Peter, Charles, et. al.
For my last few engines I've been referencing the HMEM discussion thread that began here:
Determining a carburetor throath diameter | Home Model Engine Machinist Forum
It's a really good read on the subject and included practical throat data for several successful running model engines. I didn't participate in the discussion because up until then I'd been using RC carbs in my engines and had nothing to contribute. But after studying the results contributed by others my take away was an equation for Venturi diameter that I used on my last three engines:
D = K * sqrt(c*n)
where D is Venturi diameter in mm
K is a constant .65 to .90
c is cylinder displacement in cc
n is top rpm/2000 (for a 4-cycle engine)
The results from this equation come out significantly smaller than what was in the RC carbs that I had been using in my early engines which also seemed to run well. However, the equation seemed to work for others' air bleeds and so with some trepidation I eventually started making my own air bleeds in place of RC carbs.
This equation is what I used to come up with the .180" diameter for the 289's Venturi. I should also add that in my earlier engines I gave no consideration to optimum Venturi inlet and outlet angles, as everything was seat-of-the-pants including the throat length which was essentially zero on all of them.
The included angle of the outlet taper in my last revision was 16 degrees which is within the recommended range that I came across in my reading. It can be easily reduced to bring it within Charles' recommendation, but I'm wondering if these Venturi optimizations really apply to a rotating throttle valve in a carb. The theory seems to have been derived for optimum results at wide open throttle, but I wonder how much still applies at part throttle when the inlet and outlet geometries have changed so radically.
I'm happy to try something new and am very interested to see if these optimizations will work better than my previous ad-hoc approach. I guess I really should go back and replace one of my working carbs with one with these optimizations, but I can't seem to find the energy. - Terry