I took a closer look at the NGH 9cc carb. That small set screw below the main needle is quite interesting. But I am not so sure it is really adjustable. It seems a little loose to me when it is adjusted in more, but maybe it will stay put like that, if someone does use it.

Basically the normal setting is to screw the set screw out until it just stops, do not force it. It does not screw out farther than where it stops, at approximately 3 turns out. That is the default setting for it. As you screw it out or in, the main needle outer tube will move in or out accordningly. Screwing the set screw in causes the needle to go in deeper towards the middle of the bore and screwing it out will move the needle towards the outside more. So theoretically one could adjust the low and midrange with that adjustment. But on my carb it felt a little too loose for my comfort, so I think it might move when the engine is running, but I could be mistaken.

The carb does use a rotating barrel arrangement for the throttle. The low speed needle is adjusted through the center of the barrel like many glow engines use. The needle taper controls how much fuel flows as the barrel is rotated in a spiral in and out fashion.

Now the carb does have a adjustment needle for metering the fuel into the needle and seat in the throttle barrel. So we have, sort of, two needles then. Glow carbs usually combine the main needle with the needle and seat in the carb. In this design they are separate but work together in conjunction with each other to meter the fuel into the engine.

The carb throttle barrel open shows the needle in the center.


The outer needle shown on the inside with the barrel spring too.


default screw setting where the set screw is screwed out until it stops.


with the set screw turned in until it is flush or level with the carb body.

Originally Posted by Brutus1967

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

Shorts and T-shirt weather here but blowing an absolute gale, so no flying but having fun in the workshop with a nice cold beer near to hand
Nice Heli by the way, not my thing but nice just the same.
Cleaning up my second NGH GT-9 and should have it on the test stand tomorrow.
Glenn

Originally Posted by brace

Nice Heli by the way, not my thing but nice just the same.

The heli is old (and as heli's go, not the prettiest) and the engine is even older....

But the liquid cooling is pretty unique, I think....
In the fuel heli section there's a thread going on about .50 size glow engines in helicopters being converted to gassers: http://www.rcgroups.com/forums/showthread.php?t=1476259
They tend to have heat issues along with carburation issues, but the latter seems not to be the biggest problem: most of them get away with using the original carb, if needed with heat insulation.
I am not occupying myself with glow-to-gas conversion, but have a heat-challenged fourstroke (glow fuel, spark ignition) in a helicopter that I am intending to convert to liquid cooling. From that point of view, I experimented with liquid cooling and got it working. Picked up a thing or two about gasser conversion along the way.

Brgds, Bert

Originally Posted by Brutus1967

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. Pls. remember fuel supply for all thr., link by common limiter(HSN +LSN) inline.This makes more complex. 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.Sure but if there is only one dominant venturi effect.Controlling only by air flow,not any suction.But what if there is more dominant ,reversal working suction effect?(meaning suction of cranckcase under barrel). Then all regulator principle here gets crazy.Because when engine needs less and less fuel(at lower thr.) the fuel in line begins to be sucked by engine because spray bar mouth still open to crankcase.Then regulator needle will answer this by stay open, just supposing "venturi effect" is on work.Then engine gets drown on idle.

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.My offer is "find a higher position for main spraybar over the barrel.

Brgds, Bert

Hi Bert, pls. read red lines.

Originally Posted by gkamysz

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.
Sorry but I miss that link before.I just saw and I was meaning that asking for a scecth..Now there you should notice a very effective and proper designed metering rod which proper for limiting properly the suction effect from crankcase at idling.Then when at WOT opens to allow enough fuel to flow.Remember what I was saying? I have to change LSN profile to keep proper adjustment on most Chinese glow carbs.Here on GT 9 same problem.LSN cant limit the suction enough to work properly.If one adjusts LSN for good idle then LSN doesnt allow for enough fuel at WOT.

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

You are right but still cant understand what I mean.If lower pressure (creating by fuel flow) other side of diaphragm is not accordingly for engine needs then regulator doesnt regulate properly.LSN and HSN both adjust on GT9 cant metering fuel flow properly.So my "suction" cant be prevented by LSN.(seems LSN travel isnt big enough to block suction from crankcase).

I have read the red lines, and they just don't make sense....
I am an engineer, and as such I would say (no offense intended) you are not fully understanding the design-philosophy of the rotating drum valve design.

