plugs with 7s/9s? My only performance mods are HKS intake and cat-back.