When you have a RC engine with good carb , why should it be changed to a venturi nva set up ? I have used both ways for control line back in 70’s and they ran the same . I don’t understand why it should be changed to nva set up for c/l .
I’m just getting back into c/l flying since 1980 and I have c/l and Rc engines to use . I’m not flying pattern yet but is it necessary to invest another $30-$40 to convert a Rc engine to c/l ? Please keep in mind that I’m on SS so I do have to save $ many times to get what I need .
Also does it make that much difference if the needle adjuster is in front at Venturi or in rear of engine ?
Thanks again for all the great help !

When you have a RC engine with good carb , why should it be changed to a venturi nva set up ? I have used both ways for control line back in 70’s and they ran the same . I don’t understand why it should be changed to nva set up for c/l .
I’m just getting back into c/l flying since 1980 and I have c/l and Rc engines to use . I’m not flying pattern yet but is it necessary to invest another $30-$40 to convert a Rc engine to c/l ? Please keep in mind that I’m on SS so I do have to save $ many times to get what I need .
Also does it make that much difference if the needle adjuster is in front at Venturi or in rear of engine ?
Thanks again for all the great help !

   Howdy Retro Jim;
      All good questions that have been discussed before, and lots more discussion in the various forums. I'm wide awake for some reason at 3:30AM after a long, tiring day that included cortisone shots in both knees for arthritis, so I might as well answer!!   
   
     1) R/C carb for C/L flying. Yes you can use them, especially if just sport flying. You have some pluses and minuses though. On the plus side, you can adjust where you lock down the throttle. If, say you are using an FP-40, I would suggest closing up the throttle just a bit to get better fuel draw for C/L flying especially if you want to do some stunt flying. The throttle barrel leaks air something awful, and it cuts down your fuel mileage. I forget what the typical opening size is on a R/C carb at full throttle but it is bigger that the typical C/L venturi which can range from .240" to .260". If you notice whne the engine is running, the barrel and control arm jump around a lot  because of clearances they need, and air leaks past those clearances. I have a profile model of Dusty the Crop Duster from the movie "Planes" and use an OS LA.40 on it. It is a double duty profile scale and profile stunt model. For scale I use the throttle controlled by a 2.4GHZ radio hanging on a clip on my hip. I have the throttled limits set to a max opening of about 3/4 throttle or so because during my flight presentation, I do a wing over and a loop, and at full open throttle I was getting a hesitation over the top. losing down the throttle a bit with the exponential adjustments cured that. I also have a four ounce tank on it. For my scale flight, it uses just about the whole four ounces on a flight much shorter that a typical stunt pattern. When  I fly it  in competition for profile stunt, I take the carb off along with a box that holds the electronics, and install a venturi and needle valve, and the same four ounces of fuel is more than enough for the pattern with good power. So yes, you can use just the carb.

   2) The rear needle valve is something that some people do use. You see it is speed, combat and some racing because it helps keep your fingers away from the whirling sharp prop! When my son was younger and still prop shy, we set up all his models with a rear needle valve using the components from the rear needle versions of the FP series engines when they were nearing the end pf production. The main thing with using a rear needle is that you need to choke it more when starting, specially cold, because the adjusted running mix that the engine needs is much smaller and has a long way to go to get to the engine. OS uses a smaller size fuel line for this also if you look at the newer engine. With the metal bracket and metal needle valve assembly, you have many ways to mount the needle valve to keep it out of harms way in an inverted landing and get it to where it is convenient to reach. Tank position can be critical when it comes to the end of the tank for the same reason as starting. If the pick up end of the tank is not at the correct position, the engine does a lot of stopping and restarting as it tries to run out of fuel. This is cured to by canting the back end of the tank outwards, so the centrifugal force always forces fuel towards the pickup. It can also be a sign you have too much line sweep and can move the lead outs forward a bit so te model does not yaw nose out too much. When it does that, the front of the tank is further away than the rear, and centrifugal force will push the fuel away from the pick up towards the front of the tank. When the engine sputters, the nose comes back in, fuel goes back to the pick up and engine picks back up again and repeats as long as it takes to run out of fuel. The currently available OS engines have a rear needle valve that I am not a particular fan off, but when things are right is does work. It's just not as fine adjustment as the older metal assemblies. There is a thread about this very subject in the engine set up section. This is one thing I always encourage newcomers to do, is explore the whole web site. just because you don't fly speed or carrier, you might be surprised what tid bits of information you can pick up.

    If you have both set ups that can be swapped around from engine to engine, you can see what I mean with the above, and it is educational to experiment with stuff like that. Just remember what you started with so you know you can get back to it.  Good luck and have fun!
     Type at you later,
      Dan McEntee

When you have a RC engine with good carb , why should it be changed to a venturi nva set up ? I have used both ways for control line back in 70’s and they ran the same . I don’t understand why it should be changed to nva set up for c/l .
I’m just getting back into c/l flying since 1980 and I have c/l and Rc engines to use . I’m not flying pattern yet but is it necessary to invest another $30-$40 to convert a Rc engine to c/l ? Please keep in mind that I’m on SS so I do have to save $ many times to get what I need.

