I've been troubleshooting a backfire (or afterfire) problem on my '69 350. The engine pops through the exhaust when I really put my foot in it. If I keep it in 2nd gear (th350) and roll the throttle on gradually, it runs fine up to pretty high rpms, with no problem. If I roll the throttle on a little faster, there is some hesitation ( more like a surge than a miss). If I stomp on the gas, the bucking and popping begins. I think I can see some smoke in the rear view mirror when it is popping, maybe black smoke, but can't say for sure.
This is something that happened "on its own" without any parts changes. Yesterday I checked the timing and set it down from 16 deg. to 12 and tried a run, no change. I swapped out metering rod springs to a weaker set, no change. I was about to swap back to a leaner metering rod when I saw the #3 spark plug boot arcing from the end of the boot to the metal collar on spark plug. The idle did not seem to be any different than other times when I'm standing there and not seeing that happen.
Is this plug making spark inside the cylinder when it is arcing at the plug boot?
Is this the likely culprit of the afterfire problem?
What is a good replacement plug for a mildly modified 350? I've read two posts here with some negative feedback on the bosch platinum plugs, which is what I have now.
If I go down to Autozone and exhange the bosch plug wires for new ones, is this likely to happen again?
Sorry for the super long story, any help will be great!
'69 Malibu - 413/600/180/1.6/268/1.625/2.5/X/M20/3.42/255/60/15
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