In engines where the spray bar is in the center of of the intake tube, typical practice is to have the NVA hole pointing into the engine along the same path the intake air follows.  Scribing, filing or cutting an orientation mark on the spray bar can help speed the installation and ease field checking.

In other words, the same angle as the angle of the venture's slope toward the engine.

  In still other words, just point the hole at the crankshaft.

    Brett

Therewas a Kiwi artical ,saying you needed it 45 degrees back, for hot restarts , racing .

I just used this engine for the first time today, since converting it.  It took five flights to get it adjusted correctly.  The fifth time was wonderful.  I finally have an engine to pull my second Shark 402.  I think I can have some real flying with this combo. I have to set it to almost a straight out full two cycle run.  Then after taking off, it slows down a bit.  As soon as I begin to maneuver, it runs full bore.   Does that sound normal?

    That sounds very normal. Your ground setting for the needle will sound different on the ground because you are holding the model back and putting some extra load on the engine. Once you release, it will change a bit, accelerate out, and by the time you get 3/4 to 1 lap around, you are starting to generate some centrifugal force on the fuel in the tank, and that increases the head pressure of the fuel being fed into the engine and if everything is nice, tight and sealed it will richen up a bit, that is exactly what should happen. The newer engines are pretty sensitive to needle settings, and that is why I use a tachometer to set the needle. I find it hard to hear 100 to 200 RPM difference and sometimes that is all you need to change. You don't really care what the RPM is, you just need to know the number because you are just using the tach setting for a reference. It's the same whether on muffler pressure or venting to atmosphere. Pressure can some times help settle a run down, and sometimes taking it off pressure does. Experiment with props also. Different props will load the engine differently without changing anything else. Get and record lap times. This is starting to get into the area of fine tuning your run, and some times subtle changes will make a big difference. At this point, I think you should tach you engine every run, time your engine run duration, and time every flight like it's a contest by simulating a signal and start, or at least time the engine run duration from the point where the engine starts. You are heading in the right direction.
  Type at you later,
   Dan McEntee

What plugs are you using in your FP 25's?

What plugs are you using in your FP 25's?

  Either whatever came with it from OS, or, old Thunderbolt 4-stroke plugs from the RO-Jett, with the taters scraped off. Any normal stunt plug I have tried will work OK, far as I know.

     Brett

I have some O.S. # 8's.   I can't see a clue on the one I'm using.  Not sure what it is. But, it's working!

Hi Dan,

I only had a real good run the last run.  So, when the weather cooperates again, and I can do three or four flights in a row.  I will be able to describe the run better.  It seemed to go to peek when I started to do loops and settled down a bit after the maneuver.  I'll keep on experimenting.

Allen

Please note that as any engine warms up, it goes leaner. Usually happens just before the pilot picks up the handle. I watch a lot of guys start their engine and walk to the circle center without adjusting the NV, while others (including me) carefully let the engine warm up and then set the NV by tachometer. I guess everybody has the right to do as they please, but you need to be consistent about it for best results.  Steve 

I always start out with short lines until I am confident that the ship will stay out there.  I have had some trouble getting my ships to stay taught on the end of the lines.

  In still other words, just point the hole at the crankshaft.

    Brett

        So what happened to the gospel of positioning the single hole facing rearward and 10 degrees down from perpendicular to the  venturi slope? Is pointing the hole at the crankshaft a side mount vs upright engine issue?

         Ara

One other comment.  This engine is running much better and more consistently with a venturi/NVA configuration than it was with carburetor wired open.  This is the the Enya NVA and venturi from Bob Brookens.  It was my first experience of drilling a case.  But it's way better than the other way.  I'm not sure this is the new set up that Ray Copeland is talking about.  I'm waiting to see how they are.

Brett,

Hope you don't mind if I piggy-back off this thread to ask an FP .25 question or two.

I jumped to buy an FP .25 at a good price and a day later remembered your BBTU rules say "DO NOT, repeat DO NOT, replace the spraybar and venturi with the "better ST needle" and/or a venturi from the "How I came in 4th at the 1956 East Podunk Stunt Exhibition and Chili Cook-off" venturi chart."   Well the FP .25 I bought has a Delrin venturi and ST NVA.      I haven't received it yet so I don't know the diameter of the venturi.

