I'll start off by saying I've got an '85 Nissan w/ king cab and long bed. Runs on gas and we just passed 200,000 miles about 100 ago. I went to DEQ twice today. The first time my CO levels were 1.1942 and 1.3625, passing is 1. After the fail I came home, and replaced the air filter and went back. On the 2nd test my CO levels were 1.0663 and 1.6172. I do not have a tachometer so I belive that I got the rpms too high before the 2nd idle on my 2nd trip to DEQ. I read on another post that messing with the idle and fuel mixture settings but I am not sure how to do this or what I'm actually trying to achieve to know that I'm heading in the right direction. Good thing I procrastinated and have 1 day left before my tags expire. Any help would be much apreciated!