Emissions test on a 1991 Chevrolet pick up (4WD i think)?

i have a co worker that is having a problem getting his car to p smog. the original sheet ped every thing. his fort time through the test it had elevated co% on the low speed test but every thing else was on range to p. on the high speed test every thing fell in line and ped that part of the test. he says that he did a tune up and went for a follow up test. this time the low speed test ped but the high speed test had elevated hydrocarbons but nothing else was out of range. i am trying to make sense of it but i am at a loss. if you need actual results i will continue with those on the next installation. i should mention that he stated that he adjusted timing to 2 degrees btdc before the second test but that is still within guide lines.