Edelbrock 600 carb issue?

Since you pulled the metering rods, clean the bores and pistons with carb cleaner.Litely with 1200 sand paper clean the pistons and check for free movement when you reassemble.Some guys clean and check this area every couple months as sometimes they will start to hang up. Just a thought.

Also with spark plugs you want the center electrode to have sharp edges with a flat top.Looking at yours you can see that it is rounded(normal wear). Compare it to a new plug and you will see what I mean.
Again this may not help your problem but is things to look for when it comes to tuneup.