NGH 9cc gas engine thread - Page 22 - RC Groups
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Apr 07, 2012, 01:35 AM
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Nav_Aids's Avatar

Ignition and fuel supply

Originally Posted by gkamysz
These ignitions are very simple. They retard about 20° from 4kRPM to about 1kRPM, that's all. If this engine uses 40° advance there is some possibility they have more retard programmed so idle is still 5-10° BTDC. You have to consider these engines probably don't idle below 2k like larger engines do.

I didn't think their automatic advance was much and I still believe that this feature is hardwired rather than software which is easier and cheaper to do. I'll just have to wait till I get mine to dissect it.

I was going to mention as far as the fuel pump and carb are concerned, people reading this thread have all the answers they need about small air bubbles. First if you take EarlWB's post #250 and then what you said in post #287 this would explain why you get small air bubbles in the carb fuel supply line. The fuel goes from atmospheric pressure to 5 psi to just below atmospheric which means one can sometimes get bubbles.

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Apr 07, 2012, 03:00 AM
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Nav_Aids's Avatar

More on the fuel pump and carb

I don't think most people understand how diaphragm carbs work. One big misunderstanding I think is with the fuel pump being on one side and the regulator/metering circuit on the other. The two are not separate so it is hard to see the relationship. Also these and other diaphragm carbs such as Walbro and Tilloston have a metering circuit. The metering circuit acts similar to a float system.

The fuel within the metering circuit is pulled out from the carb and like Greg (gkamysz) said in post #287 the carb supply line is slightly below atmospheric pressure. The only time the fuel is pushed to the carb is when the metering circuit is empty. Once the fuel line and metering circuit are filled with fuel and there is a slight resistance to flow the diaphragm moves away from needle lever and fuel flow stops. Actually the pressure on the needle and seat rises to what is called the pop-off pressure, usually between 10-12 psi. The only way fuel can get flowing again, is to have the carb pull fuel out of the metering circuit letting the diaphragm contact the needle lever and push back against the spring which in turn relieves the pressure on the needle and seat allowing the pump pressure to lift the needle off it’s seat thus letting fuel into the metering circuit until the diaphragm moves away from the needle lever and the whole process starts all over again.

Remember the pump supplies the metering circuit and the metering circuit acts like a float system. The needle, seat, and spring keep the fuel pressure coming into the meter circuit at about 5 psi. Here is some videos on how to check the pump and metering circuits. Yes I know I'm using bold below.
Good luck and have a happy Easter.


A Must Read:
Metering Circuit and Lever Setting: Nice short explaination

This is a Tilloston carb but is almost the same as a Walbro but better. Very good video on the basic's and some hints. Like the metering circuit and pre-soaking gaskets. Kind of boring until the 3 minute mark.
Tillotson Carburetor Repair & Maintenance (12 min 15 sec)

This guy is an excellent tech and shows the proper way to check the pump, including using a bowl of water or gas to check for leaks.
Small Engine Repair: Checking Fuel Pump Diaphragm & Inlet Needle on a Diaphragm Carburetor
Small Engine Repair: Checking Fuel Pump Diaphragm & Inlet Needle on a Diaphragm Carburetor (6 min 49 sec)

Walbro Carburetor
How does the Fuel-Pump work
Nice animation.
Walbro Carburator How does the Fuel-Pump work (1 min 50 sec)

How to Rebuild a Two Cycle/Two Stroke Engine Carburetor.
I like this one because the carb used is so much like the NGH fuel pump.
How to Rebuild a Two Cycle/Two Stroke Engine Carburetor (6 min 35 sec)

Walbro pop off 25/10/07
Walbro pop off 25/10/07 (1 min 21 sec)
Apr 07, 2012, 08:07 AM
Suspended Account
another cheaper dealer in the USA....

Rimfire spark plugs, better than stock ones....
Apr 07, 2012, 06:43 PM
Registered User
Thanks for the vids. Nav man Im just starting to get into gassers and these will help no doubt . Its good to see that the ngh 9 is starting to perform for its owners . Its getting closer to when I may get one of these but at the moment Im drawn to the mintor 22 cc . Sexy looking engine as is the rest of their line . Just waiting for a few running reports first . Cheers the pope
Apr 07, 2012, 07:28 PM
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earlwb's Avatar
I think I finally got mine dialed in pretty good now. I have most of its idiosyncrasies figured out now.

