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Old 06-28-2005, 03:55 AM   #1
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Post Mid range needle?

Hey everyone, got a HPI Nitro Star R12xs last year and accidently missed with the mid range needle. All the instructions tell me is that it's factory set and to not touch. Well I touched it and need some help getting it back to where is should be. Recommended to be 1-1.5 turns out, I'm about 1.25 turns out.

Running the engine in a GT and notice that the midrange is kinda fat? Hence my question about the midrange needle?

Any and all help would be appreciated. Basically how to set, what it does to the power band, and how it will change the high and low end settings.

Thanks
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Old 06-28-2005, 08:02 AM   #2
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There is no such thing as a midrange needle! What is commonly referred to as a midrange needle is just the seat for the low speed adjustment. You are adjusting the low-end wether you move the needle or the seat. The only way to change the rellative mixture of idle v.s. midrange is, to physically alter the needle (change length and/or taper). If your midrange is too fat after making any adjustment, then you idle is also too fat! Re-adjust from either side, it doesn't matter which (unless you move the needle or seat beyond its limmit to seal-up properly).
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Old 06-28-2005, 11:39 AM   #3
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ER??? '
What you talkin about? Most new racing engines have 4 needles. idle low mid and high. Check this link on what it does. I normally keep mine where it is. or lean it about a half a turn.

http://www.rctech.net/forum/showthre...&page=57&pp=30
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Old 06-28-2005, 11:52 AM   #4
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Carb. 101.....mid range needle

The screw inside the slide body is really the low-end needle, and it plugs into the main jet (the 'brass needle' in the opposite side, usually flush to the surface of the carb body) without obtruding completely to allow a little of fuel to pass when the needle plugs into it to allow the engine to idle.

The dimensions of the jet (brass needle), its position relative to the venturi, and the dimensions, ramps and shape of the low-end needle dictates the fuel curve of the carburetor (or how much fuel enters by each cfm of air drawn into the engine at a relative carb opening).

At idle speed, the engine is controlled by the low-end needle and how much air pass into the engine via the position of the barrel. But when you start to move the barrel to allow to pass more air, in fact, you're moving too the low-end needle, allowing at the same time to pass more fuel too. The low end needle adjustment is critical, because the engine relies on this adjustment for its temperature control when idling in the infield of the race track. This is the point where the engine does 90% of its work. If the top speed of the engine is good and the engine runs hot, you should generally richen this valve. You should see smoke when you open the throttle at low speed.

Depending at which opening of the barrel the jet is totally uncovered, and the carburetor starts being mandated by the adjustment of the high-end needle, and how much fuel this last adjustment permits to pass, rather than being governed by the low-end needle, can be varied (and the power band of the engine too, but slightly) by screwing or unscrewing the jet (brass needle) and readjusting the low-end needle. But be careful screwing the jet too further into the venturi can lead to mix and temperature problems (in fact, you're leaning the mid rpms, where the engine operates at partial opening of the carb, this can lead to problems and erratic operation) this is why on almost all the engine booklets warns you about to not touching this adjustment.

The position of the jet, relative to the venturi also changes the position on where and how much the vortex of air that enters into the carb varies how finely is the fuel sprayed into the air that enters (fuel is converted into a mist for being burned, a drop, no matter how little is doesn't burn and can create many and serious problems). This last can lead up to a conrod breakage (common) or piston breakage (not common but seen some) due to hydro lock.

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Old 06-29-2005, 05:56 AM   #5
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That was one of the best written explanation I have seen.
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Old 06-29-2005, 06:17 AM   #6
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afm,
Once again an excellent, well articulated, complete answer to a not so easy question. I have read a couple of your post in the Mtx-3 forum and all I can say is great work. Alot of us have the same knowledge in our brains but would not be able to express them into words, you do a great job! Just some kind words for you.
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Old 06-29-2005, 07:18 AM   #7
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Thanks guys. Actually most explanations are answers to same questions I made myself over the years, and found them surfing the web, reading books and then filed them, track tested, prove them, and then retyped with some practical changes.

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Old 06-29-2005, 07:41 AM   #8
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Quote:
Originally Posted by afm
Thanks guys. Actually most explanations are answers to same questions I made myself over the years, and found them surfing the web, reading books and then filed them, track tested, prove them, and then retyped with some practical changes.

