21%?!? Wow, that is a lot of schmutz!!

Since the test is just looking for soot/particulates, an It. tune-up and clean filters and fuel is a really good idea. For reference, my 84 tested at 4 to 5%, so good numbers are achievable. I had mine done at the GMC dealer on rt 3 in Middletown, fresh oil in the crankcase and Power Service additive in the tank.

Run it up and down rt 9 like you stole it a few times. Take it out to the Berlin Turnpike and drag race some kid with a tomato can exhaust.

Good Luck
Jerry