check for  fuel foaming caused by engine vibration.

Martin

This could be many things, but one thing  I see all the time, is people close the needle till it bottoms, then keep turning it until it breaks out the step orifice hole in the spraay bar, this destroys the spaybars ability to needle correctly
Check it is OK
Tells  us  what NVA  this is

Randy

what muffler  system are you using?
Is the tank touching the exhaust or engine?
Anything loose in the tank?
Muffler pressure?
Is this a true venturie setup or a  restrictor type setup?
Is the spray bar clean inside?
needle perfectly straight?
how far is the tank from the engine?
do you have a fuel fliter? what type?
profile of built up?
how is the motor mounted?

While you're looking at things, make sure the venturi to case seal is good.  I don't know if it'll cause this problem, but if you have a dried-out O-ring it'll certainly make other things weird.

Funny that the same thing happened to me this morning with my Fox 25 plain bearing and Fox tapered NVA...on a Flightstreak.  It would only run in a mid level 2 stroke and turning the needle out 3 turns didn't change it one bit.   I ended up pumping fuel though the NVA with pressure and then reversed to suction to clear it.  When started after that it was so rich it barely ran. This is set up with a standard open vent tank with overflow open but muff (stock Fox muff) pressure going to the fill.  I haven't tried capping the overflow yet.

This time of year, the  "castor slags"  are out in force, cold weather helps to fleck castor, and it gets thick gooey booger looking things that  stop up and restrict fuel systems, especially spray bars,  just another thing to be on the lookout for, they are very difficult  to see, but a coffee  filter makes them easy to see after you filter them out

Randy

Since the OP is in Australia, where it's currently Summer...it's not the cloudy castor problem, tho that's something for most members here to keep in mind. I prefer to keep the fuel warm enough to prevent the castor oil separating out of the fuel. By filtering, you obviously get less castor in the fuel.

"Battered Spraybar Syndrome" (BSS) is another possibility, but somehow, I doubt that is the problem, based on my own experience with Randy Aero NV Assy. and knowing that our OP is not a rookie to 2-cycle model engines.

I have had some weirdness from a simple clogged filter or spraybar. Not really sure which, because the problem cropped up just before an official flight. I changed the filter and all the fuel hoses, and went to the circle. Started the engine, opened the NV about two turns (battery still on), then set the NV with the tacho, as usual. This worked out very nicely, and I got my official in without problems.

A real good way to blast out spraybars is to use one of the new WD-40 aerosol cans with the folding straw sprayer, with a short section of fuel hose to couple it to the spraybar. This appears to do a very nice job...I use medium neoprene hose, FYI. DuBro still sells it, very much like the old Veco hose. In some ways, I still prefer it to the typical silicon fuel hose. Steve

I use a dubro fuel filter clunk in my gallon jug for the pump pickup. It seems to do a good job but gets eaten in a year or two after being submerged full time. When filtering gallons at a time I use pantyhose...started doing that when Red Max sent several cases of the flaky castor fuel that would clog up no matter what the temp.  There was no way to dissolve the flakes. That was a long time ago.

Ok, changed a few things,motor out of model onto a test stand,Evo tube muffler now on it, Enya 3 plug,tried the smallest venturi
supplied with the motor and have fitted a needle valve assembly from an OS R/C carby, so it now has a spigot into the middle of the
venturi throat,Power Point 11 x 4 prop, Coolpower 18% oil, 5% Nitro.It now follows the needle adjustment as you would expect.
I am going to trying a Magnum 36 XLS in the model , just to check its not something with the tank or fuel foaming.
Thanks for all the input.
Happy New year
Paul
In OZ