Carburettor performance with cold weather

[Hans Efde]

Last spring I installed a wideband lambda probe with display. In the summer I adjusted the carburettor to get the mixture between 12.5 and 13.5 AFR (grinding the needle). At higher speeds the car tends to go lean around 15.0 but without a power drop. Last weekend I took it out for a spin and here in Holland temperatures have dropped to around 15 degrees at noon. To my surprise I noticed much higher AFR's. Already at lower speeds over 14.5 and at higher speeds (70mph plus) extremely lean to 16.0 and noticeable loss in power. I have to assume the lower temperature has increased the oxygen level, causing the engine to run lean. I adjusted the CO by giving it a 180 degrees turn clockwise and now the AFR values are in the desired range again. When the injection system is running it will automatically adjust for air temperature.

So if you are still driving your carburetted Midas in colder temperatures and notice a power loss (esspecially at higher speeds), give it a bit more CO. It may need it!