need your advice with my small electrical problem

using a starter solenoid as an ignition relay is not a good idea. As was stated before, a starter solenoid is not meant for continuous duty... replace it with another relay that is a continuous duty and you should be fine. If you do want to use a solenoid, go get a solenoid from a gas golf cart. They are continuous duty. The starter runs the whole time the engine is running (actually a starter/generator). Gas golf cars use 12 volt, elec ones use 36 volt so make sure it is for a gas one.