Why do we need 2 needles (LSN and HSN)?

[Greyhound]

Why 2 needles (LSN and HSN)? I know how to use them and tune an engine to a point (to run, not to performance). I've got great tuning sometime, but not all the time. So I figure if I know how these 2 needles are engineered or why we need 2 of them (some have 3 needles), I may get the tuning great everytime.

Can someone please explain the physical of these needles on engineering point?

Thank you in advance!

 

[digger]

Hard for me to explain.
Here is a link that may help you. http://www.rchobbies.org/cars_eng-tuning.htm

 

[Diver6127]

In the simplest form of explaining, the LSN will help tune performance from idle to midrange. The HSN is for the higher rpms. In some engines, there is a "Mid Needle." This is actually a spray bar. In general, this is never touched by most. Some companies are actually doing away with this needle and only utilizing the LSN and HSN.

 

[Greyhound]

Thanks Digger and Diver. Forgive my english! I can't even ask the right question. What I'm looking for is like this "since there is a lot of difference of pressure in the fuel tank to push the fuel to carb, you really can't regulate the correct amount of fuel at difference rpm with one needle." so what makes the 2 needles regulate the fuel at difference rpm (or presure) or how they regulate the fuel? I believe the fuel goes thru both needles to reach the throttle. I hope my words areunderstandable. Thanks again!

 

[Diver6127]

Well, the LSN tip actually is inserted into the end of the HSN. Other than that, all I can say is that fuel flow is controlled by the air/fuel mixture (rich or lean). This setting will control power as well as fuel consumption.

Pressure inside the tank can also be controlled by the length of pressure line going from the pipe to the tank.

 

[Greyhound]

Well, the LSN tip actually is inserted into the end of the HSN.

Thanks Diver!
I just want to know why they need to do it that way (with the 2 needles). But don't worry about it, I'm just curious. Super thanks.

 

[Diver6127]

If you really want to get into it,the HSN is what adjusts the flow of fuel. CW=less, CCW=more. Air flow, well that comes from the carb opening.

As I said earlier, the LSN is insered partialy into the HSN. It doesn't clog it off completely, It just limits the flow during idle and lower RPMS. It also coincides with the throat opening. Look down the throat of the engine. As the opening increases, the LSN will disengage from the HSN. This is when the HSN will come into play. The tank pressure just gives a positive flow of fuel. The HSN just regulates flow.

The Mid needle is used to adjust spray pattern or atomization inside the carb.

 

[Plaidfish]

LSN controls fuel flow via a smaller opening and that is keeping fuel from flooding the mill at low RPMs...as the carb opens up the mid range needle allows you to adjust where the lower powerband kicks in to the high band by setting the gap closer to open or closed side of the carb barrel.....then the HSN allows adjustment on a progressively larger opening as more air is pulled into the carb......
my tuning isn't the best but I can get real good performance out of my mills....I look at temps in the beginning of the day and do short bursts around me for a 1/2 tank...as the tank leans out I temp it and see if it's running hot.....if it is (at this point before really getting on the pipe) I know my LSN is too lean.....then do long WOT runs and tune the top......this usually gets me closer to a good performance tune right off the bat BUT this is backwards from a lot of people, as it's easier to tune top down for most.....I learned to do it my way and it's easier for me....mostly because I'd set the HSN and when I'd see it overheating I'd fatten it up not realizing just the LSN was too lean ...then all you settings are crap and you need to start over.......

 

[rocknbil]

To put it another way, fuel is drawn into the engine by the vaccum created by the piston's movement. The fuel required for the correct mixture to keep the engine running at all RPM's is not in direct proportion to the vaccum at various RPM's, that is, at low RPM's, the vaccum is generally drawing an incorrect amount of fuel and would ordinarily flood or starve the engine.

The low speed needle generally does not insert into the high speed jet, but it gets very close. At low RPM's, this interferes with the fuel being drawn into the engine and in addition changes the way it atomizes (bursts into a mist.) The combination of these two reduce the amount of fuel coming into the engine at low RPM's.