The fuel pump works best if it is inline with the carb. You can get your best engine runs and best idle speed like that. The carb tends to spit a lot of fuel out of the intake, so it looks like it is running too rich at low speed, but it isn't. Also idle speed is actually at about 1/3 open throttle, not lower down. Which explains why one can have problems starting it with a more closed throttle position. Both of my fuel pumps did not like being bolted directly to the engine using the pump mounting bracket. As soon as I did that the pumps started letting bubbles through to the engine. The gasoline was vaporizing inside of the fuel pump. So the pumps are sensitive to the engine vibration and maybe the engine heat too as the pumps are close behind the engine right there too. I would suggest mounting the pump off to the side level with the carb and somehow isolate it from the engine vibration some. I could hold the fuel pump right behind the engine and it seemed to work Ok there too.

The piece of Tygon fuel tubing I used from the engine crankcase to the pump tended to slip off easily. So I switched to a black neoprene rubber tube for that part instead. I think the fuel fittings have barbs that are a little too small and the tubing would tend to slip off of them more easily. But for the engine crankcase I think the black neoprene tubing will work good as it tends to stick to the fittings better almost like it was glued on even. Before we had Tygon fuel tubing about the only thing that worked was black neoprene rubber tubing anyway. So it is a old fashioned way of taking care of the problem. One interesting thing that happened was the crankcase pressure tube popped off the engine fitting and the engine continued to run like there was nothing wrong. So at full throttle the engine may still run and draw fuel OK even with the crankcase tube loose. But maybe it was a fluke.

I think the RCexl ignition module only has two settings for its advance, very little or no advance and full advance, no advance curve per se. Thus the throttle winds up being about 1/3 open for idling. Now that does sort of limit the throttle range from low to high to some extent, but the engine does run Ok in between idle and full throttle. As you slowly close the throttle, you can see the effect as the ignition module changes from high to low spark advance. There is no more gradual effect to it. it isn't a big deal, but it could confuse some people.

I found that the engine tends to run at a higher temperature than the glow engines do, so air cooling is important. The engine probably won't do well if it is cowled in too much. I measured about 350 degrees F when running mine at full throttle for a extended period of time with a 11x6 prop. I measured the temperature at the base of the glow plug, so the head might be a little thin there and it may not be representative of how hot the engine is running overall. It may improve some later after the engine has some hours on it too. I was running extra oil in the fuel over the recommended 20:1 ratio as well. But my engine seems to run just fine like that too. So it doesn't appear to be overheating any.

I was running SEF fuel of which is made by Powermaster and it is sold at some hobby shops (the ones that sell Powermaster) as well as at Home Depot and Lowes Hardware stores. I really like this fuel as the gasoline doesn't stink much at all, and it doesn't leave a stinky residue that won't wash off your hands either. My family doesn't complain if I bring the engine inside the house either as it the doesn't stink up the insides either. But it is more expensive than pump gas though. But since it doesn't have ethanol or other smelly additives in it, it doesn't damage the pump or carb rubber parts. VP Racing Fuels bought out Power Master fuels a while back. ref You do need to add more oil to the fuel to bring it up to the 20:1 fuel to oil ratio though.

Here is a short video showing me running the engine. I actually had it idling for a few minutes when I was talking to my son earlier, so the carb adjustments are pretty close. I was still running the high speed a little on the rich side though, as I am reluctant to lean it out more yet.

In hindsight, I think the engine operating instructions, that came with the engine, are pretty close to being correct. About 1.5 to 2 turns out on the main needle and about 3.5 to 4 turns out on the low speed with the throttle set to about 1/3 open. I remember thinking that it was a translation error when I first perused it. But ultimately, that was about where the carb settings on the engine wound up being when I was done messing around with it. The 1/3 open throttle for idle speed can confuse people easily too. But that is where it is to get the engine to run. I can hand flip to start my engine when it is not hot and at 1/3 throttle open too. Early on when I was trying to figure out the engine, I was trying to run it at a lower throttle setting like we normally do on glow engines, where idle speed is having the throttle open maybe 1/32 of a inch or so. Anyway the little NGH engine carb for gasoline doesn't work like that.

But I think i can now plan on putting it in something to fly it now. I am digging my Ugly Stick out of the storage shed to put it back into use to run the engine on it. Anyway the reason there were bubbles in the fuel line at the end of the video clip was because it had run out of fuel. I saw it and cut power to the ignition module then.

