I would check that your brush is not hanging up slightly. It could be that the brush hood is dirty and the brush is hanging up. Also check the braided shunt to see if it is caught. Sparks usually occur if the brush is "bouncing" or not making good contact with the comm. If you have fitted a weaker spring on the negative side then the brush may be bouncing.. Check the condition and wear on the brush compared to the other side...... These are just some suggestions.
I am no expert but I would have a look at these possibilities.