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.

Questions:
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!

Dave

1. I seriously doubt that cylinder is firing when it sparks on the outside.

2. It's possible, that cylinder is dumping unburned fuel into the exhaust which can ignite in the exhaust system.

3. Don't know about plugs, what about the stock AC plugs? I've heard complaints about Bosch plugs too, takes a lot to fire them. This can cause other parts of the ignition to fail, insulation breaks down in the wire, carbon tracking on the cap.

4. No guarantees, it happened once, it can happen again. If you replace the plugs and wires it should be fine.

Sounds like the plug itself is OPEN internally. The spark is seeking ground by the path of least resistance. Nothing wrong with platinum plugs, just don't use if you use NOS or blowers.

Originally posted by d1_bradley:
Sounds like the plug itself is OPEN internally. The spark is seeking ground by the path of least resistance. Nothing wrong with platinum plugs, just don't use if you use NOS or blowers. Yes it sounds like #3 plug is shot. I didn't say that, I thought he knew that. The comment was on Bosch plugs in general, something I've heard from mechanics. I also experienced ignition failures on 2 cars after running Bosch plugs for awhile, maybe just a coincidence. When you say nothing wrong with platinum plugs do you mean Bosch platinum or platinum in general?

Just because you see spark on the outside doesn't mean it's not firing. To check this simply pull the boot with insulated pullers while the car is running, If it is firing you will notice a shake in the motor.

BUT, it should be addressed and you are losing voltage to that plug if leaking is occuring.

Well, I replaced the #3 plug with an AC, and no more afterfire. I had the temperature sensor wire from the cylinder head tie strapped along this #3 plug wire and even had both wires clamped into the plug loom. I wonder if this had something to do with the problem at #3 spark plug. Then I got to wondering if this was happening at other cylinders. I turned off the lights in the garage with the car running and could see some faint glow of sparks around the boot and spark plug at #4 and 6, but not near as obvious. Does this make sense, should I be able to see ANY kind of spark outside the cylinder? Could light from the spark inside the head be bright enough to show through the ceramic in the plug in a completely dark garage? I've probably beat this one to death, but thanks for the suggestions. One more question: is there a shorty AC delco plug?

Dave

No way should you see in any sparks. You only replaced #3 plug? I'd replace them all. Sounds like they have too high a resistance, or gap is too big. Don't know what kind of ignition you have but you're talking about at least 20,000 volts. It's going to want to go somewhere. If plugs have too high a resistance it will try to find another path to ground. Don't run any wires along a plug wire. I don't think that caused the problem but it can cause one. Don't know about what plugs for your 350.
Edit: This is the kind of problem I've heard about with Bosch. Too high a resistance and insulation breaks down somewhere else in the ignition.

I replaced #3 initially by itself just to see if that was the problem. It was. Yesterday I replaced all the others (AC R44T). Will probably burn up plug wire on #6 with the flowtech headers, may have to run a shorter plug on that cylinder. Thanks.

I ALWAYS run AC plugs in a Chevy. The ghost of the General will visit you if you don't. Besides, they are the original equipment. Like Harry said, you definitely have have bad/wrong plugs/wires if you see any spark around the plugs, lights on or off. One way to test this is to run you hand along he plug wire all the way to the plug while resting the other on the fender. No, on second thought, you probably shouldn't do that. :D

I did have a friend that would kill a running Model T by laying his hands on all 4 plugs at once. They used to used copper straps in those days in stead of insulated wires. I tried it. Haven't been right since.

Originally posted by Herb:
...One way to test this is to run you hand along he plug wire all the way to the plug while resting the other on the fender...Engine running of course, I tried that, once. graemlins/clonk.gif
You're on a roll today Herb. graemlins/thumbsup.gif

You need to replace your plug wires. They are the
cause of the misfire. If you just replace the plug the misfire will return before long. Regarding bosche platinums in a word they are JUNK .

Originally posted by Importtech:
You need to replace your plug wires. They are the
cause of the misfire. If you just replace the plug the misfire will return before long. Regarding bosche platinums in a word they are JUNK . Thanks, I mentioned replacing them in my first reply. Forgot to stress that again. Insulation in them is now weakened and they'll probably fail again.

I switched fro A/Cs to Bosch platinum tip plugs
last spring. I have noticed at 4500 rpm and up I get a rapid backfire or loud crackling sound now.
Never had that before. I wonder if it is the plugs? I have plenty of juice with the MSD coil and 6A ignition.