Exhaust gas composition.
 
The only time you give any though to what all those readings mean is when it comes around to MOT time, when you stand there with your fingers crossed hoping that everything is okay, and usually it is, but what happens if if fails? what do you do? what do all those readings mean? well hopefully this page should help you out a little and decipher those letters and numbers for you.

CO - carbon monoxide, the nasty one, odorless and colourless this is the one that destroys your immune system, this needs to be as low as possible but no too low, on cars fitted with catalytic converters this should be almost nothing, on most cars 1% to 2% is about right. High CO readings could indicate a mal adjusted carb or injection system, a worn engine, sticking choke mechanism, low CO readings could indicate a leak in the inlet tract or a hole in the exhaust confusing the anylyser.

HC's - hydrocarbon, this is unburned fuel, well you would expect to get this from a knackered engine that smokes but you can also get high HC's from a couple of other condition, air leak into the inlet tract something like servo pipe, balance pipes (favorite on CU engines) split diaphragm in the vac advance unit of the distributor or worn throttle bushes, Incorrect ignition timing or badly adjusted carb(s). Low HC's are a good thing!

Co2 - Carbon dioxide, greenhouse gas, this is what catalytic converters turn the nasties into (along with water) typical values are about 12-14%, basically you want this as high as you can get it. Low readings mean poor combustion, check valve clearances, ignition timing etc.

O2 - oxygen, we all know this one, about 22% of our atmosphere (at sea level) is oxygen, you need oxygen for combustion, a good measure of how efficient an engine is is to measure how much oxygen is coming out the lower the number the more complete the combustion process is.
You need this about 1%. high readings indicate incomplete combustion, misfire, air leak in inlet tract, ignition timing, valve clearances

Lambda/ AFR (air fuel ratio)
lambda and AFR are not gasses they are theoretical values worked out by the gas anylyser. AFR is the (are you ready for this) stoichometric value, which is 14.7:1 what this means is you have 14.7 parts of air to 1 part of fuel, this number is ratio is used as this is the best compromise for power and economy and heat generated, this is the ratio where petrol burns best, so if you get readings of say 12 : 1 this would indicate a rich mixture (too much air, not enough fuel) and a reading of 16 : 1 would indicate a lean mixture, too much air.
The lambda thing is fairly similar only instead of using 14.7 you use 1, lambda 1 = AFR 1.7:1
so a reading of 0.913 would indicate a rich mixture, 1.112 for example would be lean.

 

MOT gas requirements.

Gas Pre '75 Pre '86 pre '92 post '92
CO Carbon Monoxide visual test only 4.5% 3.5% 0.3%
HC's Hydrocarbon visual test only 1200ppm 1200ppm 200ppm
CO2 Carbon Dioxide no requirement no requirement no requirement no requirement
O2 Oxygen no requirement no requirement no requirement no requirement
L Lambda no requirement no requirement no requirement 0.97 - 1.03

 

remember, carbs and injection systems rarely go out of tune by them selves, if you check your exhaust gasses for the MOT or for any other reason and they are incorrect then it is unlikely to be the carb that needs adjusting. When the emissions are found to be incorrect and because the carb is adjustable people get into the habit of adjusting the carb to suit when most of the time there is another reason for the incorrect emissions. This is why before any adjustments are carried out to the carb or injection system the following should done FIRST!

if it has points set those, do not set the gap set the dwell angle as this takes into account worn points.

Set the tappets, make sure they are opening at the right time (noisy tappets = retarded cam timing = potential power loss and increased emissions)

set the ignition timing, this has a great effect on the exhaust emission, only a degree or 2 can throw the whole lot out.

How to set the idle mixture (taking for granted you have access to a gas anylyser!)

Make sure the exhaust has no holes in as air will be drawn into the exhaust and confuse the readings taken by the machine.

Engine warm and running, all pre adjustments carried out end engine purged by running at fast idle for 30 seconds with the sample probe up the exhaust.
I prefer to watch the AFR or lambda and set this to 14.7:1 or 1 respectively If the AFR is high (above 14.7) then turn the idle mixture screw OUT. If its lean (below 14.7)then screw the adjuster IN.
The adjust works much in the same way as a tap, you screw clockwise to decrease the flow, out or anti clockwise to increase the flow.

Then when they are set ( you never actually get bang on as the figures fluctuate, just somewhere very near) then most of the other readings will come into line ( or should!)
Please bare in mind that what you are setting is the idle mixture only, the adjustment of this on a carburetor engine has very little or no bearing on the mixture throughout the rev range, this is of course set by the main jet.

If you cannot get all the readings within tolerance and you have carried out all your pre checks then you have a problem and further testing will have to be carried out.

Other things to check if you cannot get emission into line.

Correct fuel?
air leaks?
blocked air filter?
compression's?
ignition system?

So, there you go, not a complete guide but just a quick guide in layman's terms, generally there are little or no adjustments needed to the carb if it is kept in good order with regular fuel filters replacements, but some parts do eventually wear and throttle shafts do start to drag acceptable amounts of air in and at higher mileage's will need the odd tweak now and then.
 
Other pages worth a visit.
Antifreeze - Brakefluid

 

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