There could be a number of things that cause high HC. In most cases HC is related to some type of ignition issue, but could also be related to timing, and also a worn engine. A catalyst that's marginal sometimes won't clean up the exhaust enough and can cause high HC. It's best to have a qualified technician take a look at the problem for a correct diagnosis. There are so many issues that could cause high HC it's best to have someone take a look at variables to narrow it down for you.