Take a look at the original wiring diagram for the 240 it shows how to do it....for decades now folks have been perpetuating the idea that something with diodes needs to be done to get this ignition to work on a 140 or 120....not so. Any of the more recent starters (post '75 but they fit on B18-B20s and most rebuilts by now are the later starters...check yours)have an extra contact on the starter that is only hot when the starter is turning, this goes directly to the coil, the other hot wire for the coil is from the ignition (keyed point and is hot in the on position but not the start position), this wire also goes thru the Ballast Resistor on its way to the coil....see diagram....no need for diodes didn't need'em in '75 don't need'em now.