Hi Pete - Interesting on the emissions. I can speak from experience that the CO pot is capable of setting a CO level of anywhere from very close to zero to well over 7%.
Mine was originally running at around 5% when I bought it, then I 'struggled' to reduce it through a lack of understanding of how sensitive the CO pot setting was. Now I have my own decent gastester, I can set pretty much any setting that I want, the ideal being a low CO with a lowish HydroCarbon (HC) reading. So, after setting it to about 1.5% CO, with (IIRC) about 100ppm HC, I went all conservative and made the mixture richer to give a CO level of nearer 3.5% to give the engine some insurance against lean running. This page gives some pointers on what to do - I only have a two gas tester, so can only measure CO and HC, but you can see the hints for high CO:
http://www.ratwell.com/mirror/interro/techgas.htmlWhy am I rambling on? Well, because there are some interesting observations. When the engine is running well, i.e. neither too rich nor lean, with correct timing, the exhaust products are mainly CO2 and water. On a slightly cool day, that results in quite a lot of water coming out of the exhaust, with not much of that acrid smell you get from poor running. I'm not advocating using your lungs as a gastester, but holding a hand over the exhaust and sniffing what 'sticks' to it gives a bit of an indication. On mine, after setting the idle revs in service mode, with the engine hot, you can weaken the mixture quite a way using the CO pot and the 'good' setting is about 1/2 to 1 turn back from the point where the engine seems to bog down. It's worth noting that the Oxygen and HC readings increase as well in a 'too lean' condition, but when I was playing with mine, the idle went lumpy at that point anyway and it was the resulting 'miss' that seemed to cause it.
It's worth saying that the CO rises a little if the engine has been left idling for a while, you can significantly reduce it by elevating the revs for a while before the idle test, but it creeps up and their test will wait for a stable reading. Worth knowing about when setting though.
Bottom line: If they know how to get it into service mode, set the idle valve to 20 steps as per the manual, adjust the idle bypass valve to restore the correct 950rpm, then I'm pretty sure adjusting the CO pot WILL bring it down.