i just got done putting my b20vtec together, so i crank the engine up let it ran for about 10-15min turned it off. i check the plugs and they were black carbon build-up. im running s200 that was tune 2yr ago, blew the motor so i redid the bottom end kept the head the same(stock head w/ ctr cam) w/ rc310 injectior.So what could cause the carbon build-up???