if you're having the problem while piecing, the pinholes won't matter. your seams will look just fine. if you're at the quilting phase ...
i've had my janome (different model, but still an MC) less than two months, so cannot claim to be an expert with it.
there are so many factors: weight of the fabric, loft of the batting, the thread, the needle size ... my freemotion stinks anyway, but finding the right combination of speeds of the machine itself and moving the fabric around seem to make more of a difference than fiddling with the settings.
in straight-stitch quilting, what i've noticed so far is that it seems to prefer lightweight thread in the bobbin ... 50wt or 60wt. (which may not help you at all, i know and i'm sorry, since you already have the thread at whatever weight it is.)
sharp needles of the right size for the top thread and the right bobbin size are a "must" (which really ticks me off because quilting really wears out a needle fast.) i practice on fat quarters or scraps of fabric the same weights and batting of the same loft until i get as close as i can. then i cross my fingers and pray that any remaining pinholes will disappear in the wash. :lol:
i still haven't figured out how to get rid of the pinholes when i use the embroidery mode to put down quilting designs. if i get rid of them on top, they show up on the back. grrrrrrrrrrrrrrrrrr. i guess another year or so of practice will reveal that secret. in the meantime, i've learned to pick a backing fabric that goes well with the color thread i plan to use on top so i can have the same thread in the bobbin.
how's that for a whole lot of what may not help? :?