My daughters mini still runs hot in traffic and on the freeway.
If you slow down to ~80 the temp comes back.
It has a new cast water pump and a new radiator.
I also installed a new 77 deg thermostat.
I just noticed this http://www.ebay.com.au/itm/New-Genuine- ... 233bc85616 and thought there may be different ones that open further than others.
I tested the thermostat in a pot of water and it opens at the correct pressure but I didn't think it opened very far. May be ~5mm ?

Just checked and the current one is a Dayco.

Any ideas

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I have found that fuel mixtures and the ignition timing curve can be factors in overheating.
I would doubt the thermostat is your problem (although it's not impossible that a new one is not working like it should), and it sounds lke you have changed all the common bits.

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How can it be rusty when it's covered in so much oil?

Based on what you have done already I would suspect either fuel mixture or a timing issue. What carb Are you running ?

It's an 1098 with 1-1/2" SU, bit of a cam of unknown type plus extractors.
It does not run particularly well on the freeway. It's not real bad, but just not "sweet" as it should be.

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Pull the spark plugs and check condition, the Haynes manual has a chart that will give you an idea of fuel mixture or other issue. Timing could also be an issue, pretty sure if you are to far advance it will cause over heating issues, but this will also reduce power output .

Dunno what dizzy you have, whether it's original or not. Check and adjust the points gap first.
Then I'd borrow a dialback timing light, set timing to 30deg BTDC @ 4000 rpm with the vac advance hose off. Reconnect hose, see how that goes.

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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.