What is True Running please?

Hi Jim....just dont worry about it.  in 30 plus years working with these carbs I can tell you it is best to leave it at the factory 1.5 turns, mixture screw,  then work with balance off idle, 3000 is what I like.  (pneumatic balance, adjustment on the cable to match manifold side to side) Once balanced there it should be smooth all the way to WOT.  Remember that there is no clutch to disengage like in a motorcycle to remove the load.  The lack of a accelerator pump in the CV carb is made up by leaving the mixture slightly rich.  

Cheers

Thanks Mr. Wizard, yes I don’t fiddle with the mixtures anymore. I learned (on the ground) that one can lean the mixture screws to the point where the engine will idle and accelerate just fine, but a rapid throttle reduction from a cruise power setting would cause the engine to abruptly quit.  I decided then and there to leave those screws at 1.5 turns out and never play with them again.  Being who I am though, the sooty front cylinder plugs and incomplete combustion product deposits perched atop the front piston tops in tiny heaps annoy me, I really want to fix that, but I’ve learned to just accept it.
However I am still wondering what “true running” of the engine means, and by what procedure one would check it. No guidance is provided in the manual, which is odd to me given that the reader is recommended to perform the check. I speculate it could mean marking 1.5 turns out as the baseline, turn the mixture screw in (lean) until the engine begins to stumble, note the position of the screw, then back the screw out past the 1.5 baseline mark (enrich) until the engine starts to stumble, note the position of the screw, then split the difference between the lean and rich stumble points. 99% of the time you’ll probably end up back at your 1.5 turns out baseline. Just a guess.

I think it’s just wording because Rotax manuals were written originally in German and when they are translated into English they can sound off a little bit. In English maybe a better translation for true running in German would be proper running or smooth running.

Hi again jim.

This term is a relic from the Bing information on setting CV carbs on motorcycles.  They have detailed instructions on how to adjust the mixture screw but as we know the load from a propeller is much different.  I think your assessment is near enough.  For sure you can get a very nice idle however it is at the expense of acceleration into mid range and beyond that creates the problem with the application on an aircraft engine.  

In a past life I was a motorcycle tech and we worked on a lot of constant depression carburetors.  Carbs are very sensitive in how they work based on cam, exhaust back pressure and many other factors.  In general the stock settings will be the best overall. 

Cheers

So, for all readers I have found a very nice write up from a much better authority than myself on what a CV carburetor is and how it works.

https://www.vintagehondatwins.com/forums/index.php?threads/carburetors-part-3-the-constant-velocity-carburetor.9606/

This comes from a forum on vintage honda twins, and the contributor was "Mike's Garage"  When you read it you will have a better understanding of what a constant velocity carb is and why we call it that. 

Cheers