NGH 9cc gasoline spark ignition engine test run (1 min 42 sec)

This was the temperature I measured, but that does not mean it is accurate. There are many factors affecting the measurement and it can be off. So this measurement only works for my engine. I don't know what someone else will measure. Plus my IR temperature measurement device could be inaccurate too. I had the IR meter laser pointer aimed at the base of the spark plug on the head on the exhaust side. The head is more thin there and maybe thinner than one thinks which can skew the results. My old Fox Blue head .60 with dual plugs has a really thin head on the glow plug next to the exhaust which also causes high temperature readings too.
Last edited by earlwb; Apr 08, 2012 at 09:16 AM.
Apr 08, 2012, 01:12 AM
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brace's Avatar
Thanks earlwb, nice review and I concur with what you have detailed. Will be putting another litre of fuel through mine before mounting on the plane (J3 cub) and then we shall see
Apr 08, 2012, 08:22 AM
Fly it or get rid of it
JRAllen's Avatar
I have never been SO CONFUSED about an engine in my 28 years of R/C. I gotta stop reading threads on here.
Just going to by one and hope for the best.........


Apr 08, 2012, 10:23 AM
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kerwin50's Avatar
Sorry your so confused. Myself I'm just watching reading and waiting.
Apr 08, 2012, 10:37 AM
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earlwb's Avatar
I'm sorry you are confused too.
If there is some question you could ask to help clarify it, please ask.

This is the smallest production gasoline engine so far out there in the marketplace. I sort of expected some teething problems with it, as the designer had to come up with a new carburetor and fuel pump design to get it to work. But I was pleasantly surprised to find that it does work OK. It took a while to get some preconceived notions, I had about it, out of my head before it started working OK.
Apr 08, 2012, 11:51 AM
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Nav_Aids's Avatar
Originally Posted by DoctorX
another cheaper dealer in the USA....

Rimfire spark plugs, better than stock ones....
I just got a quote from AMR (Aircraft Modelers Research) located in Longueuil Québec and as of April 8th, 2012, ($179.99US). So I can get the GT9 for $198.99 US and this includes tax and shipping. Shipping distance: Winnipeg, Canada to Longueuil, Quebec, Canada 1830 km or 1137 miles.

Now how do I convince the wife I need another motor?

Aircraft Modelers Research NGH GT09cc
Last edited by Nav_Aids; Apr 08, 2012 at 12:01 PM.
Apr 08, 2012, 12:22 PM
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Nav_Aids's Avatar
Good work earlwb. Looks and sounds like you got the motor running pretty good. Yup it does sound like it is rich laboring on the high end but, the throttle response good.

I was going to say that I agree with what you say about U.S. gas it does stink. But don't most U.S. states have oxygenated fuel? On the trips I have taken to the states I noticed that the fuel smells like pure oxygen, a burnt rubber smell, Amoco seems to be the worst.

When I measure my engine temperatures, the glow plug or spark plugs are always the hottest. IMHO, On gas engines this is where you want to measure the temperature. I like to see 350f - 375f for a safe tune.

Apr 08, 2012, 01:06 PM
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turk1's Avatar
Thanks guys for very valuable info and hard tests.I recommend to
1- open backplate of engine,check for some casting flakes may stay on port sides(mine has one flake and easy to remove with a little tool but be careful not to drop in the engine).That is because of JBA factories' confusing conditions at present.When Michael was there, there wasnt any sign of such flaws.Many, many JBA 56 s has passed from my hands and never met such a thing.Those 9 CC engines still comes from JBA 56 based blocks.
2- Blow into&suck both pump and carb nipples using some fuel tubing and try to hear working diaphragm noise and feel valve function..Also help system to move freely.I think this will help your equipment will work better at the very beginning of first run.
Apr 08, 2012, 05:54 PM
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earlwb's Avatar
NAV_Aids, yes our modern gasolines are ozygenated and have all sorts of other additives the different gasoline company consider proprietary, not just ethanol in it. They even use stuff like propane or butane mixed into the fuel. Thus the gasolines have a limited shelf life. So you can't just store a can of the stuff for any length of time. But it is definitely nauseating to smell for me.

That is good to know about the temperatures I wasn't sure at the time.
Apr 08, 2012, 06:56 PM
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Nav_Aids's Avatar
earlwb I forgot to ask you what temperatures are you getting at the rear side of the cylinder head ? I like mine to be around 225f - 275f when the air temperature is 40f - 85f by adjusting the oil/gas ratio and never higher than 325f. Anything above this temp and the oil starts to breaks down.

Apr 08, 2012, 09:25 PM
Registered User
earlwb's Avatar
It was running around those temperatures you mentioned, 250 degrees F or so. I was mainly concentrating on the hot spot next to the glow plug as it was the hottest there.

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