912 UL hesitation

Thank you for the comment. I will report back after my next flight. I have been flying behind this engine for many years And having never experienced this particular hiccup I was rather surprised. You are correct about the technique being necessarily smooth. 

Jay,

There is nothing in the Operators Manual about avoiding rapidly advancing throttle to prevent causing the engine to “choke” or stumble. Quite the contrary, there’s a note to avoid reducing, then abruptly advancing, throttle to avoid stressing the gearbox. This is because the engine responds instantaneously to throttle advancement.  

A fully warmed, properly designed, properly tuned, normally aspirated Otto cycle engine, carburetors or injected, 3hp or 300hp- should not stumble upon rapid application of throttle. My push lawn mower, my 1967 Alfa Romeo with dual Webers, and my Rotax 912UL all respond smoothly and instantaneously to rapid throttle input, and your engine should too. 
There’s a problem, it’s there- find the problem and correct it.

I tend to agree with Jim's statement, but details matter. A simple normal old fashion carb has a fixed diameter throat into which is sucked atomized fuel from the bowl through a fixed-diameter main jet. A sudden opening of the throttle, after an initial pulse, causes the air flow velocity to drop, starving the engine of fuel until the motor rpm increases and flow picks up. Back in the days of carb'ed cars, an accelerator pump was added to squirt in extra fuel to cover the gap. They could squirt in too much or too little, but it fixed the problem, sort of. All Lyc/Con carbed engines use this simple arrangement, some with accelerator pumps and some without. Hence, they will stumble or even quit when the throttle is rammed forward.

This was a real problem for motorcycles of the 50/60/70/80's where a sudden rush or burst of power could toss you onto the ground. An improvement was had by eliminating the butterfly and attaching the throttle cable directly to a slide which also controlled the mixture via a needle poking down from the bottom and into the main jet orifice. A final solution was to again use the butterfly, but retain a slide that was opened based on pressure drop. These CV (constant velocity) carbs work very well.

The Bing is a CV carb ==> the engine should not stumble, even if the throttle is suddenly opened!

Since the appeared after a carb rebuild, I suspect something got put back together not quite right. Is the diaphragm rotated correctly and seated on the notch? Or perhaps, as I discovered after my first rebuild, somebody, while installing the needle bent it while getting the clip on? Or perhaps a slide is sticking for some reason. IMO, further investigation is needed.

A reference: http://dansmc.com/cv_carb.htm

Hi Jeff & Jim,

Never too old to learn - thanks for the update on throttle / choke action, especially the reference, very clear/well written.

My apologies to Jay for promoting/misleading obsolete knowledge.

Sean Griffin, that was a refreshingly nice apology.