how do you lower your Hydrocarbons

My 97 4.3L blazer failed emmision test x2. First time was just sad. I replaced the cat the muffler the o2 sensors. Everything is good but the Hydrocarbons. It reads 0.7143 g/km. It need to be lower then 0.500 g/km. What can i do. I was told a higher grade of gas. Is this true.

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