A lot of us convert the bypass solenoid to manual control. The whole thermal time switch method of triggering it seems to be a point of constant problems. With you controlling it, through a momentary contact switch placed somewhere on the dashboard, you can eliminate the tendencies to provide either too much fuel when the engine is still warm or too little fuel when it's cold.
I placed a momentary contact switch in an unused hole in the dash right next to the steering wheel on the right. It even looks like it might belong there.
Being that you are selling the car, however, this might not be an option if the buyer is looking for originality. Probably more something you would do on a "keeper".