AFM
AFM,
you seem to be the most knowledgeable on this forum. you must have been racing for a long time. do you race in world events?
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Old 06-29-2005, 07:43 AM   #9
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Thats exactly what I do and I wondered if you do the same. I believe on your website you have some of this info as well?
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Old 06-29-2005, 08:22 AM   #10
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Quote:
Originally Posted by afm
But be careful screwing the jet too further into the venturi can lead to mix and temperature problems (in fact, you're leaning the mid rpms, where the engine operates at partial opening of the carb, this can lead to problems and erratic operation) this is why on almost all the engine booklets warns you about to not touching this adjustment.AFM
Also the booklet tells you that infact it is not for adjusting the MID range, but that it is used for fine tuning the BOTTOM of the engine......


btw, most engines don't have this setting anymore.....
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Old 06-29-2005, 08:31 AM   #11
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AFM
A very well written explanation of carb function. I do disagree on a couple of small points. I don't think that the engine does 90% of it's work in the infield! In off-road, yes. On-road is another story. Especially as it relates to the low-speed adjustment. I have found that tuning for on-road requires that the balance between low-end mixture and high-speed mixture, must be very close. That is, operating temp is mostly governed by the high-speed adjustment, and low-speed needle must be adjusted for smooth transition from low to high. Where as off-road allows for a richer low-end adjustment to keep operating temps down. The additional RPM experienced in on-road racing, has a profound affect on my tuning approach. Driving style has a lot to do with it... If you pin the throttle comming out of corners, your low-speed adjustment has little affect of anything (unless it is way-off one way or the other) as the low-end needle exits the seat at approx 1/2 throttle. If you use a lot of partial throttle movement in the infield, then low-end setting would have a larger affect on operating temps.
Now, on to mid/spray-bar/seat postioning in the venturi. First, I don't think you can move it far enough (and remain within practical limits of the o-rings to seal, and other physical limitations) to provide any change in fuel atomization. The opening of the spray-bar would have to be very near the wall of the venturi, to have an adverse affect on atomization. Relative pressure in the venturi does not change drastically with lateral movent. It does change drastically with vertical movement.
Any attempt to change the point at which the low-speed needle exits the spray-bar, relative to throttle position, is conteracted by, the need to re-adjust the low-end needle. In other words; if you move the spray-bar to the left .5mm, you will have to move the low-end needle .5mm to the left (to keep the same mixture at idle). The needle will still exit the spray bar at the same throttle position as before.
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Old 06-29-2005, 08:48 AM   #12
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Yeah I've found some of AFM's replies scattered around the internet. But he sure does put them together nicely. Meanwhile, I'm always saying to myself, "Where the hell did I see that...?" whenever I wanna go back and find it.
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Old 06-29-2005, 04:25 PM   #13
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Default Great replies

hey guys

Thanks for all the great replies, I think I know more than I ever wanted to know about the fuel mixture in the carb. But unfortunately, none of what you have said has really helped me with my problem.

As the instructions for the motor said, 1- 1.5 turns to work with. So I have a half turn of adjustment to mess around with. How is it going to affect the trucks speed and power. i.e. Too rich? too lean? Too far in or out? I am an experienced racer but no one locally really understands how to use the mid-range needle to tune the engine, or how to get it back to stock?

So far, from the posts, I get that the mid range is just for fine tuning the bottom. But is can also be made to have no affect if you reset the low end after it. Just want to know if it will make a big difference or not much of one and how to get it right.

I don't want another long explaination as to the minute details.

THanks
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Last edited by Giuseppe; 06-30-2005 at 05:50 AM.
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Old 06-29-2005, 05:44 PM   #14
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Quote:
Originally Posted by joemugen
AFM,
you seem to be the most knowledgeable on this forum. you must have been racing for a long time. do you race in world events?
Well, actually my experience in RC racing is 5 years, but my background in 2 strokes comes from racing Karts for 10 years, and after retirement tuning engines and managing several racing teams.
I did go to several Southamerican Karting Cups and two world championships. The last time in 1990 as Team Manger of Team Peru in Jesolo Italy, working with official Tony Kart Team and famous Roland Marechal engine tuner.

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Old 06-29-2005, 06:00 PM   #15
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Quote:
Originally Posted by duneland
AFM
A very well written explanation of carb function. I do disagree on a couple of small points.
1.- I don't think that the engine does 90% of it's work in the infield!...........Driving style has a lot to do with it......

2.-Now, on to mid/spray-bar/seat postioning in the venturi. First, I don't think you can move it far enough (and remain within practical limits of the o-rings to seal, and other physical limitations) to provide any change in fuel atomization.....................

3.-Any attempt to change the point at which the low-speed needle exits the spray-bar, relative to throttle position, is conteracted by, the need to re-adjust the low-end needle. In other words; if you move the spray-bar to the left .5mm, you will have to move the low-end needle .5mm to the left (to keep the same mixture at idle). The needle will still exit the spray bar at the same throttle position as before.
1.-- You said yourself. It depends were you race and how you race. In our reality which is Rubber Tire Only Class, with small to medium technical tight tracks, you are braking a lot and accelerating from very low speeds all the time. There is no pin braking and thwrow tyhe car turn fast as wiyh foams on large open tracks....so yes we are most of the time on low to mid speeds.

2.- Off course, but the expalnation was done on terms of extremes for better understanding of the concept. All carbs are desibned within usable limits.

3.- Not necesarily. Not always as a rule do you have to move the LSN the same amount you move the brass needle. That is why as M7N well said, You use the brass needle to fine tune your bottom end. In other words you tune your low end statically at the bench with the LSN at the slide end, and you use the brass needle to fine tune yopur bootoem end response on the track.

AFM
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