I have an early 1975 Mini Clubman, 1098cc, unmodified, that until recently was running well. It's driven only once every two or three weeks.
Given the sudden onset of the problem, I assume it's not a question of adjustment but more likely something broken / stuck / blocked / fallen off.

The engine starts easily, idles smoothly, and driving gently up and down the road at 30-40km/h it feels normal.
Go for more and around 2500rpm it stops accelerating, splutters and puffs of black smoke are visible in the mirror.
It feels like load is the issue, because you get a few more revs (not much) with a lighter throttle and fewer revs up hill.

It revs smoothly in neutral, and on the timing light it comes progressively to full advance and holds steady. So far I have checked the points gap, compression (even across all four and stronger than another 1100 that runs OK) and that the piston in the carby is moving freely and has oil in the damper. I have replaced the needle and seat, coil, spark plugs, leads, and distributor (with another 29D). While it's possible the substituted parts could also be faulty, none of these changes has affected the car's behaviour.

All I can think to try next is driving without the exhaust pipe and air cleaner assembly.

I hope that to someone more experienced my problem will be obvious.
Can anyone here please tell me how to fix my Mini?

Could it be a lack of fuel or poor quality fuel? Do you have a fuel filter, and has it been checked? Is the float in the carb set correctly?

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I reckon your coil is buggered - swap it for a known good one to test

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did I tell you that I won a trophy?

I'll put money on a stuffed air filter..

Cheers, Ian

Black smoke indicates unburnt fuel/way too rich. So, either it can't get enough air in, it has too much fuel, or it is not setting it alight properly.

As it is fine at idle/low revs it is unlikely to be float level. First and easy check would be the air filter... if in doubt, take it for a short drive with no filter. If it is not the filter, have a look at the ignition... coil, condenser as a start rather than plugs and leads or cap.

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Don't listen to what I am saying... just understand what I mean.

Graeme, as you say sudden change indicates something blocked up on inlet or exhaust.

My money would be on exhaust. Had this happen to me quite a few years back on a Metro, I think the inside baffle in one of the boxes rusted and then wadding moved about and partially blocked it.

Symptoms were similar to what you describe, it would start and run at idle OK, but as soon as you put any load on the engine the power dropped right off.

New exhaust fitted and it was all sorted.

Thanks all for your helpful advice.
It feels better having a clear plan for what to do on Sunday. I'll let you know how it goes.

Check that the choke isn't stuck on. By that I mean check that the Jet is properly seated in its run position against the adjustment nut.
Dave

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It was the coil, so the spare that I tested must have been bad as well. My mistake here was assuming that two bad coils would feel different from each other.
The choke and air filter are OK, and I'm glad I didn't have to take the exhaust apart.

Thanks again to all who helped me. Credit to simon k for spot-on diagnosis and Tim I for explanation and process.

WINNER!!

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did I tell you that I won a trophy?

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Find a job you love and you will never have to go to work !

Your lucky week Simon, time to buy a lottery ticket.

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1972 Leyland Mini Clubman
2000 Honda CBR929RR