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Question about the so called "mid range" adjustment

Question about the so called "mid range" adjustment

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Old 12-30-2018, 05:53 PM
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Default Question about the so called "mid range" adjustment

Hi there guys,

I've been “lurking” here for a while and this is my first post.

I recently got my first racing platform along with a “3 needle carb” (not sure they are called this way though...), namely with main valve, low end needle and the so called “mid range” adjustment screw.

I have a question about these type of carbs I was hoping someone could explain out to me. I searched for threads here on RCTech and I do get that the general response is “just don't touch the mid range screw”. Now I see that, for a good tuning, you probably don't really need to mess up with this adjustment, but my doubts are more about the physical functioning rather than tuning. I hope this won't sound too abstract.

So, two are the variables for any given throttle opening (and of course a given HS valve's): A) reciprocal position between the needle and the spraybar nozzle (“how much the needle obstruct the nozzle”), and B) spraybar position in respect to carb throat walls.

On 3needle carbs, these two are adjustable indipendently. On 2needle carbs, it depends on where the screw is: if it's on the needle side, then only A is settable and B is not; if it's on the spraybar side, then you set A and B at the same time without a choice. (Probably most 2needle carbs have the screw on the needle side.)

So in other words, basically what 3needle carbs allow you to do, that 2needles don't, is to keep the same mixture changing only how much the spraybar nozzle is “sticking out” in respect of the throat walls -- which is to adjust A and B separately.

To do this, one would have to operate on both the needle and the "mid range" screw at the same time: opening one and closing the other one by the same turns (assuming of course that the screw threads are the same size...) to get the same “nozzle obstruction level” for a given throttle (=the same mixture) but a different nozzle final position with respect to the throat. This of course can be done only on a 3needle carb.

The question is though... why would one want to change the position of the spraybar in relation to the throat? Which kind of effect, exactly, the “depth” of the spraybar has on the mixture at all in relation to the air flow? Especially since we are not talking about a total travel of several millimeters but probably barely one.

I get that the fuel coming out of the nozzle of the spraybar gets hit and atomized by the air flow, but is a slightly different nozzle position alone capable of changing how well the fuel “atomizes”, let alone the amount of it that can physically pass through it? Remember we are assuming that the mixture is the same.

Is this screw really adjusting the level of fuel atomization and nothing else?

Thanks!
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Old 12-31-2018, 06:54 PM
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You've got a knack for visualizing and understanding these things, clearly. You pretty much got it figured out. I suspect most mid "needles" just serve as an easy-to-manufacture method of building a carburetor, as opposed to machining into a blind hole.

As far as functionality, it's minimal at best. Any changes made, are to be done on the LSN as well as to not change the relationship between LSN/seat engagement and throttle position. This way the only variable changing is the physical location of the LSN seat (aka, the spray bar) within the venturi.
IN THEORY, the different areas within the venturi will experience different pressure changes and this MIGHT affect how much fuel is pulled through the spray bar at a given throttle position.
basically, leave it alone. I consider "mid needles" to be a convenient way to plug a hole from the manufacturing process, nothing more.
Your mileage may vary of course... some people will swear they adjust the spray bar postion to some benefit... whatever.
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Old 12-31-2018, 10:20 PM
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Originally Posted by timjs View Post
You've got a knack for visualizing and understanding these things, clearly. You pretty much got it figured out. I suspect most mid "needles" just serve as an easy-to-manufacture method of building a carburetor, as opposed to machining into a blind hole.

As far as functionality, it's minimal at best. Any changes made, are to be done on the LSN as well as to not change the relationship between LSN/seat engagement and throttle position. This way the only variable changing is the physical location of the LSN seat (aka, the spray bar) within the venturi.
IN THEORY, the different areas within the venturi will experience different pressure changes and this MIGHT affect how much fuel is pulled through the spray bar at a given throttle position.
basically, leave it alone. I consider "mid needles" to be a convenient way to plug a hole from the manufacturing process, nothing more.
Your mileage may vary of course... some people will swear they adjust the spray bar postion to some benefit... whatever.
I wonder why Nova has gone to 3 needle carbs now.
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Old 01-01-2019, 03:50 AM
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It is indeed placing the spraybar in or outside the airflow depending the opening of the slide of the carburator. If you have a good feeling with the engine you can notice a change of if you play a bit with the position you ca find a spot with a slight stronger midrange RPM.

But to be honest, there is nothing wrong with a 2 needle setup, there is no performance loss.
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Old 01-02-2019, 01:55 PM
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Originally Posted by Cardiak View Post
I wonder why Nova has gone to 3 needle carbs now.
it gives them a method of adjustment not able to be had with typical 2 needle style which the "needle seat" can vary from the manufacturing process as the metal inserts are placed in an injection mold and the plastic is molded around it .....


as for what the needle seat adjustment effects ...... kind of mixed opinions but you have it correct ..... almost a timing setting of fuel atomization by changing the path in which the air flows .....


carburetors for our rc engines have been changed very little for a number of years..... OS carbs are at the top for tunability .... if you look at the design there is very lil adjustment in all actuality as the low speed needle or spraybar adjustment can change graduation very little throughout the adjust ...... the high speed needle also has very broad channel for fuel to pass through...... OS hsn has a "sweet spot" for sure ......

my opinions .... i might suck .... idk .....lol
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Old 01-02-2019, 04:16 PM
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A true "mid adjustment" would be a variable-taper LSN. Or several different needles with different tapers to swap out.

In fact, the taper of the low speed needle is probably one of the most important aspects of the carburetor design. Especially for fuels that require more precision such as 0% nitro fuels or especially gasoline.
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Old 01-02-2019, 05:20 PM
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Originally Posted by timjs View Post
A true "mid adjustment" would be a variable-taper LSN. Or several different needles with different tapers to swap out.

In fact, the taper of the low speed needle is probably one of the most important aspects of the carburetor design. Especially for fuels that require more precision such as 0% nitro fuels or especially gasoline.
see if you can find info on the old flat blade slide carb ....... it was an odd one
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Old 01-02-2019, 06:32 PM
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Originally Posted by houston View Post
see if you can find info on the old flat blade slide carb ....... it was an odd one
I can't find anything on it. I'd be super interested to learn about this carb!
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Old 01-02-2019, 11:45 PM
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Picco had one, it was called the torque carb
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Old 01-03-2019, 08:39 AM
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On the OS, I run the Mid needle farther in than a factory setting. Which in turn you have to fatten the Low speed and Lean the High Speed. Mid range needle is also the forbidden fruit to 90% of racers. But , once adjusted to your driving, you will never go back.
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Old 01-06-2019, 05:46 PM
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Moderator,
The OP has messaged me, says that he's been waiting for his posts to get approved in this thread for several days. I figured I'd post on his behalf, just a heads up.
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