The fuel is 11-11-5.
and
The fuel is 20% synthetic with no nitro
Out of idle curiosity, which is it?
In any case, that's incidental, because this is very likely to be the problem:
This venturi has a throat diameter of .250" and a spigot that projects only until it is flush with the inside of the wall of the throat.
A typical range of choke areas for a wide array of stunt engines from 20 to 75 is 0.016 square inches to maybe 0.024 square inches. A 1/4" through hole is
0.049 square inches, more than
twice as big as is normally used on stunt planes
This is supposed to correct.
According to who? It's VASTLY too big. It might get through a flight and be able to be 4-stroking at about 12,000 rpm, which would require something like a 9-4, theres no way it's going to draw fuel (particularly through a very restrictive fuel passage) from a flush inlet at low revs. I would not expect that to work, and I consider it a minor miracle that you even got it started.
That may well have been what came from the factory (maybe, although I don't recall there ever being a CL version I could be wrong), however, I will categorically state that it will not result in a satisfactory 4-2 break performance, the venturi is FAR, FAR TOO BIG.
Continuing on:
I do have another nva with a .281” hole in the Venturi and a ST spray bar to try
That has a choke area of about 0.020 square inches, or only 40% of your current setup, and because of the bluff body in the stream, disproportionately better fuel draw. That will probably work (assuming nothing else is wrong), that's in the midde of the conventional range. although larger than I would have started with.
A Venturi with a .312 throat appears bit excessive for stunt
Good that you recognized that - also too big (presuming a ST spraybar), unless you do something like Aldrich and stack in 1/16" of head gaskets. Aldrich engines were fairly well-known for either sagging or quitting outright on the takeoff roll from fuel starvation. But it's still VASTLY SMALLER than your current setup. A .312 venturi with an ST spraybar is the equivalent choke area (~0.029 square inches) of a .195 straight venturi, which is well beyond the capability of an ST46 in normal circumstances. You are trying to run a .250.
You are all over the place here. We typically adjust engines with increments of .005", which is about .002 square inches of choke area differences. You are jumping around by a factor of 2 1/2.
A .281 with an ST spraybar is about 0.020 square inches, which, should run OK at these revs on 10% nitro.
A .250 straight hole
will not work, period, for stunt.
A .312 with an ST spraybar is far too big for these revs and reliable flights, but will probably let you get it into a 4-stroke.
If you want to run FAI fuel, I would suggest slightly smaller, just to keep the needle from being so touchy. But I think this is a big mistake, there is a very good reason no one uses it unless they are forced to.
Brett