Join Date: Aug 2001
Location: san jose ca usa
Ill assume you have no cat or other smog era devices. A lean idle can cause a slight lean missfire which will cause HC to rise while CO will remain low, this is assuming that the car is in good running order and has no other problems. A common cure is to either richen up the air/fuel mixture to get rid of the lean missfire or retard the timing so the combustion chamber gets hotter which allows the fuel to burn more completely.
For best idle quality I would say richen the mixture up a little. 13.5 to 1 is where most engines run the best.
70 Camaro, 355, th350, 4.10 posi.
74 Trans Am, 400, th400, 2.73 posi.