Well, pummelos should be very sweet. When are you picking them? Usually, most pummelos are ripe in California around March through June. They will often drop off the tree when fully ripe. Sprouted seeds are fine, sounds like the fruit is over ripe, then. But, if your fruit isn't ripe by now, I am wondering if what you're got is not a pummelo put perhaps just the rootstock having taken over? Can you post photos of your tree and see if you can take some photos of the trunk where you think the graft line might be? As well as a photo or two of the fruit, both whole and cut in half? We might be able to tell you if you've really got a pummelo or just fruit of the roostock, which will certainly be very sour and inedible.

Patty S.