My heater sucks!

[AirSpeed]

I recently bought a 72 GMC 4X4 and I cant get any heat out of the heater. the fan blows fine, the defrost/heat lever works fine, the temp lever works fine, water is flowing through the heater core, everything seems to be in order except I get lukewarm air only. My 69 puts out so much heat I have to roll down the windows, I cant figure out why I am getting no heat out of my 72. I have read other threads concerning heater problems and none of them seem to be the answer. Anyone have any ideas? Thanks!