If you are running faster in the up than the down, you have to strengthen the series field by removing shunts, or in your case moving taps in the series field. I would not change the compounding until you have formed a glaze on the generator commutator. Also, make sure you have the correct brushes in the gen. The compounding is usually not the problem. If the brushes are new, look at the bottom to make sure they are completely shiny all the way across. They may need to be sanded in.