To test the thermostat, just drop it in some water on the kitchen stove and see at what point it opens up.
Generally speaking, you're not supposed to run without a thermostat or restricter plate. The reason being that if the water is allowed to go through the radiator too fast, the radiator won't have time to pull the heat out of the coolant, and you will overheat, or at least run hotter. The rate of flow is often overlooked. A huge radiator may not help, if the flow rate is all wrong, for example. If you are running fine without a thermostat, then I would suspect that possibly there is allready a restriction of some kind at work?? and that adding another one in the form of the therm. is putting it over the top. Of course, I could be totally of base too...
Are the belt pulleys the right size?
How close is the fan to the radiator?
Does the lower radiator hose have a spring in it to prevent it from colapsing?
What shape is the radiator in?
Check the thermostat. Make sure it's installed the correct way. Add a small hole in the flange to allow air to escape for when you are filling it up.
You may want to add an overflow tank to the system. I have used those stainless ones from the magazines before, but have found them to be too small. If you have the room, a large factory plastic one mounted out of sight under the fender would be cheaper/better, in my opinion. Make sure you are using the right kind of radiator cap also.
