Malsal is correct with his info. Do Not adjust the idle speed. Only can be done with reader. Replace all your elbows. You can buy tight fitting rubber hose and make a gentle curve with them . Elbows are neater and shorter. Make sure they are all connected. The water temp sensor can detested with a Volt Ohm Meter. If you have resistance, it's good.

I'm not sure about clearing codes, never heard of that.

As stated start with the vacuum elbow connectors they crack and leak with age. There are eight of them if you replace on replace them all.
These are crucial to the running of the car as the computer needs vacuum for a signal otherwise it goes into rich mode.
There is a fuel trap on the firewall if it is blocked or contaminated with fuel clean it or better still replace it.
The coolant temp sensor is another failing point. If your temp reads low the sensor is bad and will cause it to run rich.
If your spi is a UK car it is not uncommon to find the grounds rusty and corroded, clean them up and refit them.
This should get you started. 
If you can find someone local with an Sikes pickavant spi code reader you need to run the codes. Also the codes do not clear themselves you have to clear them with the code reader. This means if you do the repairs the codes will still be stored and need to be cleared especially if you are trying to pass emission testing.
The spi header flows really well sometimes better than aftermarket ones you may have altered things when you changed the header and cat out.
As Bill said theminiforum.co.uk has an excellent thread written by Sprocket which points out all the trouble spots.

I would start with looking for vacuum leaks, broken, cracked lines and connectors. Dirty electrical connections, air filter, throttle body. Has it run any better for you in the past? Or is it new to you?

good morning. you might get more info on the www.miniforum.co.uk  website. as 93 SPI are  quite NOT legal in the USA. yet.. unless manufactured before March 25 1993... later bc 

To the kind folks of Mini Mania,

My 1993 Rover Mini SPi failed its MOT last week due to too high emissions of carbon monoxide (10%), and a lambda-value of around 0,7. 

Things worth mentioning about my Mini:
- I have changed the exhaust-system to one with a banana manifold. During that operation I also changed the catalytic converter and the lambda-sensor. 

- The mini is idling at around 1200rpm (too high) 

- It is spitting a bit out the exhaust especially under startups, but also during idling.

 

Is this a familiar problem to any of you? And do you have any good tips and tricks for bringing down the emissions? 

All help will be welcomed with open arms! 
Thanks a lot in advance.