Just re-taxed the 2CV this morning and happened to look at the emissions page from the last MOT in September. The CO2 gives a max allowance of 3.5%, the car is pushing out just 0.145. Out of curiosity, is this a normal reading for a M28, or is it too low, suggesting that the fuel mixture is perhaps too lean? Ta muchly for any input.

Does the engine run hot? That would equal too lean would it not?

emmision-test is done at iddle (at least here in Holland) so the car is running on iddle-jet, when driving this jet is not used, so for iddleling along emmision can be as low as possible (car needs to pick up rpm smoothly)......

Don't think it runs hot, I'm just wondering if a figure of 0.145 is a healthy number or whether it's suggesting I should look at the mixture whilst things are still fine (if indeed they're not).

Ah, okay, nothing to worry about then.

yes its a bit lean it may hesitate coming off idle

aircooled engines need to be set richer than the ideal but the emissions ( allowance in the test) is so generous that if you were at the top end youd be stinking of petrol

Okay. The HC emissions have a max of 1200ppm but are 128ppm on my 2CV, so with both those figures, what should I look to do? Is 128ppm too low or is that a good number?

Had two tests on Celeste and they were 238 and 302 - is the HC test one of making sure engine is burning fuel completely and therefore not emitting much NOX?

"NOx = Oxides of Nitrogen (This is only seen by a 5-gas analyser) Only seen with dynamometer or engine under load. NOx emissions rise and fall in a reverse pattern to HC emissions. As the mixture becomes leaner more of the HC's are burnt, but at high temperatures and pressures (under load) in the combustion chamber there will be excess O2 molecules which combine with the nitrogen to create NOx"

Citroen's 'target' setting for CO emissions at idle is between 0.8% & 1.6%.
Too low and the idle is uneven, too high and there's that pong of unburnt petrol from the exhaust.