Join Date: Jul 2006
Location: Northfield, Illinois
I did exactly the same on #4 on a race engine many-many years ago (1966?). The engine needed warm up plugs (this was then) due to big piston to liner clearance, and I normally started it with 2 step warmer champion non race plugs, then warm installed 4 N57G fine wire plugs to race.
I had an opportunity for low speed track familiarization at dusk, warmed it up quickly, fitted the cooler plugs and drove off. There was a reverse camber turn running uphill where I was no-good, so drove it harder each lap for three laps. Going up-hill on lap three, my engine became a 3 cylinder.
Back in the pits, it was clear I had removed and reinstalled the street champion in #4 instead of the N57G as 1-2 and 3 had those, but not #4. It looked EXACTLY like yours. I heard no detonation (helmet) and felt nothing unusual until it became a 3 cylinder, under load at high rpm.
So.. Check your plugs, then with a jet wire gauge, check the ACTUAL main jet and air correction jet size. I never use drilled or leaded and drilled jets but some do, and marking can be WRONG. Look for air leaks at the insulator / isolator rubbers between Webers and manifold for deterioration or gasket failure. An air leak will do this at high engine speeds.
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