Japam in and of itself does not wipe out good karma. What Japam does is purify the body and soul. Sarada Devi, the holy consort of Ramakrishna Paramahamsa said:
As wind removes the cloud, so the Name of God destroys the cloud of worldliness...Do you know the significance of Japa and other spiritual practices? By these, the power of the sense organs is subdued...One has to suffer the consequences of one's deeds. But by repeating the Name of God, you can lessen its intensity...The effect of karma can be counteracted to a great extent by Japa and austerities...Japa will eventually bring spiritual realization.
By subduing the sense organs, realization comes. The Katha Upanishad (II. iii. 14-15.) says:
- When all the desires that dwell in the heart fall away, then the mortal becomes immortal and here attains Brahman.
- When all the ties of the heart are severed here on earth, then the mortal becomes immortal. This much alone is the teaching.
Many people want to get rid of the effects of bad karma, but good karma can bind one to rebirth also. Thus when all desires, good and bad, and all ties of the heart, good and bad, are wiped out the soul is purified. When the soul is purified, then realization comes and both good and bad karmas are wiped out. Good karma, just as bad karma, lies within the realm of maya.
Krishna says in the Gita (II. 50.) "Endued with evenness of mind, one casts off, in this very life, both good deeds and evil deeds. Therefore strive for yoga. Yoga is skill in action."