[johnsjmc]

High HCss can be caused by fuel contaminated engine oil. .A rebuilt engine often lets more fuel past it,s unseated rings and add some fuel to the oil. The PCV system will suck up the fumes and increase unburned HC in the tailpipe. Small amounts can make enough difference to fail . A good long hot highway run will burn off most as will an oil change (smell the dipstick and change the oil if you smell any fuel).
Timing off a couple deg. either way will also affect HC.
If your HC was high caused by timing then your NOx would likely be on the low end of acceptable.