'84 Accord won't heat up

I've just got a '84 Honda Accord and the thing wont heat up. It only gets about a quarter off the gauge and blows only lukewarm air. I replaced the thermostat and that didn't help. I also checked the little valve that the hot/cool slider knob controls and it appeared to be working properly. I also blew air through the heater core and there doesn't seem to be any blockage. If anyone has any ideas I would sure Appreciate it. Thanks,

Off the top of my head, I would flush the system. I also wonder if the car has the original radiator or has a bigger replacement. Another thought is have you felt the hoses when the car is at normal operating temperature? Do they feel hot or warm? If they are hot, then you have some blockage, but if the hoses only get warm, I again would look at the radiator, try lowering the amount of fluid, or even try blocking the front of the radiator most of the way to see if the car gets any warmer.