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Old 05-30-2005, 09:55 PM   #1
prangled prop
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G26 Problem

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I have a new G26 in a Neuport 17 and have had about 11 flights. The problem I am having is that I can not get a satisfactory transition from idle to high speed. I have tried several props and find that a 16 8 works the best but still not good. I have to advance the throttle fairly slowly to get the engine to speed up or else it will quite. Invariably on the ground when trying to give bursts of power to turn the plane it dies. I have tried adjusting the low speed jet and high speed jet to peek the high end and as I said I have had moderate success. The problem seem to occur in the middle portion of the throttle movement.

Any comments appreciated, This is the third Zenoah gasser I own and have not had any problems with the other two.
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Old 05-30-2005, 11:16 PM   #2
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What muffler do you have on there? I have had restrictive mufflers cause this same problem. It is nearly impossible to get an easy starting good transition compromise with very restrictive exhaust. Try running it with open exhaust just to see what happens. JMTB
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Old 05-31-2005, 12:42 AM   #3
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A. Idle Adjustment:
1. Adjust the idle stop screw so the engine idles at a moderate idle speed, just a bit fast.
2. Turn the Low Speed Needle Valve (LSNV) clockwise until the engine idles smoothly.
3. Re-adjust the idle stop screw to reduce the idle speed to a suitable level. Repeat step 2.
4. Continue to close the LSNV slowly until a slight decrease is noted in the idle speed.
5. Open the LSNV until an optimum smooth idle speed is attained. If the idle speed is still too high, repeat
steps 3,4, and 5.
B: High Speed Adjustment:
1. With the throttle set at full open, the engine should run rich. Listen for the four cycle sound. If not
heard, open the High Speed Needle Valve. (HSNV)
2. Close the HSNV slowly until the engine begins to smooth out. (Two-cycle sound)
3. Continue closing the HSNV until engine speed begins to decrease. STOP!
4. Open the HSNV 1/a to 1/4 turn so engine speeds up again and is smooth and steady.

Certainly active R/C modelers are well versed in operating the common `Model' or glow engine. Now, with the
change to the larger industrial engines, most may find a little more knowledge might be needed in order to
understand just how these larger engines function.
Except for the physical size differences and the addition of CDI ignition, the operating principles of the
chainsaw engine are exactly the same as the more familiar glow engine. If you `old timers' think back a few years,
this is where we all started when the first model engines showed up. This just might reinforce the thinking that,
"The more things change, the more they remain the same!"
If anything has changed over the years in the small two-cycle engine, it has been the carburation systems.
And, if there is one facet of the chainsaw engine not understood by most modelers, it is the principles of
operation of the modern day diaphragm carburetor. Hopefully, I will be able to shed some light on the deep, dark
mysteries of this marvelous little device. I may just help you `Quarter-Scalers' to understand just how the
carburetor is designed to operate. This, in turn, may make adjusting these carburetors a mite easier.
For the benefit of those of you who have had the willpower to refrain from taking your carburetor apart to see
what is inside, my applause for your infinite wisdom. For those others who didn't manage to control that urge, I
do hope your engine still runs. If not, read on. Who knows, you may find enough information here to get the
engine running again. Just be sure you don't permit a little knowledge to become more dangerous than none at
The two most common diaphragm type carburetors used on our engines are the Tillitson HU series and the
Walbro WA series. Their basic sizes and shapes are very similar, as are their principles of operation. In fact, we
might well wonder who copied whom. In many installations, they are directly interchangeable. Of course there
are many other manufacturers, models and sizes, but their functions are identical. In passing, it might be as well
to advise you that it is NOT a good idea to simply install a larger carburetor on your engine in the hope that it will
enhance it's performance. As you might suspect, each engine requires a carburetor that is specifically engineered
for it and its ultimate uses.
The diaphragm carburetor is really quite an engineering marvel. Within that small, cube-like object are
contained the basic metering devices which assure the engine receives the proper amount of fuel/air mixture.
It is, also, a very effective fuel pump to transfer fuel from the tank to the carburetor. In addition to these
functions, this little marvel can do so in any position and at almost any altitude as well.
The modern diaphragm carburetor can be broken down into two basic parts; the fuel pump, and the fuel
metering sections. To best understand how these elements are designed to operate, we will examine each section
separately, using the cutaway drawing in Figure 41. As the fuel pump section is the least complex, let's start
The fuel pump is located in the upper portion of the drawing. The pump, as it's name implies, uses a
diaphragm to move the fuel. The flexing action of the diaphragm (4) is caused by pressure pulsations from the
crankcase acting through the impulse inlet ( and its associated channel. The impulse inlet is directly connected
to the crankcase by alignment with the small hole in the crankcase at the carburetor attachment pad. Stated
simply, each time the piston moves up the cylinder, it creates a negative pressure in the crankcase. This negative
pressure acts on the pump diaphragm, causing it to flex upward. This flexing creates a low pressure area directly
below the pump diaphragm in the fuel chamber (3). Fuel is drawn through the fuel inlet fitting (9), its channel,
and through the flexible inlet valve (6). Conversely, on the piston's exhaust stroke, a positive pressure is built up
in the crankcase and the resulting impulse causes the diaphragm to flex downward, forcing the fuel in the
chamber out through its channel and the flexible fuel outlet valve (2). As fuel leaves the outlet valve, it is under
positive pressure as it passes through the filter screen (1) and into the fuel inlet supply channel (27). At the end
of the channel, the fuel flow is restricted and controlled by the main inlet needle valve (24).

