Nissan Repair: idle, nissan 4x4, flowmaster muffler

Question
I have a 97 nissan 4x4 with a 4 cyl. Sometimes it idles rough below 1 rpm and at times it idles at 1.25 rpm which then it's fine. I noticedthat the inside of the tail pipe is black. What could cause this? And also, do you have any tips on how I could get more power out of my truck with out spending a fortune?
                           Thanks! Dave

Answer
First I would do a tuneup to baseline the engine and that includes cleaning the throttle body with carburetor cleaner.  You will do this with the engine off and the intake hose removed so you can spray the throttle body down on the inside.  Then hook the hose up and start the engine up.  Yes, it will be difficult to start at first due to the carburetor cleaner richening the mixture.  But, this will make the idle much better.

For horsepower I would go with a K&N filter and a free flowing exhaust like a Flowmaster muffler.  You do not have to change everything out.  But, just go to the local muffler shop and get a three chanber Flowmaster installed.  Both of these will increase horsepower without hurting gas mileage.