If you have multiple carbs, then get enough Colourtunes to do each mixture branch (I have four, for doing Rover V8s with twin SUs / Strombergs). The ability to independently see the mixture is a _huge_ advantage over sniffing it from a single tailpipe.
on the gunson website i read the products manual and found that yes you do have to calibrate it to 2% in air before starting.
thx
It's because its not measuring CO per se but the thermal conductivity of the exhaust gas, so what they are saying is that the thermal conductivity of air indicates the same as the thermal conductivity of the exhaust gas ( a mixture of nitrogen, steam, CO2, CO, 02 and other traces) containing 2% CO. They must have a map that enables them to assume that CO changes inversely to O2 content.
If so it won't work with different fuels.
Also as it depends on near stoichiometric fuel:air it won't work at all with diesels.
AJH
ok i think i have found a reference to this 2% in the pdf manual.
"NB: The calibrate position represents what the instrument should register when the probe is in the air. It is coincidental that air should measure the same as exhaust gas with 2% CO). When the probe is subsequently inserted into the exhaust pipe, the display of the instrument may increase or fall from the calibrate condition"
I dont understand the theory behind it though.There are some graphs in the manual too about stocastic ratios etc. Any ideas about why a 2% is the initial calibration and not 0%?
thx
oops AJH was answering my question as i was still writing it!! thanks
Note also that even the tiniest leak on the exhaust will throw out the reading. Dunno why.
In addition tyo what AJH wrote, the gas is already present in the air=20 we breathe as well so being accurate, it should never be at zero.
--=20 Conor
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out in which direction? i.e. leak causes a lower CO on the display or higher CO on the display?
vehicles with cats have to have a sealed system with no leaks as it upsets the Lambda & CO readings making them higher readings then they should be.
i can test one with a few pin holes in it - a fail. patch it with a bit of exhaust paste straight through as a pass. mind you its not always the rule, a blowing exhaust can pass on one vehicle & fail on another !
Higher. Also tends to give a rapidly changing reading starting out low and climbing.
I've not tried it on a cat equipped vehicle with a leak. Only really my old SD1. I'd say if one of these Gunson analysers showed up a problem with a cat equipped car, you'd know all about it already.
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