Our old Challenger electric garage door will not stay closed when using wall switch or remote. The light blinks also. There is a nut either side of the main unit which adjusts but it will not tighten any further and loosening the bold makes the door too heavy to open.

If you mean it closes then immediately revereses and goes up, it sounds like it believes a sensor is telling it that it senses and obstruction. I don't think its an issue of balance or weight.

I think you need to clarify a little more about what exactly you see it doing.