I have quite a few 1/2a, .35/.36 combat and stunt .35~.52 Fox/ OS/ ST/ K&B/ Enya engines where the venturi area and supplied NVA did not have the recommended open area
On Randy Smith's and Brett Buck's advice ---I DID NOT turn down the NVA diameter in a mill/lathe or drill press....each of their cautions was " too thin and liable to not tighten properly without stretching or breaking" so I followed the advise to file flats on both sides of the FAT ST NVA spray hole... to reduce / or perhaps...increase the flow volume
IMO this method works well ----BUT!!!
Ken Cook and I go back long way---- and he did NOT suggest hacking...he said...CAREFULLY ...RE RE-READ ---CAREFULLY sand and file the diameter down to increase the throat volume
I know if I had a good controllable method, I WOULD try his suggestion
I myself,, and I know Ken Cook uses, Permatex Anaerobic Gasket maker --- just about every where a model engine needs an air tight seal
carefully applied---with the Anaerobic Gasket maker, YOU DO NOT need 8~14 Inch pounds of torque to seat a NVA too be air tight, and not move, due to normal vibration
So there is a way ...or many ways.... to achieve the suggested --or recommended ----Venturi/ NVA volumes... proper venturi hole and several different NVA----spigot, sprinkler, remote, local through pipe, filter, panty hose, wood spacer, sleeved down adapter....there are a lot of variations that do work
And just to acknowledge a facts or two
When a 16 YO kid, I had no such internet resource ---and the hints in magazines of the time were deliberately false so I was lost and failed a lot....
Since 2012 when I came back to the hobby----I have followed the Fuel, Plug, Prop, NVA/Venturi...advice.... pretty much lock step with Brett Buck, Randy Smith ,Dirty Dan, Ken Cook, and Jeff Dawson for all my vast array of glow engines
Must be blessed....3 years of playing airplane now--- and no burps, run aways, flame outs, or too much tinkering with props and fuel mixes
I have bought NVAs and Venturies from Tower, Randy Smith, Lee Machine, Brodak, Texas Timers, and Doug Galbreth all after the good advice on This and other excellent forums
None of this was possible in 1971 no matter what magazine you read