How to troubleshoot a misfire?

[Xavier]

Picked up a '68 C20 a few weeks ago, have been slowly working on it to make it a solid driver. The motor has been running fine, until last night. I put new adjusters in the headlights last week and did a quick adjustment but haven't driven it at night since. I decided to take the dog and make a trip to a nearby Sonic for some late night shakes and see how well aimed the headlights were. On the trip there the 327 was running fine. After we had finished our drinks I fired the truck up and quickly noticed a miss that wasn't there before. It was definitely down on power, too. This miss was happening at all RPMS, seemed to be consistent in the cycle so I think it's a specific cylinder.

The thing is, this is my first time trying to troubleshoot something without an ECU to give me a clue where to begin. So I'm asking for some advice on how to best approach this or maybe what some common things to look for may be.

The truck in question has a 327 with a quadrajet and the 4-speed manual transmission.

[geezer#99]

Could be as simple as a plug wire leaking or a cracked plug. Or a fouled plug.
Run it in the dark. Watch for errant sparks or arcing from the wires.
If you see none then carefully move the wires a bit where they attach to the plugs. Use a broom handle.
Check that first.

[Chris_oz]

Hi,

Does it have electronic ignition?

Have you checked the battery voltage with the lights on and engine running?

I have seen many vehicles develop a miss caused by low voltage at night.

Check all wiring at the battery, look for loose earths, check voltage to the coil with it running and the lights on.
If it has electronic ignition, some don't like to function well on under 12v
Maybe you have a tired coil and it doesn't like the lower voltage with the lights on.

Start with the basic's, it was running fine though the day, so look for voltage drop, poor connections on the battery, check earth to the block and body.

What type of ECU does your 68 C20 have?

[Xavier]

Originally Posted by geezer#99

Could be as simple as a plug wire leaking or a cracked plug. Or a fouled plug.
Run it in the dark. Watch for errant sparks or arcing from the wires.
If you see none then carefully move the wires a bit where they attach to the plugs. Use a broom handle.
Check that first.

I'll take a look when it gets dark, thanks.

Originally Posted by Chris_oz

Hi,

Does it have electronic ignition?

Have you checked the battery voltage with the lights on and engine running?

I have seen many vehicles develop a miss caused by low voltage at night.

Check all wiring at the battery, look for loose earths, check voltage to the coil with it running and the lights on.
If it has electronic ignition, some don't like to function well on under 12v
Maybe you have a tired coil and it doesn't like the lower voltage with the lights on.

Start with the basic's, it was running fine though the day, so look for voltage drop, poor connections on the battery, check earth to the block and body.

What type of ECU does your 68 C20 have?

I believe its got an HEI ignition.

I'll check voltage with lights on.

I took it for a quick trip today and it seemed the miss was still there.

Battery connections should be good as I just wired up a remote starter solenoid. I'll double check the ground straps from the block to the firewall.

No ECU, made a typo, meant to say first time trying to troubleshoot something WITHOUT an ECU.

[68gmsee]

ECU? Has your truck been upgraded to new computer controlled engine?

If not a missing engine can be a hit or miss on these old trucks. Most of the wiring is old and you don't know for sure when ignition parts were installed. I've owned several 10 years or older vehicles in the past and one of the first things I do is replace all plugs and ignition parts that look suspect.

As heads up, one of the easy checks is what geez mentioned above. I did that on an old 68 Javelin I had and used a spray bottle on the wires and they lit up like a Christmas tree. Another time I found a hairline crack on the distributor cap.

One other thing you can do to see if it's spark plug wire or spark plug itself is to use a timing light on the plugs. Watch the firing of each one. If it's skipping on occasion you know it's wiring or ignition. If all fire and it still misses you can suspect a spark plug, valve problem, bad fuel, vacuum leak.

[cadillac_al]

You're gonna need to learn about ignition points soon. I'm just now starting to miss working on them. It used to be yearly maintenance on the farm trucks and almost yearly on my daily drivers. There was nobody happier than me when HEI came out. You should play with yours (filing/adjusting) or try some new ones. Don't forget to replace the condenser at the same time. That will bite you if you try to cheap out.

[geezer#99]

He’s got an hei Al.
So we’re to assume you were a grouch in 1974 and got happy in 1975 Al!

[cadillac_al]

oops, missed that detail. Geezer, I didn't know I was a grouch before HEI but HEI showed me how much happier I could be. One less thing to worry about when diagnosing a misfire and one less yearly chore.

[Xavier]

Thanks for the suggestion geezer, I was able to see the wire for #7 arcing to the header. Ordered some new wires and worked on the routing in the meantime and it's been running fine.