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Old Nov 09, 2012, 10:33 AM
Complete RC Idiot Savant
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Originally Posted by turk1 View Post
Are you sure?
Yes....
I have never, ever had a carb, also not the chinese copies, that was not adjustable or leaking sad I could close the needle, or render it unadjustable. Some carbs are better adjustable than others, but all were at least useable.
In the early years of the hobby I had some problems, but they were caused by my own inexperience of those days.

Brgds, Bert
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Old Nov 09, 2012, 10:35 AM
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It works just like any other carb. As I've said before, the carb doesn't care what sort of fuel is flowing through it. The air/fuel ratio is only a minor change. The needle tapers make a carb for glow or gas. This one is just poorly executed. One needs to understand how pressure and pressure drop influences flow in order to design a good carb. Differences in fuels around the world is not an excuse for a product intended for the world market from the beginning.

Turk, I'll accept everything you say as fact if you have pressure measurements to support your statements.

Greg
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Old Nov 09, 2012, 12:45 PM
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Originally Posted by gkamysz View Post

Turk, I'll accept everything you say as fact if you have pressure measurements to support your statements.

Greg
Sorry but no way for me.Also I remove the GT9 from bench.But during bench and other one on my friend s plane,I certainly sure problem is reaction of regulator needle against glow type carb s feedback.
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Old Nov 09, 2012, 01:41 PM
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Can you explain why Walbro can make "glow type" rotary barrel carbs using the same pump and metering system?

Greg
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Old Nov 10, 2012, 12:54 AM
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I didnt see any example so I cant say anything.Can you put a scetch drawing about it?The fuel jet and throttle valve placing is the only key factor.
Pls. think again how regulator needle(common principle for all) works...
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Old Nov 10, 2012, 03:33 AM
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Originally Posted by turk1 View Post
I didnt see any example so I cant say anything.Can you put a scetch drawing about it?The fuel jet and throttle valve placing is the only key factor.
Pls. think again how regulator needle(common principle for all) works...
Yep.... that's easy: the regulator keeps a constant pressure relative to what it regards as the atmospheric pressure, before the orifice of the needles. If it doesn't manage to keep that pressure constant relative to the pressure in the atmospheric chamber of the regulator, it is strained beyond its capacity and that would be very unusual.

It is purely the pressure difference over the needle (regulator pressure minus venturi pressure (yeah, Bernouilli effect and all that...)) that determines the flow of fuel.
If the throttle setting changes, the venturi pressure changes, but the regulator will not notice anything about that. It will only see a change in fuel mass flow, and supply new fuel with exactly that same pressure. As the air mass flow and the venturi pressure do have a relation, but not a linear one, you need a highspeed needle (full flow limiting orifice) and a low speed needle (fuel metering orifice, mostly a tapered needle, sometimes a real variable orifice like on OS or Perry carbs) to keep the fuel mass flow and the air mass flow in the proper ratio to each other.

If the propwash somehow gets into the atmospheric chamber of the fuel pressure regulator, the fuel pressure before the needle will start to change.
For example, you are running the engine on idle (low propwash) and all is adjusted OK. Now you give it full throttle, and the propwash increases pressure in the atmosperic chamber of the regulator to above atmospheric level.
That means, the fuel pressure also rises, with increasing RPM, and the mixture richens up: Power and thus RPM drop because of that, so propwash decreases => fuel pressure decreases => mixture leans out => power and RPM increase again, etc etc, and you have an engine that will not respond to the needles the way you want it to: at full RPM you need a lean setting to compensate for the higher fuel pressure, but that setting will not allow the engine to reach full RPM as long as the propwash, and thus the full fuel pressure, is not yet there. So effectively, the engine dies when you open the throttle. If you open the main needle more, the engine will pick up on the throttle and as soon as RPM picks up, immediately start to run rich.

Find a different location for your pressure regulator, OR connect a hose to the atmospheric opening on the regulator and route that hose to somewhere inside the fuselage or another spot where airspeed or propwash do not have influence, and your problem should be solved.

Brgds, Bert
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Old Nov 10, 2012, 03:58 AM
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I have been wanting to try out a Perry carb but I ordered the wrong one and so waiting for the correct one to arrive, should be interesting to see how effective it is. Have heard that the Super Tiger Gt 40 carb fits and runs well on the motor.
My thoughts on this is similar to what earlwb has pointed out and that is the fuel air bubble that forms at the mouth of the carb is too exposed and is disturbed by prop wash making it hard to keep the motor running at low speed. To keep it running we have it tuned quite rich so that at idle fuel is spitting out the top. When the motor is WOT and tuned correctly for that it runs just fine and pulls like a demon but then the idle suffers as mentioned above. I have tried a velocity stack (very crude version) as has earlwb. I would like to try again with something a bit more refined and then play with the tuning some more.
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Old Nov 10, 2012, 04:36 AM
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Keep in mind that the "optimal" length of the velocity stack has a direct relation to the RPM you are running.
Just a tube, or more a trumpet shape does also make a big difference in some cases, where the trumpet seems to perform better over the whole RPM range and prevents backspitting more effectively for the same lenght (in other words, it can be shorter)

Brgds, Bert
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Old Nov 10, 2012, 11:11 AM
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Originally Posted by turk1 View Post
I didnt see any example so I cant say anything.Can you put a scetch drawing about it?The fuel jet and throttle valve placing is the only key factor.
Pls. think again how regulator needle(common principle for all) works...
You've ignored my previous posts about Walbro rotary barrel carbs and my comments about the Cline regulator (made with Walbro regulator diaphragm and needle) in conjuction with "glow carbs" regardless of the type of fuel.