If the spraybar is centered in the rotating drum valve, the valve not only closes above the spray bar, but also below, closing off the spray bar from the suction of the crankcase.

That is the whole idea behind the rotary drum valve with its dual variable orifice: the area between the upper and lower throttle area is supposed to be subjected to only a change in air-velocity, not a change in pressure (that is in reality not completely true because you get a dual pressure drop, which necessitates the airbleed screw on older designs and the LSN on more modern designs, but the idea is there)
Therefore, at closed throttle, what you describe simply does not happen, as the LSN is closing off the spraybar opening, at least partly, and the lower throttle opening is closing off the spraybar from the crankcase suction.

You can test this principle quite easy with a normal glow engine: just fuel the plane, but leave the fuel hose to carb empty. Apply the starter with open throttle, and repeat the same with throttle in idle position, and with fully closed throttle. You will not notice a big difference in fuel draw between full open and idle, and with the throttle fully closed, there is NO fuel draw, because the lower throttle opening closes off the spraybar completely from the crankcase pressure.

No offence, but what you are saying is simply NOT happening, otherwise glow engines could not work either: they have the same working principle but no fuel pressure regulator to start with....

If there is a problem with the profile of the LSN as you describe in post #1010, that is a design flaw that has nothing to do with fuel pressure regulator, and also nothing with suction pressure in the carb throttle, but just with.... the design of the LSN profile....

Brgds, Bert

I have to admit Turk1 I too am some what confused. the fuel flow is adjusted by the combination of crankcase pressure and the slot in the crankshaft. On a standard gasser there is usually a reed valve preventing back pressure from escaping the crank on this it is the same as a glow with a slotted crankshaft. As you know change in crankcase pressure is created by the piston moving and this pulse is what pumps the fuel at the pump/regulator and draws in the fuel through the carb. As the piston moves faster the pulses increase in speed and the suction through the carb increases. This works in reverse as well. I can only guess that you are missing the point of having a slot in the crankshaft as it is not only there to provide fuel to the crankcase but also to minimise positive pressure in the crankcase blowing back through the carb thus acting in the same way as a reed valve. Where I see the main problem is that the fuel/air mix is too close to the top of the carb and so can be disturbed making the engine run lean at times, we compensate that by running it extra rich which of course makes it idle poorly and spit fuel. this is why I am still exploring the velocity stack idea.

It does not work that way AT ALL....

The pressure pulses in the crank operate a pump. That is true. but the pressure of the fuel, as delivered by that pump, is completely independant on engine speed or anything else, it is (should be, if the pump is functioning correct) ONLY determined by the atmospheric pressure. That is what the small hole in the pump body is for: to give atmospheric pressure access to the pressure regulating membrane. The exact pressure is not even that relevant, but the fuel pressure should have (within a range) a fixed overpressure compared to atmospheric pressure.

That sets fuel pressure before the HSN orifice.

Now the engine starts drawing in air, and this air movement creates a local low pressure at the spray bar opening.
The vacuum at the spray bar opening is determined by 3 factors: atmospheric pressure, air velocity and throttle opening.
Variations in atmospheric pressure is cancelled out by the fuel pump delivering at a set pressure above atmospheric.
Variations in throttle opening should be compensated by the co-operation of HSN and LSN needles and the LSN needle profile, so there remains only the air velocity to draw in the correct amount of fuel.

That is in theory.

In reality, things are always a bit different of course, but in order to solve possible carburation problems, making sure your pressure regulator works OKand is not influenced by for example airspeed or propwash, is the biggest factor, as the carb is just similar to any glow carb, the only difference being that gas is much more sensitive to small changes in mixture strenght, compared to Methanol, and this is complicated by the fact that gas requires much smaller volume flow of fuel in relation to air, compared to methanol.

The fuel pressure regulator is nothing more, than a device that makes it possible to achieve the desired accuracy and consistency, as the carb would for example function without too much problems on methanol without the regulator.

So, if you have any mixture problems, and your carb is not leaking/faulty, your problem is 99% sure located in the functioning of the pressure regulator.

