My 89 N/A only gets up to 100mph on the highway, and I only get around 15mpg. Some ppl have told me that its probably becuase the 6th port is stuck or somthing. How can I check to see if this is true, and if it is, how do I go about fixing it?

 

Aaron Cake has a wealth of information on his sight. Here's a link for the 6port test,

http://aaroncake.net/RX-7/6porttest.htm

Browse around his sight for more info.