I have a 91 Nissan Maxima and my car is taking a long time to pick up speed and it I keep hearing some type of knocking, puttering noise. When I get up to about it 45 mph it seems to be fine but it takes forever for me to get to that. I have had the spark plugs and spark plug wires changed but that didn't help. Could you please tell me what this could be?

Try using a higher octane of gasoline. That could be your problem if it knocks like that.

Sounds like a rod is knocking I would take it in and have it checked out before you totally trash your engine.