79 spirit,heater core,just warm air!

AMCs have a water flow control valve somewhere. Start by following the heater hoses. If it's not in the hose or on the firewall look under the dash where the hoses go in the firewall. It definitely sounds like you're not getting water flow through the heater core, but it could be the mix door. When you move the "AC/heat/bi-lv/def" lever a "door" moves that redirects air flow. That able could be loose or broken. Are you getting hot air out of the defrost ducts? From the center floor ducts? Just not getting it from the dash vents? If getting hot air from one of those the door cable is the issue. Also, is the fan running at all speeds? I'd check all this before the heater core. If all that is good, well, I'd replace the heater core. If it's that stopped up flushing probably won't help. Won't hurt to try, but take both heater hoses loose and see if you can run water through the core with a garden hose first.  I'd get a repair female hose end and put it on one of the heater hoses so you get a good connection with a hose.

I'm assuming this is a six cylinder 79 Spirit...

It was common for the foam insulation to fall off of the door and block the air flow.

If your Spirit has A/C then the valve the controls the hot water to the heater core is a device shaped in the way of the letter H and is located in front of the fire wall and basically shuts off the hot water to the heater core when the A/C is on. And it is prone to failure which means it does not work and it is made of un-obtainium if it is actually no repairable by some mystery means I have yet to discover.
The letter H has four water lines going to it, two from the engine and two from the heater core. This should describe what you are looking for. If you do not have A/C I don't know what it looks like but it is different.
But in any case that is where I would start looking for your problem.

Since my '79 Spirit (Casper) is out getting changed to an automatic,and the tech has my TSM, I cannot go look at my set up. I too  just looked at the Rock site and see the valve. Since you do not have one and your hoses are hot, then your core is clogged.

Last autumn I hooked up the heater in my Hornet (it was disconnected from the previous owner) to keep warm during the fall AMC show. I had already bought  a new core but since that is a chore to replace, I decided to see if the existing core had a leak so I took my garden hose and flushed out the core. Not much came out at first (what did come out was rusty sludge) but the flow got better and better (and clearer and clearer) until it ran clear and full flow. Then I looked to see if the carpets were wet. They weren't so I hooked up the hoses, topped off the radiator and let the car run with the heat on full. I let the car run for fifteen to twenty minutes and I got fantastic heat out of the heater and still the carpets were dry. So I concluded that the previous owner disconnected the heater to keep the interior cooler in the summer.

So I would suggest you use the same method and see what happens. I am convinced that your core is clogged. By flushing with the hose, you will see what comes out. Once you have finished that process you can use the radiator flush to clean the entire system. Once you have good heat and the system is clean, do not forget to add antifreeze.

Hope this helps.

Later!
Mr. Ed

Hooked 2 short 24in hoses to the core,poured 1-2 bottle clr in and let set 15-20min.
Poured about qt of water in then, quart of fluid came out other hose,kinda green(clr ).
Then blew through hose with my mouth, blew fluid out other hose, wasn't hard to blow through,
no trash or dirty water came out. Think it's ok. Both levers are working,I can c the cool to
hot lever on the heater box moving when I move the slide.let it run 15-20 min, it's getting better,
but still nothing to brag about, the longer it runs the Better it gets.with a rag completely covering
the radiator running 20 min the temp gauge only gets about 1/4 in into the green normal. New 195 tstat
Just installed.