Give it a FULL tune up, plugs, points, cond, oil and filter change, if you have a oil filled air filter put in new oil to the line or replace the paper filter and adjust the valves. Take it for a long drive at higher speed then to air care and see what numbers come up.If you pass, good, if not you will know its weak spot. On my truck it failed the idle test (hydro carbons off scale) all other tests were very good. I tried some different settings 3 times but no good so I gave up and took it to a shop in Maple Ridge for set up and it passed (in 2007) and it has passed every year since then. My truck is a 1/2ton 1949 chev with a 216 L6 stock bore and 135,000 miles on it.
