[Bruce Edwards]

The Edelbrock RPM is a split style manifold. When you installed the carb you may have noticed a divider in the center of the manifold. Also called a dual plane manifold. It splits the air fuel mix into two sections. to take advantage of longer runners which give better bottom and mid rpm mixing.
Sounds like you just need to work the fuel mix a bit.
That manifold will split 1-4-6-8 on one side and 2-3-5-8 on the other. So if you are going to check with you sensor you will need to compare the correct pipes. Remember that the different length pipes will also change the temp so you want to check it as close to the head side as you can.