Thread: Spark plug burn pattern?

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I am trying to diagnose a backfire problem. The engine starts and idles good but under load it backfires. All the valves seem to be good we pulled the valve covers and all move the same when running and they seem to be fine. I think that the distributor is bad and I have one on order.
I pulled the plugs and they have a strange burn pattern like they are really lean only burnt on one side. The carb is an Edelbrock 1406 and is running .095 and .098 jets I have the Metering Rods # but not with me at the moment. Could the carb also be to lean what would I do to richen this carb. I plan on buying a Edelbrock Carburetor Calibration Kit and change to larger jets. is there anything else I could try?

If it were my carburetor, I'd probably want to start with the Owner's Manual...
http://www.edelbrock.com/automotive_...ers_manual.pdf

Travis,
A little more information might help here. Is this a new problem, that is, was this engine running fine, but just started this "backfire under load" problem or is this a new configuration of parts you're trying to sort out? What is your ignition system/distributor that you expect is bad? Is the burn pattern shown consistent on all cylinders? And just for grins, what engine are we talking about here, and is it generally stock or heavily modified? Welcome to CHR, by the way

As rspears says need more info. Backfire under load would first look at cap and rotor for crossfire arcing. Try separating plug wires to see if makes a difference.

It's lean!!!! Get out your Edelbrock book, order the tune and tweak kit, and study it carefully. The information, instructions, charts and graphs are superb! Backfire through the carb is either timing or lean....the plugs say lean.

Sorry for the delay and the lack of info. My post did not show up after I posted it and did not get back to it until today. Hope this gives more info.

The engine is a rebuilt (less than 600 miles ago) SBC #3970010 1969-79 350 based on the casting numbers but it was completely rebuilt. The old distributor was a converted HEI they took the coil pack out changed the dust cover to Modified HEI Dust Cover External Mounted Coil. Everything else is MSD Blaster 2 coil and a MSD Ignition 6200 - MSD 6-Series Ignition Box. Mid cam and GM 487 SBC heads Edelbrock 1406 carb.

The problem started just hearing the backfiring every once in a while thinking it was a bad or stuck lifter pulled the valve cover everything seem OK. Replace a old header that had a small hole with new headers. The problem got worse after the new headers.


I installed a new Pro-Billet MSD 85551 distributor last night it runs much smother didn't get to do a road test yet but will soon. The strange thing is I think that the old distributor was wired backwards (not the spark plug wires). With the new one you turn it counterclockwise to move the timing mark to a higher BTDC number the old was opposite.
The distributor cap has a strange burn pattern for a new cap. This is a new cap less than 100 miles (see Picture).

The new distributor looks nice at least.

The carburetor metering rods springs and jets come in today. The springs and metering rods may help the lean plugs, but I suspect if the wires (not the spark plug wires) on the distributor were crossed and spark degradation occurred possibly causing the plug burn pattern. The jets in the carb are stock also metering rods are stock the springs for some reason were changed to the Plain 8" ("Hg)