I recently was given a 1977 Class C motorhome with a Dodge 440 that last past California smog in 2008. At the time the mechanic stated he didn’t think it would pass smog in the future as it was very close and the carburetor had to be tweaked (and left tweaked) to pass.
In 2010 it failed smog and was left parked until last month. I was given the motorhome and drove it home on a one-day operating permit. It ran fine but the exhaust pipe was very sooty. I burned out the old gas and refilled with premium gas and some fuel additives.
I put in two bottles of Guaranteed to Pass, one bottle of Seafoam and a cup or two of Lucas fuel treatment then took it for 7 hour ride and burned up a full tank. I also burned some Mystery Oil in it when I got home.The exhaust pipe is now soot free and with only gasoline in the tank, it easily passed the smog test with ample margin to spare. I expect it to pass in two years when it comes due again.I wasn’t sure if the additives would work, but apparently they do.