[silicon212]

Look for vacuum leaks. Also, look for a plugged air filter.

It sounds counterintuitive, but if the engine is running too lean, you'll have increased HC. The reason is that the fuel mixture is too lean to support proper burning in the cylinder, which in turn leads to increased HC (unburned gasoline) in the exhaust. Of course, too rich can also increase HC, but will more likely negatively impact CO first.

Any sort of misfire will result in a fail due to high HC. You want to make sure that your engine is running on all 8 cylinders. If the engine is in good mechanical shape, then things to look for are plug wires, the plugs themselves, the distributor cap, module, carb tuning i.e. idle speed (low idle speed will cause failure on both HC and CO), air filter, evap system, vacuum hoses etc. If you're getting a check engine/service engine soon light, find out what the trouble codes are and that will give you an area in which to look.

Note - if an engine is running too rich for an extended period, then you will probably wind up needing to replace both the O2 sensor and catalytic converter as neither of these will last too long under rich conditions.