All motors with aftermarket high compression head run much better with high octane fuel. Always use premium. And I always add 1 ounce per gallon of octane boost. It's inexpensive and can be found at Walmart or Auto Zone. USE 1 OUNCE PER GALLON ONLY. If you have not already installed it, my aftermarket ignition system will smooth out engine vibration. https://www.pedalchopper.com/hd-lightning-cdi-kit/ And to smooth out engine further, my carburetor solution is the ticket. See it at https://www.pedalchopper.com/carburetor-solution/