As background, I have a fair bit of experience with carbs in general. I've installed overhaul kits in Bing carbs (on airhead BMW motorcycles), and have a decent basic knowledge as to how they work. To summarize, here's my impression:eddie, are your carbs set up correctly? i find that 7,500 msl is the sweet spot for best performance and best range seems to be around 10,000'
my plugs always look good, i think the auto-mixture works pretty well.
The throttle is hooked up, not to the slide, but to a butterfly valve, only controlling the amount of air flowing into the carb. This air, via Bernoulli, causes a drop in pressure when constricted. This lower pressure air is channeled to the top of a rubber diaghram, which in turns lifts the actual slide. The flow of fuel into the venturi is metered by a tapered needle which rises in the main jet with the slide - the higher the slide, the more fuel.
Now, to my way of thinking, the volume of fuel remains constant for any given slide setting, but since the density of the incoming air decreases with altitude, the mixture gradually gets richer and richer.
So, I don't see how the Bing carb can be said to have "auto-mixture".
Here's the Cruise Performance chart for my Sky Arrow with as ROTAX 912ULS2:
This is a very different situation from a "normal" aircraft engine - note what happens to range at 6,000' compared to 2,000'. In an aircraft engine with mixture, its a truism that both TAS and range increase with altitude for any given power setting/fuel flow. Clearly not the case here, and my assumption has always been that its due to a necessarily "too-rich" mixture at altitude - necessary because otherwise it would be too lean at low altitiudes.
Subjectively, my Sky Arrow does seem to "coarsen" up a bit with altitude - never rough, but it just feels like I'd reach over and lean the mixture a tad if I could. I think 8,500' is the highest I've had occasion to climb so far.
With a performance chart like mine, I tend to fly low. I'll often fly 1,500' or lower, often just following highways. I know the higher I go, the less range I'll get, significant if I'm trying to fly close to 300nm non-stop.
At the last annual my plugs looked normal.
So, is there an "auto-mixture" feature* on the aviation Bing's I'm missing? Or is it just my Sky Arrow? I'd love to see the Cruise Performance chart for a ROTAX engine that shows best range and best speed at 7,500' or 10,000'. Not arguing, it could clearly be the case - more curious!
*Cirrus SR20's DO have a real "auto-mixture" that progressively leans the mixture with altitude, so such a concept is clearly possible.