But, I wasn't the only one suggesting you look at the identical Walbro design.

You say the walbro pump works on vacuum. You misunderstand that the amount of suction is not what controls the fuel rate. There is a fuel metering circuit after the regulator diaphragm and needle. Without the metering needles or jets, the carb would supply a very fuel rich mixture because the fuel regulator circuit can supply far more fuel than is required. The regulator diaphragm has one job, supply as much fuel as the engine needs at a fixed pressure. That is all. In real life this pressure varies slightly depending on fuel flow.

To get a functional carb one must look at the entire system. This is why randomly choosing a carb for an engine doesn't work. The carb is very dependent on the engine characteristics for it's calibration.

Greg
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Old Nov 10, 2012, 11:35 AM
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I think you all forgot that they more or less copied the Saito FG14 or FG20 carburetor to make the carb for this engine. They did break out the pump/regulator part from the main carb body, making it a two piece carb setup. Anyway I would think that if there were problems with the carb, Saito would have found it and corrected it to some extent. Now they did leave off a few obscure parts, such as the little check valves. But one has to wonder if those check valves were really needed or not.

So we would really need to study the Saito carb closely and see what differences there are as compared with the NGH carb.

I think it still boils down to issues with manufacturing for the pump/regulator unit. Subtle things such as the tooling marks causing air leaks, etc. The carburetor or throttle body seems to work fine for me, albeit it could have probably been done better, but I honestly can't say what one could do to make it better though. I am no authority on the intricate and subtle details that make a carb work good. But sometimes I can see something and I am able to correct it.
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Old Nov 10, 2012, 12:27 PM
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I certainly have not forgotten. They began by copying the Saito carb. By failing to understand how it worked they salvaged the casting and turned it into a standard rotary barrel two needle (or three needle if you count the adjustable idle needle seat) design. Manufacturing issues aside, I think there is a problem, not specifically with the design, but mating of the carb, engine, and pump/regulator. The carb and regulator have components which must be "tuned" to make them work together.

Greg
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Old Nov 10, 2012, 04:15 PM
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Must be cold in the northern hemisphere. A lot of interesting theory but few are putting it to the test by buying a motor and actually giving it a go.
Gentleman, the motor runs, not perfect but it is certainly more than suitable to fly a sport, trainer or fun flyer and is a lot of fun to fly at the field for hours with little cost.
I agree that it could be better and I will, as time permits, put my limited efforts to this with trials of various sorts. There are people on this forum with far greater knowledge and experience who I am sure could make this a better motor but I suspect will only do so from the safety and warmth of the computer desk.
As for the velocity stack? working around in my head how to make a more trumpet shape that will fit on the motor.
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Old Nov 10, 2012, 04:37 PM
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Must be cold in the northern hemisphere. A lot of interesting theory but few are putting it to the test by buying a motor and actually giving it a go......

.......There are people on this forum with far greater knowledge and experience who I am sure could make this a better motor but I suspect will only do so from the safety and warmth of the computer desk.
Yes.... it IS cold in the northern hemisphere.... and yes, my laptop is currently located in my nice, warm and heated livingroom .... but I am not only sitting behind my laptop in my comfy chair: It's not gas (yet) but this is what I have been occupying myself with in the last week and a half....
From the dim burning bulb in the back of my brain, to a functional prototype in 5 days, to a full functional and useable system in 5 more, made from scratch....

And I have been testflying it this afternoon in an 8 deg C, 3 Bft drizzle.... so, yeah, its cold up here....

Brgds, Bert
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Old Nov 10, 2012, 07:52 PM
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Quote:
Originally Posted by brace View Post
Must be cold in the northern hemisphere. A lot of interesting theory but few are putting it to the test by buying a motor and actually giving it a go.
Gentleman, the motor runs, not perfect but it is certainly more than suitable to fly a sport, trainer or fun flyer and is a lot of fun to fly at the field for hours with little cost.
I agree that it could be better and I will, as time permits, put my limited efforts to this with trials of various sorts. There are people on this forum with far greater knowledge and experience who I am sure could make this a better motor but I suspect will only do so from the safety and warmth of the computer desk.
As for the velocity stack? working around in my head how to make a more trumpet shape that will fit on the motor.
Yes I can concur, I did put my money where my mouth is, I am currently fielding three of the little gas engines in three different airplanes. They are working OK for me. Granted they could have done things better maybe. Or maybe I just had the bad luck of the draw, so to speak, on the engines I got. But I did get them to work and run OK for me. Plus I have gotten quite a few flights out of the engines and planes so far. Now I do have two more engines as well that I am planning to fly in a twin engine plane. It has been a lot of fun along the way, playing with these engines too. If could have been much better if they were plug and play, but I have had much more worse engines to tinker with in the past though.
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Old Nov 10, 2012, 07:57 PM
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Hey Brutus1967, that is a pretty neat heli setup there with that really cool radiator setup.
I had conversations with one of the GenesisHobbies.xxx people a while back and they were going to mount a few of the little 9cc gas engines in their helicopters and see what happens. I haven't conversed with them lately though. If I remember I'll fire off a email to him again and see how it is going. I think that with the fuel tank being mounted close to the engine and pretty much level with it, that they could dispense with the pump/regulator in their case too.
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