Fat Italian NVAs

A common engine change for OS (and other) engines is to install Super Tigre NVAs. What is the advantage? Personally, I like the OS “click” adjustment, as it is not dependent getting a friction nut at a tightness that holds a setting and is not too difficult to turn.

I know, STs aren’t really Italian today, but still have a pasta shape instead of a fortune cookie shape.

The additional restriction of the larger diameter spraybar might be a plus or a minus, but can be adjusted with venturi size in any case. So what does everyone see that I’m missing?

Larry Fulwider

Edit: Previous wording implied negative hands-on experience with ST needles

Larry,
Both work very well in reality. The ST's have been more available for a longer time and most folks just got used to using them. They're also a little easier to duplicate (no spring to install).
Some folks think that the spring on the OS can resonate and give problems with consistent settings but I've never really been able to prove that with a lot of testing.
Also, theoretically at least, the ST style is more "anolog" since it doesn't require any clicks to hold the needle in place.
The only real difference with the size issue is that the "asian engines" require enlargement of the needle valve hole to allow the use of the larger diameter ST.
Personally I prefer the ST Style (I actually use PA needle valves from Randy Smith), because the needle is a little easier to deal with in my opinion than the spring with the knob. But admit that there is simple preference involved in that choice and no real value added.
For what it's worth I have had the needle come loose in the knurled housing on a couple of OS's but think that was a matter of individual poor quality rather than any endemic problem with the design.

Randy C

>Larry,
>Both work very well in reality. The ST's have been more
>available for a longer time and most folks just got used to
>using them. They're also a little easier to duplicate (no
>spring to install). . . .
> . . .
>For what it's worth I have had the needle come loose in the
>knurled housing on a couple of OS's but think that was a
>matter of individual poor quality rather than any endemic
>problem with the design.
>
>Randy C

Good point about the robustness of the ST. The OS has more places it can break, particularly in a profile crash. It is not uncommon to see see OS NVs with a shorter (or bent) spring. As pointed out below, STs are probably easier to accommodate around cowling.

Leonard's point is a good one -- a small change in spraybar size also requires a change in venturi size to keep everything same-same. I am not convinced that just doing the math to keep the net cross sectional area the same keeps it same-same in the turbulent environment down in that dark hole.

Larry Fulwider

The OS NV Assy does seem to have a failure mode where all looks good, but it won't needle right. I've only seen that on one, crash damaged. Maybe bent, or something else? ST NV's seem to either be fine or they're obviously broken, nothing in between. Like Randy C., I much prefer the Randy Aero/Nelson NV Assemblies. You can tighten the poo out of the collet nut, so it won't move by vibration, yet the needle is always easy to adjust. Steve

Steve Helmick
Renton, Washington
USA

>A common engine change for OS (and other) engines is to
>install Super Tigre NVAs. What is the advantage? Personally, I
>like the OS “click” adjustment better, as it is not dependent
>getting the friction nut at a tightness that holds a setting
>and is not too difficult to turn.
>
>I know, these aren’t really Italian today, but still have a
>pasta shape instead of a fortune cookie shape.
>
>The additional restriction of the larger diameter spraybar
>might be a plus or a minus, but can be adjusted with venturi
>size in any case. So what does everyone see that I’m missing?
>
>Larry Fulwider
>

On OS engines, I prefer sticking with the OS needles. I have about a hundred or so left in stock, so availability is not a problem. As you say, with the click you know exactly where you are with these. IOn fact, with our set-up we could adjust lap times by the number of clicks. Much easier than the "guess" on the ST type needle.

Also, as you mentioned, if you do go to the ST needle, then you need to open up the venturi a corresponding amount. It is not just a matter of changing needles. (People do this all the time and they are losing power in the process).

For the engines with a .155 cross hole, then, yes, stick with the ST (or PA style) needles. Use of the smaller OS style can lead to leaking situations in addition to making the venturi choke area too large.

But...to each his own. When the OS engines were popular with the top stunt guys, some would put ST needles in them. Later when the ST engines became the thing, some put OS needles in them. (There were some larger OS needles made). It is probably just what people got used to.

Aayy, wasamattayou? Theya worka justa finea. Who a you callin a fat, eh

I, too prefer the OS type, but often the length of the needle dictates using the ST type with its longer needle. Depends of course on the cowl, but that has been my take on them. No other reason [photo not recovered: sipcof.gif]