Plugged are fouled in Cylinders 5, 6, 7, 8 - Suggestions?

[bigboi]

I pulled my plugs yesterday and here is what I found...

Cylinders 1 & 2 - Plugs were nice color. Tint of caramel to them.
Cylinders 3 & 4- Little more toasty but still an acceptable color.
Cylinders 5, 6, 7, 8 - Black as Coal!

Why are the last 4 cylinder spark plugs fouled?

I am running a AED Performance Holley 750 - Mech Secondaries - 4 Corner Idle - No Choke. I think I am jetted 78 front / 80 rear.

I know I really need to take the car down to the Dyno Shop to get it tuned to perfection, but until I do I would like to get rid of this problem.

Any suggestions?

[MrQuick]

what intake? could be a bad intake seal or carb tuning issue if its a single plane.

[bigboi]

what intake? could be a bad intake seal or carb tuning issue if its a single plane.

Intake is a Edelbrock RPM - Dual Plane.

Engine was freshly built. Prob have around 500 miles on the engine. Intake was bolted on and sealed by the builder. What seal you thinking about? Intake to head?

[MrQuick]

Dual plane could possibly rule out the carb since it runs a split pattern feed. ie left side of the intake may feed 3,5,2&8 This is a strange one but i'd tune that carb to rule it out. Might be as simple as a rear corner adjustment.

[bigboi]

Dual plane could possibly rule out the carb since it runs a split pattern feed. ie left side of the intake may feed 3,5,2&8 This is a strange one but i'd tune that carb to rule it out. Might be as simple as a rear corner adjustment.

I am completely crappy at tuning a carb... LOL! I probably just need to take it to the Dyno Shop and have them do it for me.

But... In the meantime... I should probably play with the rear corner mixture screws you think? Maybe give it more air?

Couldn't be a problem with the jets? Could it? Probably not because cylinders 1 and 2 are ok... and 3 and 4 are slightly ok.

Am I right here?

[MrQuick]

Jets should be fine.

Is it running bad? To tune via plugs you gotta have a nice full throttle run then a clean shut off. Most likely an idle problem.

Try less air bleed in the rear. Turn them in a bit.

[Shaker455]

Where your initinal timming at and how many turns out is your mix screws?

[minendrews68]

Not trying to seem stupid but have you tried looking at some of the most obvious? Like spark plug wires (either being bad or not on the plugs good enough to get a good spark ), look in the distributor cap and make sure the inside is in good shape as far as spark contact is concerned, you might check cylinder pressure on those cylinders and see if they're ok, could be a bad valve seat problem. Just a couple of things to look at..

[bigboi]

I knew what my initial timing was, but then a good' ol boy down at Total Performance in Wichita KS bumped the timing up a bit and adjusted my carb for me.

I have the gas tank out right now because I am swapping rear leaf springs.

You guys have some good suggestions and once I get the tank back in I will fire it up and do what you said.

- The engine is fresh. About 600 miles on the motor since I got it from the builder. Heads are new.... But could very well be a valve seat like you say.

- Spark plug wires are brand new.

- I will check the distributor cap.

- I will turn the mixture screws a bit.

Thanks for the suggestions... Once I try these things I will post back in the thread.

Brian

[bigboi]

I played with the idle mixture screws on all 4 corners. Fronts are 1/2 turn out and back are 1 turn out.

I switched to a Autolite 2934 plug (I was running a 2933) so the plug is a little hotter - Not sure if this will help or not??

I also gapped the plugs at 35 instead of 30.

I started it and ran it for about 10 minutes or so and all seems well. I will know better when I take it for a spin.

[Vegas69]

If you are fouling plugs your main jets are the last thing you need to worry about. Your idle circuit and intermediate will cause the fouling. You should turn the idle mixture screw in until it stumbles and then back out until you find your highest idle. Since it's only on those cycliders I would be inclined to think it's idle mixture not jetting.