As I menioned, I too had a lack of heat and needed to change my T'stat. I put in a new 195 deg F thermostat, filled it up and was very pleased to feel heat almost immediately in the heater return hose. As the engine temperature rose, the thermostat cover stayed cool until the thermostat opened, then presto! (actually water and Prestone) flowed through the top of the rad with just a bit of mosture vapour.

Part of my dilemma was to figure out how to drain enough coolant - into a pan, not onto the floor - so I could reuse it. I tried separating the heater hose from the branch of the lower rad hose, but it was well stuck. Since I have the return hose routed through the alloy intake manifold, it ws asier to disconnect it there. I then found I could bend the pice of hose down lower than the top of the head and direct the stream down into the catch pan under the car. Success!

I also found my old t'stat was rated at 165 deg F. Way too low!

 

I use Redline water wetter in all my cars now, it sounds like a gimmick but it really does work! it's not going to solve any mechanical problems you may have but it does work very well to keep the temp's down when it gets to be high ambiant temps or your just pushing it along a bit.....

Early 60s cars use a different temp sender - i got caught out with that once and thought he car was overheating when it wasn't.

Concerning your comment about quickly overheating after idling awhile: How are you determining that it is actually overheating? Is it boiling over or venting from the overflow tube? Or are you going by the dash guage? Mini temperature guages are not particularly accurate AND need to be matched to the temp sending unit. You may have either a bad guage, the wrong sending unit or both.

 

You have several ideas of things to check for the heater issue, but another issue you have is that the car is overheating.  Have you checked to see if the fan is on the correct way?  If you have the plastic one, the smooth side faces the engine.  You'd be surprised at how many folks install them backwards and that causes the engine to overheat.  Good luck on tracing the heater/water issues, but also check the direction of the fan.

It is early here, and before breakfast, but I don't see mention of an operating thermostat. The purpose of a thermostat is to keep coolant in the head until it reaches the t'stat operating temperature - 180 deg or 195 deg. F. If you have a failed t'stat or none at all, the engine will take forever to warm up and will cool down quickly under way. It also kees the engine at the temperature for optimum combustion efficiency.

When the t'stat is closed (or partially closed) the water pump can't pull from the rad, so pulls from the heater hose, which should bring warmed coolant through the heater, giving you warmth sooner.

I have the same problem of no heat because I know my t'stat isn't working properly.

 

to begin with, thanks for all of the great responses, lots of good things i can double-check. i know the block and head were originally clean, i assembled the engine myself and both looked pretty good to me. i know one line to the heater goes from the valve at the back of the head and the other goes to that funky molded thing that is part of the lower hose, so i think the hoses are physically correct. i know i tried opening and closing the heater valve with no difference, but i do not remember if i took it off and actually looked inside to verify it moves, my memory says i did, but memories can fail me. i completely disassembled and restored the heater when i restored the mini, but that was in 1984. it would be my standard procedure to at least run garden hose water through the core to flush out anything left inside it. the heater never worked from the day i got the car running thirty years ago and it remains dead to this day. it was sometime in the few years after i got it running that i found the flow to be non-existent while troubleshooting the heater. i currently only drive it during winter months, so the topic came to life again recently. perhaps another fact will shed light on the real problem. during the winter, the engine never warms up while driving around, there is so little temp in the coolant that the heater would probably do little even if it had flow. it will only warm up if i let it sit and idle a long time . Even it winter, it goes from too cold to too hot if i let it idle long enough. it has always done this as well. simply put, the cooling system has been an overall disappointment ever since i got this guy running years ago. thanks again for the abundant, thoughtful responses.

Check the return hose FROM the heater... Your "hybrid" may have plumbing from a later model. I've seen return lines going to the thermostat housing or the top of the radiator, where there is "pressure" from the water pump.  It needs the suction FROM the water pump where the return hose connects to the lower radiator hose just before the water pump.

If your hoses are standard, then I agree with others that a good flush of the water jacket is in order! If your head is full of rust, it'll overheat, and the heater won't get a good supply of hot coolant.

I worked on a Mini once and the PO had used a radiator stop leak which had plugged up the heater valve and inlet hose into the heater core. I unplugged the stoppage and flushed the system and it worked fine.

I've never had to bleed the cooling system - it is a self bleeding design.  As long as the heater tap is open and nothing is blocked, the water pump sucks the coolant through the heater. 
It sounds to me like the heater tap has clogged up internally.

When you fitted the 997 carbs and manifolds, did you change the needles and retune the mixure to suit a 1275 ?
Running very lean will cause the engine to run very hot (and burn things like valves and pistons) and the cooling system can struggle to keep up.

I'm with CupCake here. I've had to burp my system numerous times and that's the best way that I personally know how. I have more heat than I can handle sometimes. Doubtful it's your water pump's problem or anything with the engine. You should purge the whole system any time you play with the hoses. It will also keep your block from creating air cavities and forming hot spots. Might even help some with your overheating issue.

Hydraulic lock? Bleed the heater core by disconnecting both lines in the engine bay, then pour coolant in one until it flows from the other.

How is the condition of the heater control valve that connects the heater hose (to the heater core in the cabin) to the cylinder head above the #4 cylinder?  Is it in good working condition?  Does the heater cable that connects to it actually do anything to the valve or is the valve frozen up?  You say you disconnected the hose from it and virtually no water flowed - perhaps the valve is stuck closed.  Just a guess ...

my '63 Austin Cooper has a hybrid 1275 engine from an Austin America. i say "hybrid" because internally it's a stock AA engine, but externally it has the original 997 carbs, intake and exhaust header, distributor, along with a bunch of the other original details. the engine was completely rebuilt and the block is internally clean. it also has the original 997 fresh air heater. there is no heat from the heater at any time and the engine overheats all year when idling. i took off the hose from the back of the head to the heater while the engine is idling and there is no flow to speak of, hardly any coolant comes out of the hose at all. it has the "heavy duty" water pump with the deeper impeller, obviously that is not doing much pumping. there was some flow when i revved up the engine, but it was so tiny compared to any other car i have ever seen to be insignificant. i have no idea why the flow is minimal. any thoughts?

thanks, Dave