How to adjust fuel mixture?
 

I recently bought an 88 b2200 which is a great little truck ... except for the fact that it recenly failed an emissions test and has very poor fuel economy.

After trying to narrow down some of the symptoms to get a cause, I'm thinking that it's running too rich. I haven't checked the plugs yet, but will shortly.

My question is, how do you (or can you?) adjust the fuel mixture on these trucks? There doesn't seem to be an adjusting screw like other vehicles have, only an idle adjustment screw. Does that mean you're not supposed to mess with the fuel mixture?