Ruby, the 850 with a 1098 has been running hot ever since we fitted a mechanical temperature gauge, that's how we found out she runs hot. I've checked the temp with my laser temp gauge and the mechanical gauge is spot on.
In traffic she manages to stay at about 90, but on the freeway she gets over 100.
We decided to run some Wynns cooling system cleaner but you need to drain the glycol out first.
I removed the plug in the back of the block near the flywheel housing and no water came out
I suspected that there may be significant mud build up in the block so I shoved a tiny screwdriver in the hole and it was full of black crusty stuff
I poked and prodded the screwdriver in about 50mm and still no water.
Even started it with the radiator cap on to build pressure, to force the crud out but still no luck.
Has anyone had this issue.
Could there really be that much crap in there

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Was the block chemically cleaned when rebuilt ? Unfortunately mini engines didn't have the best cooling system in the world so they need all the help they can get .

Never been rebuilt by us.
Don't know when it was rebuilt either, but she actually runs really well. Just hot

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You could pop out a few welch plugs and have a better look and dig out the scale.

My temp goes down on the freeway? Even at full tilt. Hates the traffic. How can it go the other way?

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If I'm carrying on like a pork-chop, I'm having a great time.

If I go very quiet there's a very good chance I broke something.

There is little heat generated idling in traffic and if the radiator is clean the fan should flow enough air to keep it cool.
At speed on the freeway, more heat is generated and if there is significant sludge in the galleries the water flow is restricted and cannot carry the heat to the radiator, hence it runs hot.

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Yes we could. The welch plugs are only on the front (aren't they) and would only provide access to part of the galleries.
I'm considering filling it with a molasses mix for a few weeks if the Wynns doesn't do the trick.

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Do you have any additional things plumbed into the cooling system - heater, auxiliary radiator, manifold etc?

Have you tested to see if the thermostat is opening fully?

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No Timmy, no additional item. Just the bare engine and radiator. Super simple.

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If I go down the molasses path, what is likely to be eaten by the molasses.
If I replace the water pump with a steel blanking plate and remove the thermostat housing I can fill the block and head while it is all still in the car.
Is there anything else that may be a problem ? Head gasket, welsh plugs........

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It will only eat alloy parts. Head gasket will be fine. Re the blocked rear drain hole. Stick a 1/4"? drill bit in there and twiddle it with your fingers. Usually clears the muck out. It should go in around 70mm or more.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

As Doc said the mollasses will only affect any alloy parts but realistically the block needs to be treated and flushed to help the system operate to its max
capacity .

Maybe try giving it a good flush , then reverse flush , maybe get the rad cleaned out ( rodded ) and see how it goes .

Thanks Doc and goodie
We have removed the radiator and water pump and thermostat housing.
I made a blanking plate to replace the water pump and we have filled the block and head with a 50/50 mix of water and molasses.
It only required 1.5 litres to fill it up to the face of the head where the thermostat fits.
We will leave it a few weeks and then give it a good flush.
We tried to cure the temperature issue 18 months ago with a new water pump (cast impeller) and radiator and 77 deg thermostat, but it ran exactly the same temperature.

Time will tell if I'm on the right track.

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once the molasses is out, it might be worth screwing a fitting into the drain plug hole on the back of the block and putting your garden hose on it with a hose clamp... there will be a LOT of crap sitting in the bottom of the water galleries. I recommend popping the welch plugs out and poking around with bits of wire & using a magnet

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Don't forget there is a Welch plug on back of the block. As Simon said I would remove them all, then hit it with garden hose or a water blaster, with water pump hole open.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.