Tantrums can be a very good way to get the point across. No worries, we've all been there for some reason or another. I took a set of machine quilting classes and one of the exercises we did may help you too. Make a small sandwich of plain muslin and mark lines at 1" intervals across the sandwich. Mark each line in succession starting with 1 and go up to the highest number of your machine's tension setting.
Then set up your machine for FMQ. (I agree with the others that the Bottom Line is more for the bobbin than the top and if you use 50/3 weight in the top, try a size 80 topstitch needle.)
Then turn your top tension to 1 and sew within the first area not worrying about anything other than laying down some FMQ.
Now turn your tension to 2 and sew in the second area. Then turn the tension to 3 and sew in the third area. Until all tension settings have been tried.
Now inspect your piece front and back and see where your machine has the best combination. That is the tension setting with THAT needle size and THAT thread combination that will/should work on your real sandwich as well.
Yes, it is a little tedious but it should help you determine what's up with your machine.
I'm also not sure about the Janome and if the stitch length has to be set to 0 or not.
Don't give up. I know that Janome fans are very loyal and there are a lot of them. They can't all be wrong. Give it a try.