My guess is your fuel mixture.

Here are some before and after on the stock exhaust before I added the tuned pipe. First is stock exhaust and gasket. Second is stock exhaust. Third is exhaust port. Fourth is new gasket. Fifth is new exhaust manifold. So I start by creating a new exhaust gasket using the exhaust port as a template. Then I use the new gasket as a template for the exhaust manifold. Then repeat for the intake side.

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