MikeE: If you could stand another opinion; I'm with GMCpanel, get the correct motor. What you're attempting to do can be done, but usually it ends up being a little "backyard engineering-ish." Essentially what you are doing is putting a fixed resistance (the line dropping resistor) in series with a variable load (the motor & associated circuitry). If both had fixed resistances, then the line voltage to the second resistance (motor) would always be 6 volts as intended. However, the motor & its circuitry actually vary in resistance relative to speed & control settings. The result is that the voltage the motor circuitry sees varies depending on the resistance/current drop across the motor & the resistor. In short, the fixed resistance of the in-line resistor can not change relative to the change in the motor resistance and maintain a constant 6 volt drop. Basically that is what voltage regulators do, they maintain a specified output voltage if if the load changes.