Engines without a low speed needle, such as some of the HPI NitroStars, are often different by design and generally don't have as much "wiggle room" to make errors in high speed needle adjustment. With a low and mid speed needle, you can run the engine extremely lean in racing situations but offset that wilth low speed adjustments to keep the engine running at idle and low-mid speeds.

Edit: Plaid where'd you come from? You beat me to it.

 

[Plaidfish]

I was sitting in your mind (you my friend need to seek help) watching you type on your monitor...then I made you sleep for 2 mins while I posted mine...

 

[Greyhound]

Thank you all for clearing that up and and mostly for being so patient with me.

One more thing, does this means tuning the LSN and HSN are totally independent then?

 

[rocknbil]

The LSN (generally) is independent from the HSN but not the other way around. When you adjust the high speed mixture, it adjusts the fuel proportion across the entire power band. I see it as the main fuel metering valve. When you adjust the low speed mix, it only affects mid to high RPM ranges. Generally once you get the low speed set and get the best performance across the entire power band, when you tune you should only have to adjust the HSN. The LSN will only need adjustments as the glow plug begins to age.

There is an argument out there that as you set the HSN you have thrown the LSN out of adjustment and it also needs to be tweaked when you adjust high speed mixture, but I've never seen this.

 

[truthseeker]

ok, i'll give it a try here, too. the high speed needle is metering fuel for when the carb is open wide and there is a lot of air present. lots of air = lots of fuel. at idle, however, the carb is almost closed and there is NOT a lot of air. less air = less fuel. if you have a lot of air and just a little fuel, you will run lean. if you have a lot of fuel and a little air, you will run rich. on a single needle carb, all you can do is try to split the difference between good performance at both ends. with dual needles, you can tune to get good performance everywhere. everything Plaidfish and rocknbil says still applies, i'm just stating it a little different. sometimes it helps to hear it explained a few different ways.

 

[Greyhound]

Thanks all! I ran my HB stadium pro this morning and I tuned it seperately (get the HSN done first and then LSN later) and it feels a lot easier to get it perform. I was better for me to look at each needle seperately. I couldn't run the Savage 4.6 becuase the sliding throttle were sticky (I don't know what cuase it). I just used after run oil to loosen it and it should be ok. I was thinking about ezilla, but now nitro only sir. Now I can enjoy it. Thank you for all the help here. This is a great community.

 

[truthseeker]

glad you got it working! now go have some fun!

 

[Bouzouki]

You need the LSN and HSN; however, IMO the mid range is useless.

Some RB engines don't even have it anymore.

 

[Jeep]

Sorry if someone already said this, but,
The LSN is necessary to regulate the fuel mixture at idle and just above idle, and there is such an extreme volume difference in fuel demand at idle as compared to midrange rpm and max throttle, that the HSN cannot possibly be used to regulate the mixture at idle.

Here is an analogy:

Suppose you have a water valve that can flow 10,000 gallons of water per second(HSN). It would be impossible to use that valve to adjust the water flow in terms of drops per minute, so if you may need 10,000 gallons of flow per second at sometimes and then almost instantly go to drops per minute at other times, you'd need a smaller more precise valve(LSN).

That make sense at all?


Jeep

 

[retarded pilot]

Sorry if someone already said this, but,
The LSN is necessary to regulate the fuel mixture at idle and just above idle, and there is such an extreme volume difference in fuel demand at idle as compared to midrange rpm and max throttle, that the HSN cannot possibly be used to regulate the mixture at idle.

Here is an analogy:

Suppose you have a water valve that can flow 10,000 gallons of water per second(HSN). It would be impossible to use that valve to adjust the water flow in terms of drops per minute, so if you may need 10,000 gallons of flow per second at sometimes and then almost instantly go to drops per minute at other times, you'd need a smaller more precise valve(LSN).

That make sense at all?



Jeep

NO

























do you have an explanation that uses beer as the example? i think a lot of us here would understand that better