Why a double spark on left side of 4.2 liter 1997 F150?

Answer 1

Well, I'd be inclined to ask how you came to this 'double spark' conclusion. Did you actually have a diagnosis done which told you this, or do you simply hear a knock in your engine? If you're hearing a knock in your engine, then you're probably getting detonation in the cylinder, which causes the fuel/air mixture to combust prematurely. And possible reasons for this are abundant. A clog in the fuel injector line or fuel injector, a bad poppet valve in the fuel injection, incorrect timing, damaged distributor, worn timing gear and/or chain, worn camshaft, contaminants such as water or fuel getting mixed in with the fuel/air mixture in the cylinder (in the case of water, a bad head or intake manifold gasket is the likely cause; in the case of oil, a worn scraper ring in the piston is a likely cause), fouled spark plug, bad spark plug wire... it's going to take a little more troubleshooting the narrow down the cause.