Colthebrummie said:

Apologies, I didn't notice you are in France. Take out the spark plugs and look at the colour of the tips, they should be a light tan colour if the mixture is right. Also check your air filter, if it's dirty or clogged, the engine will run rich.

Col

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TS2304 said:

I always found getting my ’81 petrol through its MOT emission test one of the biggest hassle so I feel your pain. Clearly something in the set-up of the engine is not right and regardless of whether you need an MOT or not (as Col says in the UK you don’t but maybe in France you do??) you will want to get things right eventually anyway.
The answer lies in the relationship between your valve clearances, your ignition timing setting and the settings on your carb (idle speed and mixture screw). Getting things set up right properly really involves working your way through things - if you search for ‘setting ignition timing’ you’ll find plenty of guides online. You should really start with setting your valve clearances to the right gap, then adjusting your ignition timing mark to the right number of degrees BTDC (Before Top Dead Centre), then when your idle speed is right you can adjust your carb mixture screw to achieve the right engine note which should then result in the right emissions. But to do all that if you’re not used to how the engine should sound and run, and without a timing light, a way to measure the idle speed and your own gas tester i always found it tricky to get right. TBH even with all that, I have set mine all up so i thought it was right, and my little gunson gas tester read right but still the CO measurement was too high come MOT time.
If your valves are about right, and your timing is about right then usually it’s down to the carb setting and if it’s close with luck the MOT man will let you adjust things while he has his probe running and you can simply twist the carb mixture screw (slowly and by small amounts) to change the CO reading and get it to pass.
Without a gas tester or an experienced ‘ear’ to the changes in engine note you’ll be hard pressed to know if it’s right on your own but you will find plenty of guides and suggestions on how to do it.
That all said, one thing to check first would be that your choke isn’t stuck ‘on’ - mine did that once and although the knob was pushed in, the cable snagged and so it was still operating at the carb which led to a very dodgy running!
Good luck!

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rob1miles said:

You MUST do this in the order TS says above. Don't start with he carb, you will go down a rabbit hole into a morass. Do the most mechanical and exact first, that means tappets, then points (clean and gap) then dizzy cap, then static timing, and check the advance and retard moves, its under the plate with the points on, if not oil it and free it.. Check the vacuum advance works, pull the tube off, suck it, if get any movement its OK, check the pipe to the vacuum advance, make sure its got no leaks. Check the gasket on the base of the carb, make sure the air-filter is clean, check the breather is connected right.. Put new plugs in. NOW at last, you are ready to start adjusting the carb. How to do that is a whole other post, but it has to be done in ranges, idle, pick up, mid range and full throttle. The plugs are your guide and there is lots on the web about the colour. but is not easy, you have to drive, cut the engine, coast to a stop and pull the plugs. Yes that's faff but if you slow down with the engine running and the idle is rich it dusts them in soot and fools you. If you can get it on a gas analyser do it. Because this is for an emissions test set it to the lean limit and drive carefully. Once its passed richen it up slightly to protect the pistons. Old engines can pass emissions tests but the settings have to be spot on.

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