Hello All:

          I purchased an original venturi and nva for my HP .40 engine.  The spigot threads into the Venturi only far enough to be flush with the inside wall.  There is no projection into the venturl.  Will this work?  I also have aftermarket nva that are adjustable.  I have been advised that these are designed to be used with the spigot placed in the center of the Venturi.  Which is correct?

          Comments/suggestions?  Due to a few problems, I have not had a chance to run this combination.

                                                                                            Tia,

                                                                                            Frank McCune
 

          Hi Lauri:

           Thanks for the reply.

            That was my thoughts.  I remember spigots in motorcycle carbs that were centered in the air flow.  These were at about a 45 degree angle to the airflow and the end of the tube was cut at the same angle making it parallel to the airflow.  I am not convinced that being flush with the side of the Venturi would be good, but I was assured that indeed it was correct.

                                                                                   Be well,

                                                                                   Frank McCune

         Hello All:

          I purchased an original venturi and nva for my HP .40 engine.  The spigot threads into the Venturi only far enough to be flush with the inside wall.  There is no projection into the venturl.  Will this work?  I also have aftermarket nva that are adjustable.  I have been advised that these are designed to be used with the spigot placed in the center of the Venturi.  Which is correct?

   It will work, both the PA and the Jett come that way stock. There is an argument to be made that this arrangement, which Randy refers to as a "true venturi", has better fuel suction and/or total air mass flow for a given fuel suction, given the same manifold vacuum, which it probably does in an ideal situations.

    I think that may be accurate, but is a moot point, because I can put a *piped 88* with a potential power capability of maybe 5 HP in a 600 square inch and only need 1/2 horse. I think a vastly more important factor is making it run the same way all the time, through maneuvering and the varying air flow direction across the inlet. The fuel post/spigot definitely improves that, and so does some sort of diffuser over the inlet. Also look at any automotive carburetor and none of them have a "flush" arrangement, the closest I know about being the SU/Stromberg/Mikuni¹ types that have a little projection above the wall of the venturi - and also a huge bluff body (cylinder for the plunger) that takes up most of the area at anything but full throttle, messing the purity of the "true" venturi.

   It only really needs to stick out about .020-.030 from the wall of the venturi to get it well out of the boundary layer. Further just causes the choke area to go down since the spigot takes up some of the space - which is a good way to fine-tune the choke area if you think you need to.

     Brett

 ¹Before I get "corrected", yes, I know that the SU/Stromberg type works fundamentally differently from a Mikuni type, the Mikuni slide forming the sole throttling effect, and the Stromberg responding to manifold vacuum. It doesn't make any difference to the effect I was talking about. RC carbs work more-or-less like a Mikuni, and they all have something like a fuel post or spraybar to put the fuel in away from the walls.