Guys,
I've put ST NVA's in many older engines and was wondering if because of the very long thin tapper could the tip cause setting problems? What I'm thinking about is in something like a Fox 35, that vibrates a bit could the thin tip of the needle start vibrating (like a tuning fork) and cause some erratic run characteristics? Has anyone done modes to clip the end off to shorten and seen any improvements?

Best,         DennisT

Any unsupported needle end in an engine will have a natural resonant frequency.  It is highly unlikely that its resonance would coincide with engine RPM.  Even so, its "Q" (or resonance bandwidth) would be so tight that only a particular RPM, within + or- a few cycles would cause problems.

F.C>

I have used Super Tigre NVAs for many years with no problem.  I have noticed that there may be a polished section down toward the tip of the needle. Maybe the result of vibrating against the spraybar.

What I have seen a couple of times on Cox reedie engines, was the tip of the needle breaking off and causing all sorts of adjustment problems. 

Combat flyers used to have problems with the ST NVA if the lock nut was not tighted down well.  18,000 rpm would do strange things to the setting.  Stunt troops don't have to work, methinks.

  Youthinks wrong. 9000 is half of 18000. People with ST46s and ST60s found the same thing happening, and Big Jim (among others) discovered that screwing down the collet as tight as possible fixed it most of the time. The OS needle is a lot better because it is supported all the way down until the taper starts, instead of right at the very end of the collet. The PA and RO-Jett arrangements are better because the point of contact is a damping material, and, they generally fit better.

   BTW, the PA and RO-Jett needles are interchangable. I used a PA needle in a RO-Jett spraybar in the engine I used at the 2006 and 2007 NATs. The spraybars are NOT interchangable, a RO-Jett Spraybar is bigger around than a PA.

    Brett

Supertigre has or had a shorter version that came with G.15s.  When used with a bladder with a pressure of about 10 psi, they would abruptly shut off the fuel flow to the engine.  I didn't have a problem with the bigger ones.  I am skeptical about the resonance idea.  I, like you, am too lazy to do any actual calculation to support my skepticism, but I remember the Vibra Tach, a piece of wire with a calibrated slider that read RPM when slid to the wire length that resonated with the engine.  As I recall, the wire diameter was about the same as a needle valve, and the resonant length for 11,000 RPM was 3 or 4 inches. 

.  Seems to uphold my long held belief that standard single cylinder, front valve engines all have a major vibration mode at around 8000 rpm.  Another one occurs at about double that.  The old G-21 would just start to run well, hit 16-17000 rpm, and then break the shaft after a couple of runs.  

     I would be surprised if a variety of engines had the same flex properties - from a wimpy Fox 35 to a 40VF the difference in the stiffness is radical, just looking at the cross-sections of the parts, and the relative weight. Could be, but it would surprise me.

    I forgot the other trick with the ST46 (and presumably the 60) - use the Harry Higley "Long Strong Needle". It had two excellent qualities for this particular problem. It has a sort of curved taper than ended with a significant flat spot on the end instead of coming to a point, which presumably made it less likely to go crazy from vibration. The other far more interesting feature was that the diameter of the threaded part was much larger than the stock needle, and was a very tight fit in the spraybar. This kept it supported for the entire threaded length instead of just right at the end where the collet pinched it.

    Brett

p.s. of course you cut the needle as short as you can and still be able to turn it.