340 Gets Terrible Gas Mileage

My 367 runs a square-top, Accell Supercoil, that you can practically weld with. Best coil I ever bought.The ignition system runs off a relay triggered by the key-on, so as not to pull that load thru all the usual trouble-spots.
My charging system runs at a tic over 15 volts. The Optima battery doesn't seem to mind.
Champion coppers are the bees-knees.
I worked in small-engine world for a few decades.
Here NGKs were the plug of choice. They worked ok. As long as they continued working. If they got gas-fouled, you couldn't save them. Two-stroke Snowmobilers always carried spare sets.
Champions were standard in marine engines during those years, cuz when you're fishing miles from shore...... it better run. No worse feeling than when the engine will not restart, and you've pulled yur arm off. Unless maybe if you didn't pack an oar.......
I'm with;
A plug is a plug is a plug. If YOU can’t make a plug works that’s on you.

I think it was Smoky Yunich who once said something like;
after 6500 the plug is just there... that's over 50 sparks per second........
I suppose that's why my 367 spends a generous amount of time up there..... lol.