Hope you can help.

I've got a heavy carbon on my plugs. Not enough to cause fouling or misfiring. Also have carbon in the tailpipe. How can I determine whether the cause is due to plugs that're too cold, or jetting that's too rich?

crate sbc HO350
plugs are AC R43LTS, as recommended by GM
Edelbrock 1406, 600 cfm
Primary .098, Secondary .095 (unchanged)
Edelbrock Performer manifold
tube headers
Holly mechanical pump
fuel pressure @ 4½ lbs

Any ideas would be welcome!
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I would change the metering rods to lean it a bit. Clean the plugs and go from there.
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WPoC

Is it gassy at idle? Can you get a lean drop with the idle screws? If not the throttle plates may be open to far and idleing on the primary circuit. This will cause an overly rich condition. May need the go up one jet size on the primary circuit.
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Mike J
Unchartered DSRA Member
If it ain't broke, fix it anyway...

Mike, If my memory serves me well, The Edlebrock 1406 has metering rods, Not jets. Going down a size in rods will lean the mixture, Up will richen it. The same with Holleys that have jets, Up in size jets richens. The 600 Holley comes with an .060 from the factory. Good point on the throttle plate adjustment.
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WPoC

What I found with mine was: I could not adjust the idle with the idle screws.The throttle plates were used to get it to idle. With the larger jets, I went up two sizes,it broke the siphoning that was occuring through the primary circuit. This allowed me to close the throttle plates,no more run on, and set the idle with the screws. Lean drop 20 rpm per screw. This also took care of my overly rich conditions.
I'm sure this could also be corrected with the metering rods if he is not overly gassy at idle. Could also be in need of a spring change. Edelbrock has a great site that will help with the adjustments. I also emailed them and their tech got me started in the right direction.
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Mike J
Unchartered DSRA Member
If it ain't broke, fix it anyway...

Makes sence. Right on with the tech department. They definitly help out.
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WPoC

What kind of cam are you running timing-wise?
If it's a bit rumpty there are a few things you can do to clear up the low end.

It's my understanding the Edelbrock/Carters come from the factory jetted about 5% rich for the intended use.
(True for the street Holleys as well.)

You may want to go to the Edelbrock site and look up their jet, metering rod springs and metering rod recommendations so as to go about 2% lean.
I just did this one to the 750 Carter on my mildly cammed 462" Buick and other than deviating from their metering rod spring recommendation I find it runs quite well.

As far as plugs go, almost always the stock recommended plugs in your preferred brand are the way to go.
The colder/hotter plug bit is usually for racing.
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C9