While I am on a roll, has anyone had any luck running the latest Fox NVA pictured below?  I has tried and tried to get a goos needle, only to find it too sensitive.

Brew   

             Harold, the flattened needle was  the earlier needle made available from the 70's to the late 80's. The last run to use that style needle valve was the 40th anniversary Fox . It was said that this flat spade actually prevented the needle from turning. However, the tapered point version you depict is the replacement and works just as well. The threadsize is 3-48 which  are formed on both and not cut which can be a bit of a downfall as air leaks are prone here which is why tubing is needed on the needle valves themselves to seal any air leaks. If 3-48 screws could be sourced long enough this would be a easy fix to grind a taper on the end and have threads that actually fit.

                    Depending on where the indexing of the threads begin in the spraybar can ultimately have a great impact on the newer style tapered version over the flat. Sometimes, the needle is barely in the spraybar and I mean barely. You will find the newer one requiring to be backed way out further than the flat style.  The only solution to that is to try another spraybar as there was many changes in terms of some were plated, some where black oxide, and all of them combined while do fit the engine may have slight differences as to where the threads start and stop. The last version was the one used on the Super Stunt .35. This is the knob like needle valve you also show. I do know that one problem that is a large one is that the assembly can't be swapped to the opposing side. This can be problematici  if the plane demands it. I found that one thing to do with that needle is oil the o-ring and then screw it on. The internal o-ring can be very grippy not allowing for fine adjustments. But the taper on that needle is not real fine therefore one isn't going to get a very fine adjustment as one would with a tapered point.

settings.  The only drawback that I see is that the spraybar is larger in diameter as cpmpared to the Fox.
By my calculations, the reduction in area is about 30%, which will probably affect power.  I'll fly it this week and find out.

  That sounds about right, if you do nothing else, that's about how much the peak power will be reduced. But generally, you are not running the Fox near the peak to begin with. What the effect will likely be is to make you have to run it leaner in level flight. If you can keep it in a 4-stroke, the break will be reduced and delayed and weaker. That may be a good thing, if it doesn't start burping in corners due to the even-more-reduced flow rate. In some cases it even starts burping in an inverted mount. As long as it's not on a profile, the bypass stuffer modification is probably optional, but reduce the power enough and it *will* start running funny even with a conventional mount.

    This change would be a good combination with a higher-compression head like that described, before it got derailed, in the other thread. This will increase the overall power, let you run it richer in level flight, and increase the power boost when it does break. The break will be more abrupt and the return to a 4 will be faster. There's no real danger in going "over the top" on compression, don't worry about that.

    I would also suggest at least 10% nitro, maybe even 15, to recover the power without any real ill effects on anything else. Above 15 and you are running a serious danger of breaking the crankshaft, in fact, on 25 I wouldn't expect it to last more than 100 flights. You can put all the nitro in the world in it and it will still run well, but it wasn't intended to take that sort of pounding.

   If you want to recover the stock performance, then you will have to do some combination of filing the sides of the spraybar down (be CAREFUL because you can easily go through or make it so weak it will break), and  enlarging the venturi. You can also retrofit an OS20/25 spraybar, as long as you sleeve the ends where it goes through the case to take up the excess space - it's the same problem with the same solution I see all the time with "improved" 25LA with ST spraybars, and in that case, it really does impact the power by, not coincidentally, about 30%.

    Depending on your application, doing nothing and seeing what happens is OK, too. If it's on a profile, for sure do the bypass stuffer mod, but the power will be OK for most airplanes you are likely to put it on as long as you can keep it running through the maneuvers. It almost certainly *does* put 5" of pitch off the table, you will probably need the standard 10-6 - because the power curve has been cut off slightly and you need to make it up with a more efficient prop.

     There's really nothing that you can do to a Fox that someone hasn't already done, so there's not a lot of mystery associated with it.

     Brett