The alcohol makes part of it run cleaner, but it also leans out the mixture, and can cause other numbers to go up. I can't remember which is which, but it should be a couple Google searches away. It's a balancing act, and you should know what you're doing before you just start throwing solutions at it. That said, a decently tuned up motor (good plugs, wires, cap) and good cat should take care of 90% of the problems you can run into.