Brgds, Bert

I guess you used to use nitro blended fuel on your glows so it compansate some problems I met repeatedly.On many glow engines(mostly Chinese built) in years, I always met adjustment problems(especially .90 and bigger size) if low speed adj. doing well then engine stops during take off or even at nose up tests independent of how much HSN open.If WOT adjust put well then idling bogs engine down eventually to stop.This is only because LSN profile isnt appropriate with LSN travel to restrict enough and to allow enough fuel needed by engine.When I remake LSN profile to block enough at idling but to pull out fully at WOT from spray bar,then engine gets running like a champion.So unfortunately I cant agree with you,barrel keeps spray bar from engine vacuum.There is big pressure difference at spray bar mouth (maybe a little venturi effect at WOT but certainly a big vacuum at idle position).In GT 9 if LSN profile/travel designed properly then that vacuum would be ignored.But now simply we cant.
Proof is simple.Please adjust your GT9 well for idling then you will have a great low speed adjustment but impossible to have WOT. Maybe engine seems run good but at the very first second you take plane nose up,engine will immediately stop.
Contrarily,if you adjust it to have good run at WOT then you will see its impossible to have a proper idle mixture(because LSN cant block enough now and regulator also allows more fuel).This is because barrel cant prevent vacuum as you think.
But to change LSN profile more aggressive then may give you a good working system.

Proof you are wrong, is even simpeler.
Remember the old carbs with the air blee screw?

They have a main needle, that is adjusted at full throttle, and there is NO LSN that is closing the spray bar.... the full spray bar is subject to the "vacuum" you claim there to be.... still, a very small bore of mostly only 1 to 1.5 mm, partly closed off by the adjustment screw, is sufficient to relese that "vacuum", and the fuel flow through that wide open spray bar is just sufficient for that engine at idle....

I am very sorry, but the vacuum at closed throttle that you speak of, does not exist!!!!! If that was true, an air bleed carburettor could never have worked properly. Of course there is a lower pressure than at WOT, but that is logic, the air has to pass two restrictions, so there are two pressure drops. But the crankcase vacuum is NOT present at the spray bar, not by far.

What ever might be wrong with the chinese engines you have worked on, it is NOT that vacuum. Needle profiles, wrong fuel choice (some engines will not allow good adjustment with for example some synthetic oils due to their viscosity) I don't know, but it is NOT the vacuum.

Personally, I couldn't care less, I find my way around any engine I have met so far, but if you are making wrong presumptions, you will come up with the wrong solutions, and that would be a shame....

Brgds, Bert

....
Here we mostly use no nitro,full castor homebrew fuels.
If you have spare time,pls check this loonngg thread.The problem was only wrong LSN profile.They tried many ways except it.
http://www.rcuniverse.com/forum/m_22...tm.htm#2308584

Hi all, may i know that this GT9 using spark plug: 1/4 x 32 Rcexl. is it same as this
CM-6??
http://www.hobbyking.com/hobbyking/s...idproduct=8867

Small gasoline engine look interesting. Wonder anyone try it on Raptor 50? and Will it direct 'drop-in' into the Raptor 50??

Originally Posted by Kamenrider

Hi all, may i know that this GT9 using spark plug: 1/4 x 32 Rcexl. is it same as this
CM-6??
http://www.hobbyking.com/hobbyking/s...idproduct=8867

Small gasoline engine look interesting. Wonder anyone try it on Raptor 50? and Will it direct 'drop-in' into the Raptor 50??

No the CM 6 is a larger plug

Originally Posted by Kamenrider

Hi all, may i know that this GT9 using spark plug: 1/4 x 32 Rcexl. is it same as this
CM-6??
http://www.hobbyking.com/hobbyking/s...idproduct=8867

Small gasoline engine look interesting. Wonder anyone try it on Raptor 50? and Will it direct 'drop-in' into the Raptor 50??

As GadgetEric stated the CM6 spark plug is larger than the tiny 1/4x32 size spark plug. RimFire sells them as do RCexl and some others too.

Yes, I beleive it would be a drop in replacement. But you have to keep the drive washer as it has the timing magnet on it. That might cause a problem with the flywheel, fan unit spacing though. The crankshaft has two flats machined on it to match the drive washer, so you could have a problem trying to get the flywheel farther back on the engine, plus the timing magnet won't work right if the flywheel is steel either. But you might be able to adjust the timing more to compensate. You would then need to shift the timing sensor to pickup the magnet properly then too. But some heli's may allow you to move the engine back farther to fit the flywheel on the engine with the drive washer attached. You would also need to come up with a larger head with more cooling fins on it as well. But that could be a clamp on head cooling fin extension too.

I'd expect a 9cc gas engine made to run 10-2kRPM to fall far short of the power made by glow engines for 50 size helicopters.

Greg