I have a RO-Jett .65 recently repaired by Dub Jett. something I noticed since I got it back and re-broken in is that it's got a really sensitive needle. It has been a bit hard to set because very small movements in the needle seem to make very bit changes in the RPM. Once set it stays there and doesn't move around, but getting it set can be challenging.

Anybody know what causes an engine to have a very sensitive needle?

Oh, fuel is 10-22 Powermaster and the plug is your garden variety Thunderbolt. Prop is a 12.5 x 4 two blade with slight undercamber.

I have a RO-Jett .65 recently repaired by Dub Jett. something I noticed since I got it back and re-broken in is that it's got a really sensitive needle. It has been a bit hard to set because very small movements in the needle seem to make very bit changes in the RPM. Once set it stays there and doesn't move around, but getting it set can be challenging.

Anybody know what causes an engine to have a very sensitive needle?

Oh, fuel is 10-22 Powermaster and the plug is your garden variety Thunderbolt. Prop is a 12.5 x 4 two blade with slight undercamber.

   Is it on pipe pressure?  When I first got the new airplane going, I was running pressure like I had the year before. But it was way too touchy to needle reliably.  I ran open uniflow vent, and then no problem, the rest is history. I started trying to figure out what the deal was after the fact. The *only* change I had made was to use a different needle, since the old one was bent a little too short. I compared all the needles I have (mix of PA and Jett, since they are interchangable) and the tip end of them was quite variable. The one I had used before was the pointiest, and the one that was too touchy on pressure was the bluntest one. I just filed/sanded the blunt one down, and it then worked dandy on pressure.

    Brett

I think Brett is correct about the tip taper.  Blunt needle tips are nototriously sensitive.  Another important factor is the thread coarseness.  It would seem like this would not vary from one NVA to another, but you never know.  We had a very difficult time trying to set R/C carb settings on Magnum engines a few years ago.  Someone finally realized the threads were much coarser than other carbs on the market. 

  In this case I think that the PA and Jett needles are certainly the same thread pitch. But the needle tips from both were all over the place. It was only a few thousandths, probably, but the fattest was only about .015. A few thousandths would make a huge difference.

     It also depends on how much fuel you are using, and the fuel suction. More suction has the same effect as pressure, it makes you close the needle more and run the tip closer to the seat.

      Brett

Nope, not on pressure. I will check the needle thickness. I figured the next step was to just change out NVA, but I'll look to the needle "pointiness" first.

Thanks

With the needle issue solved and both pipe pressure and open uniflow working successfully, do you have a preference for one or the other with the RO-Jett?

    I think it runs just fine in any wind you can stand up in on open vents. We were out flying a couple of months ago, in a pretty good wind, and all I could say afterwards is "I wish I had had that engine run at the 2003 NATs".

      Brett