With the head laying on the back, move the stitch length lever while watching the motion of the connecting "rods" from the feed dog lever down through the pillar, and left across to the feed dogs. Make sure each of the movement junctions move freely. Your machine may have been sitting unused for a few decades. Even sewing machine oil gets sluggish over time.
If you note some sticky spots, either oil again, or use a few drops of kerosene (lamp oil, charcoal lighter fluid), turning that handwheel well to work it in well. Follow up with a drop of sewing machine oil, turning and distributing well.
I also like to take an old pin, needle, or fine pick, and clean out between the feed dog teeth.
As to your test fabric, I recommend using two layers of shirt weight woven cotton for test sewing. (Not felt or a knit fabric like t shirt material.) Felt is not an easy fabric to calibrate on, and most sewing will not be on felt.
Good luck!