This is a needle type that adjusts to be flush with the throat of the Venturi.
Suggestions/comments
     Tia,
     Frank

Frank, do you mean that the spray bar just screws in from one side and the end projects into the venturi?  Much like we see with some R/C carb spray bars?
My understanding is that the end of the spray bar needs to come to around the centre of the intake to creade a little restriction.  You mention that it's flush with the side of the intake, am I reading this correctly?  If it doesn't project into the venturi it won't create enough fuel draw.

  I am having a problem with a HP nva. that can not be made to adjusted to enable the engine(s) to be run only at full speed.  The needle may be turned out until it fall out of spray bar without changing speed.

     The Venturi and the nva. are stock HP parts designed for CL use.  This is a needle type that adjusts to be flush with the throat of the Venturi.  I got the idea of enlarging the in the body of the “spraybar” to permit more fuel to flow.  Am I on the correct path to solve this problem? 

    Probably not. You should never, ever, take drills, files, dremel tools, etc. to your engine unless you have it working stock first, and unless you have a specific problem with it that you want to solve.

     I would also add that most people should never take drills, files, dremel tools, etc., or screwdrivers or allen wrenches to their engine, either, for any reason, but I know that everyone feels compelled to do it.

  As before, either the venturi is too large, the fuel is too thick, or you have a blockage/clog somewhere. Any version of the HP40 is not much of a stunt motor, and this sort of venturi/spraybar setup is not what you want for stunt. But from the factory, they certainly could be needled with the stock arrangement and standard fuel.

      Therefore, there is something different about your system than it came from the factory. Remove everything, clean it out including poking wires or bottle brushes through every passage. Re-assemble the spraybar/needle off the engine, then use a fuel syringe to push fuel through it at various needle settings. It should come out easily, and adjusting the needle should control how much goes through. Note that you could put fuel through it backwards - outlet end to inlet - to make sure it is back-flushed.

   Then clean the filter, or replace it, and squirt fuel in all the tank openings and see that it comes back out the pickup with no restrictions.

   Then, re-assemble it with a *factory stock venturi* or smaller,  run standard fuel (Powermaster 10/18 "Air" or equivalent), and it will adjust properly.

      Brett

When I reread my post, I noticed that I had omitted the word hole when I described what I was contemplating to alter.  It appears that if the size of the hole were made larger, there would be more fuel flow making the engine to run richer. 

 

   That is right. However, the unmodified system flows sufficient fuel if there is not something clogging it up or you have altered something else. So you have a problem somewhere with the undrilled system.

      If you drill it out, you may or may not solve the problem, but you have not solved the original issue that was restricting the flow. So you now have two problems, a restricted flow that you never proper diagnosed, AND, you have altered perfectly good stock parts that did not need to be altered.

    Brett

   

Pictures would be good.  I still maintain that the spray bar should project part-way across the intake venturi to create a pressure differential.

Ok, not being familiar with the HP engines, I just searched some pictures and the spray bar is mounted as I envisaged. Pictures weren't detailed enough, but I would imagine there is a lock-nut which secures the spray bar in position.  Loosening should allow the bar to be turned in an extra turn or two.  If that's possible, I would certainly try that before making any permanent alterations.

     Hello Walter et al.:

     The Venturi has a diameter of 5 mm or about .195”.  There is no provision for running this engine on pressure.  I have run these engines with a conventual Venturi with an opening of .281” and a ST style needle valve. 

     Thanks for all of the replies.

      Frank

     Hello Walter et al.:

     The Venturi has a diameter of 5 mm or about .195”.  There is no provision for running this engine on pressure.  I have run these engines with a conventual Venturi with an opening of .281” and a ST style needle valve. 

      Why would you expect it to work for stunt with ***50% more choke area***? The .281 venturi with a ST spraybar has 0.020 square inches of choke area. A .195 straight venturi has ~0.030 square inches of choke area. You can't even run an ST46 on a venturi that large with reasonable props, you sure can't run a 40 on it.

   The . 281 is too large for stunt, it will surely NOT WORK WITH HALF AGAIN AS MUCH.. If you get it to needle, you are not going to have acceptable runs with that big a venturi. A straight venturi for that engine, for stunt, should be more like .150-155, not .193.

    Even with that, you should be able to needle it with the giant venturi, if you have the right fuel and nothing else is wrong. But, to be honest, it's not important, because you aren't going to have any success using it that way for stunt, the venturi is far too large.

    Brett

   

    Hello Bret:

     In the past, I have had a HP.40 set up by an “expert” and he made the throat in the Venturi .281”.  Another chap assured me FHA’s the proper Venturi size for this engine was indeed .281”.  However, I never was satisfied that the engine would only run wide open.  I should have caught on that the Venturi was indeed excessive in size!
What size of an opening would you suggest?

   .281 is at the extreme  limit for venturi size, as previously noted, and note that suggested something smaller about a year ago. *You do not want large venturis*, you want something that is tractable so you can get flights in and not have to screw around with all these problems.

     Magic is not real, and a .195 venturi is grossly excessive. I recommend .150 to start with on the straight venturi. As previously suggested, if you are going to run an ST spraybar, something around .260-265 will get you pretty close.

    For future reference, *calculate the choke area*, that is, the area of the smallest cross-section. For a starting point on an HP40, the choke area should be in the range of 0.015 to 0.017 square inches. If it is bigger or smaller than that, then, get a different one, until it is in that area.   That will ensure that you will have enough stability to actually use it in flight.

      It will not be terribly powerful that way, but it will at least work with a reasonable range of stunt props and suction feed. You aren't trying to compete, so you probably want to err on the side of easier operation, rather than trying to eke the last bit of power out of it.

   For reference, if you are using the ST spraybar, that means the venturi to start with is between about .260 (.0149 square inches) and 0.0268 (0.0169 square inches). If you are using a flush inlet, then it should be between .142 (.158 square inches) and .147 (.170 square inches), although you can probably get away with the .150.

     Note that a few thousands of an inch makes a huge difference, you can't be changing things from .150 to .195 to .250 and make any sense out of the results. If you had  a #26 drill (nominally .147) and just drilled a hole in aluminum with it, you will get about .149-.150 just because it won't be perfectly concentric.

     And, *there is still a problem with your spraybar*, almost certainly some sort of debris somewhere, that needs to be fixed as well.

     Brett

p.s. note also:   https://stunthanger.com/smf/engine-set-up-tips/engine-will-not-go-rich/msg584225/#msg584225

      Hello Brett:

     A big thank you for taking the time and effort to answer my questions! THANK YOU

     I will get several venturm made with smaller throats.  I also cleaned the nva and the entire fuel system.

     As a temporary remedy, how many layers of panty hose do you think would suffice to restrict the air flow to a point where I tame the engine enough for stunt use?

    Taking .030 square inches of choke area and turning it into .015 square inches equivalent will take a lot more than a few layers of panty hose.

    Just use the other engine with .281 venturi until you get the proper parts.  And, I still don't see where you got the original problem solved - that is, that you have cleaned and then tested the spraybar assembly for proper operation. That can/should be done on the test stand, since you don't want to fly it that way.

      Brett