[Daniel]

I suspect your answer is the most accurate I'll get, Rick  . CO is measured at idle speed. That's where the Sunbeam falls short. The idle speed cannot be set accurately enough to get a result that can be repeated. Anyway, purely out of interest, I'll give it a try and see what happens.

[Rick Parkington]

Interesting question Dani and I don't have an answer except to say that I suspect there is no answer. I would guess the carburation on old bikes was never precise enough to get a very consistent reading on a test like this. Most old vehicles were tuned on the rich side in order to  keep them cool and to overcome possible fluctuations resulting from atmospheric influences among other things. I think modern testing methods rely on carburation being 95% right in the first place - I know a dyno, for example can only perfect carburation, not set it up from scratch and I  doubt any prewar vehicles came close to that level of accuracy. When you look at carb history, it started out with very setup - the two lever Amac and similar were designed to be tuned by the rider while on the move, so a skilled rider could get the engine running perfectly by constant slight adjustments - but many more were probably running around like it was on full choke! Even into the 1930s carb settings were quite generalised, based more on engine capacity and sports or touring tune than on individual make and model and it seems it was only after the War that the idea of having a specific jetting setup for each model came along - in fairness this probably also had something to do with variable fuel quality, you can't be that precise about settings when there is no fixed standard for fuel grade.
Although carburettors became more  complex as emissions regulations toughened, digital fuel injection has been the game changer, in theory it is possible to set it up so that fuelling is perfect for all driving conditions - and then a CO meter will pick up and problem and enable it to be rectified. Brilliant...although, strangely my old diesel van never blew out the kind of black smoke you often see from more modern digital injection diesels, so maybe old school mechanical with a bit of margin for error thrown in is better than super accurate digital after all...!   
Cheers Rick 

[Daniel]

I know we're working on engines that are around 100 years old. However, for my modern motorcycles, I have a carbon monoxide meter that allows me to adjust the air-fuel ratio almost perfectly. Does anyone in this group have experience with CO measurements on such old engines? Does it work accurately, and what percentage of CO is a reasonable value?

Best regards,
Dani