Im not sure if thats going to work… the FET is across the + and - rails, so it would basically be a short circuit. and im not sure if you can hook a pot straight up to a FET and get good results, things may get hot if only half on…
Is this for a manual control? I would keep the dosing pump running PWM for the extra torque at low speed. this could be done with the spark, using a pot on an analogue input and connecting the FET to a PWM output. Or for an independent setup, buy a pre-made PWM motor controller (or make your own with something like a 555 timer, maybe this one http://www.dprg.org/tutorials/2005-11a/ )
there is a neat little hand drawn diagram at about post 36 in this thread, they are driving small DC blower motors.
BTW… V2 is printing now! 1st one i overcomplicated a bit, so ive redesigned and made it even more complicated!
