[spankle my wankle]

My first gen barely gets above the second marking by the C when it runs or even idles. I've changed the thermostat and that didnt work. Im wondering why this could be. I know the gauge works because I didnt have enough coolant in it and it went up towards the H. Its an odd problem to have but I need it to run a little hotter so I can tune it and it will idle better. Any advise? Thanks.