Hi everyone, so I found out I had 2 fouled plugs only on cylinders 1 and 7 (see attached, maybe this was caused by starting the car up for short periods in the winter months). Driving normal I wouldn't notice anything then about half throttle I heard a bang from the front and from the exhaust. I replaced the 2 plugs and everything is back to normal. My question is, this happened about 3-4 times (as I was trying to diagnose) and I'm hoping the chances that I damaged anything is slim?

How do the distributor cap and rotor look? How old are the wires? The bang, I'll assume is a backfire, have you checked timing? What did the other plugs look like?

Wires are new, cap is new. All the other plugs look fine tan color as expected, except 1 and 7. Hoping the fouling was just the winter short startups. I used a timing light and looked at each spark plug wire to find out which cylinder had an intermittent fire and 1 and 7 were skipping noticeably. The timing is right on as well. After I replaced those 2 spark plugs, car ran great. Yes the bang was the backfire heard from the front and exhaust. I drove the car to work today and will pull out one of the new plugs to see if it's ok or looking like it is fouling again and report back here. Thanks!

replace your pcv valve

Yes, constant too short start ups can foul plugs. My '80 Turbo T/A was in paint shop jail for 9 months and prior to the end I got a phone call telling me the car would no longer start. I had to replace all 8 plugs.

Thanks for the replies everyone. I will check the PCV, is the reasoning that I am sucking up oil into the intake? I took the car to work today with cylinder 1 replaced (ac delco R43S) and also took a pic of cylinder 2 (autolite 85), I will eventually change them all but store did not have autolites though I prefer ac delco. So first pic is cylinder 1 with new plug and ride to work, it doesn't look like it is fouling to me anymore so maybe too many short startups from winter based on my first post. Second pic is cylinder 2 unchanged plug, look pretty much the same. What I"m also trying to figure out is if R43S is a colder plug and autolite 85 is a hotter plug, why is the heat line at the same location on both plugs. The store specified R43S for "conventional 4BBL" and R45S was listed as a second choice. Which heat range should I use? The motor is from a 1970 big car 455 so it has the #15 heads and an edelbrock 750 carb, edelbrock intake and headers.

Well, if you did notice a miss on the two suspect cylinders with a timing light, then something in the ignition system should be suspected.

Right but the timing light won't light if the spark plug does not actually spark due to fouling (to complete the circuit), and the ignition system could be fine. That is why I swapped the bad plug to another cylinder to see if the problem moved with the plug and it did, which indicated the ignition system is ok.

Ah, you didn't mention that part before. Since it's only on those two cylinders, and those plugs are fouled pretty good, I'm thinking valve guide seals?

Hmmm I hope not, seeing as it was rebuilt 2500 miles ago and the fact that the new plug I put in does not look like it is fouling from the 2nd set of pics I posted? I'm going to stick with the winter startups and short run-time for now and keep an eye on it. Thank you for reading my post and for the help!

Everytime I hear backfire at the exhaust and carb I think timing chain or rounded cam lobes. Your problem sounds like an ignition one though. Could be fouled plugs due to seals or fouled as they are not firing correctly. A cracked distributor cap could cause the problem but maybe it's just two bum plugs. Wouldn't be the first time. I chased a similar problem myself and traced it to plug wires, they looked good but were shorting through the insulation to the rocker covers.

Backfire out the exhaust caused by a Rich condition. Check float level, fuel pressure, power valve, weak ignition.