Hello all,
Just re-registered the 850 after sitting idle for months. Driving it to blow the cobwebs out of the carb, it did a rough-sounding change into 2nd (not thrashing it!) and now there is a Harley exhaust note only under load (present only briefly when revving out of gear). It sounds the way my old Hyundai Sonata did when the cat blew a hole. But the mini doesn't have a cat.
Quick check under the bonnet showed the manifold is all in place, as it should be.
Exhaust hangars look all intact. Any other ideas before I have to jack it up to have a look underneath?

have a look at the manifold gasket

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1972 Mk3 Mini 1000, 1150cc supercharged.
https://www.ausmini.com/forums/viewtopic.php?f=3&t=95872

Manifold, also any of the joins and possibly the muffler box may have lost some of its baffles or a hole.
You will need to get umder and have a good look.

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Cheers Dave.

I believe 850s tended to crack the exhaust pipe where the downpipe bends to go under the car, beside the diff. Due to engine movement on and off drive and not much stabilising in that area.

I've had a check in all those spots, nothing obvious comes to light. I have not taken the gasket off, because I don't have a spare. I might take it to a tame mechanic I know and see if we can hoist it, to have a proper look. He might have some other suggestions too.
Many thanks for all your help so far, will update if I find out anything.

The other common spot is where the down pipe meets the cast manifold. The conical seal there can go bad

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Well the issue actually turned out to be a head gasket. Blown between 2 and 3.
Started blowing a lot of white smoke, and the loss of power became too severe to actually drive anywhere - all the symptoms pointed to the gasket, which I replaced and the car seems fine now.
Thabks all for your help!

You may want to check the timing, and make sure it is not pre igniting or pinging.

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Cheers Dave.