Cyclic Hardover

My Feedback: (3)

The title is a tad misleading here. Remember a few months back when we were having a healthy discussion on the 2300. One guy mentioned that peak rpm can usually happen before the barrell is fully open which I was already aware of. His thought was to set the high end wherever the position of the barrel was at when you were at peak rpm.

I forgot about that and have not tried that little trick yet. I might try it out on the 2300 in the next week or so. Anybody tried that at all?

Harry Lagman

Keep us posted Cyclic.

One of the guys at our field has asked me to tune his 2300 to get rid of the mid range richness issue. I'll be trying the spraybar rotation technique first and may try the approach you mentioned if that fails.

I'll post my findings.

Cyclic Hardover

My Feedback: (3)

It all sounded logical to me. Some of my engines have reached peak and the barrel is only opened 3/4 the travel. It will be a reduced airflow and setting the high end from that. Beats me but why not. Right now I don't see the harm in it. If you reach peak rpm , then it seems obvious there is enough airflow with the barrel partially closed.

DarZeelon

Cyclic,


It is possible for some engines to reach peak RPM in static conditions, with the carburettor barrel 2/3-3/4 open.

However, if this position is adopted as the 'full throttle' position, the engine may gain less RPM, upon unloading of the prop load, in high speed flight.

Also, you may be in the range covered by the low-speed needle's fuel metering, making this adjustment problematic, to say the least.

In this range it is the low-speed needle, which must determine the amount of fuel the engines fuel flow and you would be superimposing this as the maximum, with the high-speed needle.

I would not do that....

Harry Lagman

Dar, I doubt that the throttle setting Cyclic is talking about (the setting that is less than full throttle where rpm is at or close to peak) is greatly influenced by the low speed needle.

As for the onloading factor, that is a valid observation and as you say could affect performance where maximum speed is desired. The application I am tuning this 2300 for is a Funtana 90. The owner only uses full throttle on his vertical uplines so there should be very little unloading in normal flight.

DarZeelon

Harry,


In all two-needle carburettor, the taper of the idle needle, and/or the shape of the fuel-jet metering slot, meter the amount of fuel the engine receives from 0% (idle) to 75-80% of full throttle.

Before Cyclic edited his opening post, the question was about this particular engine model, reaching maximum RPM, with the throttle less than fully open; 2/3-3/4, I believe. This is why I expressed myself as I did.


Since at these partial throttle settings, the fuel control is done by the low-speed circuit, even if the main needle would be opened several complete turns, it would change the amount of fuel that the engine receives, by very little.
The low-speed 'circuit' is the bottle-neck.

...But closing the main needle further would reduce fuel flow, since at a certain point it becomes the bottle-neck.

Dr1Driver

In my experience, when an engine reaches top speed at less than wide open throttle, the prop may be too large. The engine simply CAN'T turn any more RPMs no matter now much air/fuel it gets.

Dr.1

downunder

I think this is a consequence of most modern sport engines having too large a carb for the revs normally turned. The correct fix of course is to sleeve down the barrel like Super Tigre do when they use the same carb on different sized engines. I see nothing wrong with leaving the barrel at whatever opening provides no more revs but the main needle would most likely need to be retuned because there's then a higher velocity of air flow through the barrel which will give better fuel draw. The mass of air flowing through will still be the same because that's all the engine can pump. Set the barrel and tune the high and low speed mixtures and go for it.

jaka

Hi!
Agree with DR 1Driver; You have a too large prop!

Highspeed needle should always be set with the throttle drum fully open. As Dar said, the low speed (Idle needle ) will interact too much if you set the highspeed needle with the throttle drum not fully open...which is no good! Second: What happens when you open the throttle drum fully??...then the mixture gets upset (Engine runs lean) when too much air is entering the engine.

Regards!
Jan K
Sweden

Cyclic Hardover

My Feedback: (3)

This is something to think abut also. I guess this could also fall under properly dialing in the engine.

Harry Lagman

It may be a good idea for some of you guys to read the post that Cyclic is referring to, if you haven't done so already. The post is a response to the often reported extreme richness of the ST carburetor when it is in mid range throttle positions of say 1/3rd to 2/3rds open.

If my memory serves me correctly, a poster suggested that the engine typically does not need full throttle to reach near peak revs with props that it is usually fitted with, suggesting that the carb throat diameter is larger than it needs to be. This poster went on to suggest that if you note the position where peak revs are attained and set the throttle servo ATV to this point so that it is then "full throttle", when you retune the main needle - no doubt, as Downunder suggests, the needle will need to be leaned considerably due to the higher airspeed past the spraybar and greater turbulence affording better atomisation - this new leaner setting of the main needle will extend its influence into the previously-too-rich midrange, leaning the midrange. In other words, because the main does not now need to be tuned for the relatively low airspeed condition that exists when the barrel is fully open, it can be set leaner which will have a positive effect on the normally rich mid range.

Whether or not the idle jet has an influence at this "corrected full throttle position" is a moot point. The problem we are trying to solve is a too rich midrange condition that exists when the low speed is set correctly. The key point is that we are trying to get more main needle influence into this midrange.

At the end of the day, this whole procedure breaks all the rules of correct engine tuning - it is merely an attempt at a pragmatic solution to what is essentially a flawed carburetor implementation.

Cyclic Hardover

My Feedback: (3)

I think it was in Not24's 2300 thread for a while. Not quite sure where it was. So if we ran the ST with the recommended prop in the rpm range and able to get peak rpm and the barrel is not fully open, then we do not have an oversized prop.

Not24

My Feedback: (1)

What I think this tuning technique is trying to accomplish, is make the maximum allowable airflow of the carb match what the engine can actually use. I haven't tried this idea, but i feel it may help. The magnum 1.08 carb with the sleeve installed is a much smaller bore than the ST carb, and i get the same rpm with it that I got with the ST carb. The difference is that the main needle is more responsive, and the low needle is less sensitive. It would be nice to see someone make a sleeve for the ST carb to choke it down to the same diameter as an OS 7D and see what it does. When I ran the Magnum carb without the sleeve I had a less responsive high needle that needed to be opened much more, meaning it was less fuel efficient. I think the intake diameter of the carb is a major factor in engine performance and operation.