You're getting after firing, not back firing. Back firing happens when the combustion flame propagates BACK through the carburetor or throttle body. Popping out the exhaust is an after fire.
You need block off plates. Google them. Or search here for countless threads on the topic. They block the fresh air injection ports on the valve cover that's allowing the unburned fuel to ignite in the header.
You need block off plates. Google them. Or search here for countless threads on the topic. They block the fresh air injection ports on the valve cover that's allowing the unburned fuel to ignite in the header.
