I've got a 1990 F150 with a 300 and I recently did a upgrade on the ignition. Taylor Wires, Taylor Cap/Rotor, Accel 48,000 Volt Coil, and a new set of Motorcraft copper cores. I also decided to side gap the plugs. Here's the issue... While driving there is a noticable miss. I've tried gapping out to .060 .050 and .046. No matter where the gap is the miss always appears. At .060 the miss is noticable around the top end of the gears with a shutter. While at .046 the miss appears when accelerating. I really didn't notice it in the 1st gear but when I shifted into 2nd and gave it gas it immediately started missing. Really need some help with this. Thanks in advance.

 

Did it miss before the transformation? I am not a big supporter of ignition upgrades since the OEM system is good and upgrading produces little if any results. All I can tell you is to start switching back components one at a time to narrow it down. It likely will miss more as the engine is "worked".

 

Had to go look up what side gapping is. A neat idea, but I wonder if you'd still have the miss with normal plugs?

Otherwise, I'm thinking a spark that goes bad with rpm could be a weak coil or the timing is off. Were you trying to do the sixlitre tune up?