    The RC carb will work OK, and solve some of the problems that others have had incessantly in other threads - specifically, when you get it in your head to start taking it apart, and changing parts, it's apparently easy to fall prey to various "local engine experts" and their "fencepost wisdom". Many/most threads here are about undoing the effects of this sort of expertise.

      The other problem is that, typically, people try putting these engines on airplanes far too small. An S1 Ringmaster, for example (that is, the regular one that everybody flew with Fox 35s) will be so fast with something like a 20/25FP or 25LA that it will be very difficult to fly. That's because even a 25LA  - which is the weakest of these type engines - is much more powerful than a Fox 35. An S1 Ringmaster goes very nicely with a 15FP. A 25 is 2/3 again larger and more powerful.

    With the carb, you can adjust the engine needle for optimum control setting, and then adjust the throttle to get the right speed. That's all that experts do with venturis, change them until you get the right amount of power at the desired "setting". Set up your carb with a spring pulling the throttle closed, and then use the idle speed adjust screw to adjust the speed. Unfortunately, this means that without doing something else, you can't adjust it with the engine running, but, you shouldn't need to adjust it much, or often, once you get a good run at a good needle position.

   The mixture needle is *not a throttle* and can only be used over very narrow ranges to adjust the speed - beyond that narrow range, you are generally much better of adjusting something else.

     The only downside to using the carb is that sometimes they just take small "shifts", like the throttle moved slightly. This changes the speed a little bit. This drove people crazy trying to use carbs on 4-strokes in competition, but works fine for sport flying or casual competition.

Also does it make that much difference if the needle adjuster is in front at Venturi or in rear of engine ?
Thanks again for all the great help !

   Functionally, it makes *no difference* in how the engine runs. The only downside to it on the 25LA is that the mount for it is molded into the backplate, where an inverted crash on a profile model will typically break it off, and ruin the backplate. I use mine exactly as it comes, but I also am not likely to crash. I can envision some fixes to avoid that, but I decline to list them here right now, because I can see everybody running off and doing it, and I haven't tested it myself.
 
    I actually prefer the separate remote needle mount from the 20/25FP RN or 46LA-RN, with those, the bracket just bends and you can just bend it back.

    Brett


   

If, and only if I could figure out how to mount the remains of the needle valve from an LA-25, and if and only if I were putting the thing onto a profile, then I would saw that needle valve mount off of the back plate and I would mount it on a bracket.

If you do go down that road pay close attention to how light the torque is on the fasteners on those plastic back plates. Those back plates work great as long as you do not over torque the screws. but the amount of torque that is right is way way less than what is right for an aluminum back plate. All of the people who complain about plastic back plates not working well have " fixed " them by torquing the screws down to the " correct " value.  If you tighten it down like it is made out of aluminum, you will warp it up.

   The newer LA.46's have the bolt on NVA and metal bracket. I don't hink I have seen a brand new later model LA.25 to know that they do. If you got some drill bits, some sheet metal and a file I'm sure you could cut the NVA off a LA.25 and come up with a mount pretty quickly. I mounted my Son Sean's rear needle horizontal behind the engine. The biggest surprise I found out was that the first time I tried this set up, I used a venturi for a normal NVA and it was a bit big. I couldn't hold enough fuel for the pattern, I think I was using a FP-40 at the time. I went in search of a stick to get me through the contest. The rear needle pray bar is smaller diameter because it ain't got no needle going through it! I think Tower still sells these and the correct size venturi.
   On the plastic back plates, and flat washer helps spread the load a bit on the back plate screws. Doesn't make them bullet proof but does help a bit.
   Type at you later,
   Dan McEntee

   Alternately, if you take the needle/spigot assembly out of the original RC carb, you have the same thing they used to sell as the OS Rear Needle conversion kit, and exactly what they used for the metering block on the OS 20FP-RN. Just need a bracket to mount it (not even necessarily on the engine). I used one many moons ago on my Fox 35, worked as well as anything else - before the bypass restrictor fix.

      Brett

       This is exactly what I have been mentioning. They were used on the 3A carburetor and some others. the eBay seller shttrman has some from an Enya .09 R/C engine that are smaller. I got some of them new in package wand replacement needle from Tower. Any old FP R/C engine that I picked up at swap meets had these in them. The bracket may still be available from Tower also  nbut can be easily fabricated. It has a 4mm thread, and I have drilled and tapped venturis for them to make spigot style venturis. Very good fit of the needle on the valve body and no O-rings to worry about. If you have a hard time getting fuel line to stay on the out put nipple, one wrap of copper wire, a touch of solder and a little buffing with some sand paper will still let the 4mm nut pass over it. When swap meets start back up again it's worth digging in those little junk boxes for stuff like this.
  Type at you later,
  Dan McEntee