Toyota Repair: 91 corolla High HC, vacuum hoses, incomplete combustion

Question
Hello Ted,
My 91 corolla (4A-FE Federal, 190K miles) has high HC and did not pass the emission test. I just found a crack in the exhaust manifold between #2 and #3 again after I replaced it 2 years ago. This crack seems to be a "popular" problem with the corolla, by the way. I am trying to decide if the high HC is somehow related to this crack and worthwhile to replace the manifold to pass emission. I quickly checked things like spark plugs, air filter, leaky vacuum hoses and ignition timing (10 deg.), and the car has ~25mpg (mix). Could that be aged O2 sensor? Could you please advise?
Thank you very much,
Sato

Answer
The crack in the manifold needs to be corrected, it will cause a lean running condition, the engine may also have a misfire, HC is unburned fuel coming out of the exhaust, it's incomplete combustion, not all of the fuel is burned and some is expelled through the exhaust.