Running rich I think - parts help

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I have a 1999 with a 5.7 engine in it and 150,000 miles. It starts right up on the first start of the day. Last week it got real hard to start when the engine was hot. You have to turn the engine over a long time before it cachets. When it dose cachet the RPM are about 400 and then slowly moves to 600. I replaced the plugs but that did not help. I noted that the tail pipe had a LOT of black soot in it. I also have 1996 5.7 and looked in that tail pipe and saw very little soot. Any ideas on how to fix the starting problem? Any help or ideas would be great. Thanks Dave