What type of fan are you using? (i.e. 12 VDC, 120 VAC, etc) What are you using to vary the input to the fan?I suggest some testing is in order. Take some variables out of the equation.Set the flask aside. See if you can get the bar to spin while sitting on top of the plate. see if you can control it from zero RPM to throws the bar. The stir plate I made I had plenty of room on the top end. In other words, it always threw the bar with plenty of room to go in the "turn up" direction.Question. Can your speed control go from zero RPM to max?This will probably give you some scuff marks on top of the plate to let you mark the center point of where the stir bar likes to spin. This is where the center of the flask wants to be. Depending on the shape of the magnets you used, the magnetic center could different than the geometric center .If you can spin it on the plate, next add the flask but no liquid. Can you get a good zero RPM to throws the bar?If so, next step add water. Can you go from zero RMP to throws bar with a gradual steps up?Lastly, do a spin test with wort. I was surprised by how much 1.050 OG wort loaded down my stir plate.
FWIW This is with a 12 VDC muffin fan and a 0-12 VDC power supply.As far as stir bars are concerned, my empirical observation says that short and fatter stir bars work better than long and skinny stir bars.Let us knwo what you learn. I might be able to provide soem additional help in the way of a reliable speed control.zymot