Pot Carbonate is an accelerator and increases the energy of the developer. When mixed 1 to 1 with solution A and during extended periods of time in developer can increase film base fog. I have reduced Pot Carbonate up to 40 % of A with good results.
When designing negatives for projection I would also try a small amount of Ascorbic Acid in conjunction with reduced Pot Carbonate. Try 3:2:200 with 1 part of Ascorbic Acid for contact prints and evaluate.
I got Ascorbic Acid from a local health food store. The brand name NOW is said to be the most pure and comes in easy to dissolve powder form.
Keep us posted. Steve