Originally Posted by Mickey2

I don't trust the amp numbers on these motors, they can be stronger than the basic Singer at 0.6 amp, but often not noticeable difference at all. 1.3 is about as powerful you get on portable machines. On the other hand, if it's extra speedy and with a good needle punch you probably have a stronger than average motor.

Formosa means Taiwan. It fits with the date, I don't know if model 15 clones still were made in Japan that late, they had long since moved on to zigzaggers with fancy stitches by then.

You have a nice machine, the irregular stitch length is usually caused by gummed up parts, and it needs a thorough oiling. The other commmon reason is it needs a good cleaning under the throat plate (feed dog and bobbin area).

Give it a good cleaning and then oil. Detect all oil points, give them a drop or two of oil, run the machine abit while you move the stitch length lever up and down. Then repeat. Pay particular attention to the parts from the stitch length lever (reach through the lid on the back) and all moving parts under the base. If it's a bit stubborn, don't give up, keep up oiling and and test sewing a bit the next few days; it can take time to dissolve dried up oil and grime, it will gradually flush out and be replaced by clean and fresh oil.