I found a source on ebay (shttrman) who notes there are no more OS NVAs for the OS FP .20 - .46m, and says he has a newly made direct replacement for the original OS FP .20 - .46 Needle valve assembly. He also says, "No more drilling."    [He also sells Enya NVAs for OS FP .20 -.46.]    He also has original OS venturis for the OS FPs .20 - .46 in both 6 mm and 7  mm. 

Question #1: Is the ST spray bar a larger diameter than the original OS spray bar?  And if the ST is larger diameter, is there a way to go back to a smaller diameter OS spray bar?
Question #2: Which OS venturi is the correct one, the 6 mm or the 7 mm?  I'm guessing the 6 mm, but I'ld like to buy the correct one the first time.

Thanks,
Joe Ed Pederson

    The correct *stock* OS venturi is 6.5mm/.257". The stock OS spraybar is 3.5mm/.137", and the Supertigre spraybar is 4.0 mm/.157". Changing to an ST spraybar and a stock venturi results in about a 25% loss of power. An equivalent, hopefully, venturi bore diameter for the Supertigre to give the same choke area (0.0184 square inches) is .274. The closest equivalent US drill size is a letter I drill. A metric drill that is kind of close, but on the wrong side (larger) is 7.0mm.

    If anything, on the 25FP, I would err on the side of making it smaller - a letter I drill is nominally .272, it will make a hole about .274 under most circumstances. An adjustable reamer appears to be required to get it smaller than that, but bigger than the next smaller size (letter H at .266 and 17/64 at .2656).

    For these tiny airplanes we are talking here, maybe a letter H drill, which will make a hole about .268 or so. It will reduce the power slightly (~9%) which may be helpful to keep them subsonic. Note that even 0.008" (8 thousandths, two thicknesses of paper) changes it by 10%.

     For reference, a typical "adjustment" for a full-size stunt engine is .005", and this is a very noticeable effect. I have seen 0.005" change improve the engine so much that it probably raised the pilot's score by 40-ish points. So these changes need to be *subtle*  - changing it from , effectively, .257 to ~.240 by stuffing a much larger spraybar in it is a HUGE change that has very drastic effects on the power and characteristics of an engine run.

     The way to restore the original configuration, assuming you have a proper 3.5mm spraybar, is to sleeve each end of the spraybar to make the diameter larger where it passes through the case and venturi. It looks like a "dogbone" when you are done, because you want the middle 6.5mm of it to stay .137, but about .155 or so for about 1/8" on each end. This is not terribly tricky as long as you are comfortable cutting small bits of tubing. You can cut a kerf along the axis of a piece of 7/32 OD tubing with a razor saw, compress it to close the kerf back up to fit tightly around the spraybar, then cut it into short lengths and J-B Weld them to the spraybar so it will line up with the case/venturi. It doesn't have to fit all that well, since this is just there to take up the space created by drilling the hole in the crankcase, but if necessary, file the kerf wider so that you can make it a spring fit on the spraybar. Keep the glue out of the obvious spots, or you will probably have to toss the entire thing and try again with a new one.

   I might post some dimensions and pictures (but not with glue, because my engines don't actually have the holes drilled out) later or tomorrow.

    Brett

        So what happened to the gospel of positioning the single hole facing rearward and 10 degrees down from perpendicular to the  venturi slope? Is pointing the hole at the crankshaft a side mount vs upright engine issue?

         Ara

  What "gospel" is that? Tilting the spraybar hole definitely does something, but to first approximation, I want to run the engine in a stock configuration, so we know where to start. It comes from the factory with the hole straight down the venturi bore, so unless there is some reason to change it, leave it alone. We know it works that way.  Once everything is working, then and only then, change it, and then you will have some idea what the effect is.

   As always, experimenting is fine, but you have to start with a baseline working system (which running it stock will provide), then and only then, make one change at a time, fly a bunch of flights to remove random effects, then evaluate it. If it's better, leave it, if it is worse or no different, put it back. It's really not magic, just a matter of careful and controlled experiments and objective evaluation of the effect.

    Brett