Adjusting the metering lever. The metering diaphragm gives movement to the metering lever which opens and
closes the main inlet needle valve to regulate the fuel flow according to the engine's demands. The metering lever
of the Walbro Model WA carburetor is correctly adjusted when its upper surface is aligned with the floor of the
circuit plate as shown in Figure 42. Use the end of a steel rule to test. Corrections are made by gently bending
the lever as required.
With the engine running anywhere between 2000 and 10,000 RPM, you
can see the little pump diaphragm really has to do considerable flexing as it
must do so with every piston stroke. In most carburetors of this type, the fuel
pump diaphragm and the inlet and outlet valves are all contained on one
small rubberized sheet. The inlet and outlet valves are small flexible flaps
which act as check valves. The drawing indicates these valves in the closed
The diaphragm fuel pump is about as foolproof as is possible to design.
However, as with most of mankind's designs, there are many ways a fool can
upset the best of engineering. The following is a short list of some of them:

Re-install the carburetor to the engine with its associated gasket reversed.
This will effectively block the impulse transfer port.
Re-install the diaphragm so it is improperly located and the flap valves do
not seat correctly over the transfer holes.
And that ever popular favorite, the pinched or kinked fuel supply or tank
vent line.
The diaphragm pump generally works best when left alone. I do
recommend the fuel screen be examined now and then if no filter is located upstream of the pump. The
diaphragm pump used on our engines can be expected to lift fuel a distance of 12 to 18 inches, a real plus for
our side as the fuel tank may be located almost anywhere that's convenient. I try to place my fuel tanks at or
near the center of gravity of the model which eliminates any in-flight trim changes caused as the fuel is
With some insight into the workings of the diaphragm fuel pump, it may make it easier to understand why it
takes a little choking and engine cranking those first few times out to get the fuel line and the pump itself primed
before the engine can be started.
Now let's examine the real `heart' of the carburetor; the fuel metering section. This is the more complex of the
two sections as it contains the majority of the components making up the carburetor. To understand the
workings of the metering section, let's begin with the basic starting, or choking, of the engine.
The air inlet to the carburetor is first closed off, either with a finger or some mechanical device. In the drawing,
the throttle shutter (10) is shown in the `cracked' or slightly open position as it should be for starting. The engine
is now cranked over. Again, the piston's movement upward on the compression stroke creates a low pressure
area or suction, within the air/fuel passage of the venturi. (26) As the engine continues to be turned over, this
suction is transmitted to the diaphragm fuel chamber (17), through the primary idle port (12), the secondary idle
port (11) and the main fuel discharge port (25) creating a low pressure area on the fuel side of the main metering
Atmospheric pressure acting on the opposite side of the diaphragm entering through the atmospheric vent
(19), forces the diaphragm upward, actuating the small button on the diaphragm to depress the larger end of the
inlet control lever (20). This movement overcomes the tension of the inlet spring (1. As the control lever pivots
on the fulcrum point pin, it moves the fuel main inlet needle valve (24) off it's seat, allowing fuel to enter, under
pressure, into the fuel chamber side (17) of the metering diaphragm, up -through the idle and high speed fuel
channels, through the needle valve orifices and out through the discharge ports (11), (12) and (25), into the
engine's crankcase.
The quantity of fuel allowed into the engine is controlled by the high and low speed needle valves (13) and (23).
As the throttle shutter (10) is opened, the amount of air/fuel mixture is increased and the engines RPM
increases. The velocity of the air passing through the venturi (25) creates a low pressure area at the venturi
throat and diminishes the suction on the engine side of the throttle shutter. When the pressure at the venturi
throat is less than that within the main diaphragm fuel chamber (17) fuel is drawn up through the discharge
ports and into the air stream entering the engine intake, in accordance with the throttle setting.
When the engine is idling, the throttle shutter (10) is at a more nearly closed position than shown, so only the
primary idle discharge port (12) meters fuel to the engine. As the throttle shutter is opened further, more fuel is
supplied the secondary idle discharge port (11). At a full power setting, with the throttle shutter fully open, fuel is
drawn up through all the discharge ports. The two most important components within the fuel metering section
are the inlet control lever (20) and its associated tension spring (1. Their movement, acting on the movement of
the metering diaphragm, control the inlet needle valve (24).
The tension of the inlet spring has been predetermined in accordance with the size and power
output of the engine. Under NO circumstances should the tension of this spring be altered.
In most carburetors, the larger end of the inlet control lever is positioned so it is parallel to the floor of the
main fuel chamber (17) when the inlet needle valve is seated in the closed position. See Figure 42 for
It would be impossible to describe all the follies which can and do upset or even cripple the operation of the
metering section of this type of carburetor. The best advice I can offer is that any repairs needed be left to the
The principal complaints received concerning the adjustment of the carburetor are those involving getting a
good solid response from the carburetor when the engine is being operated at intermediate throttle settings. As
you may now understand which discharge ports open at what time, you should now also understand that both
the idle and high speed ports are involved. The settings of these two needle valves, interacting at intermediate
throttle settings, may require some `tinkering' to get them adjusted 'just right'. Take it easy, as a little at a time is
a whole lot better than twisting the needles a half turn at a time. Adjustments of one sixteenth of a turn at a time
are more in keeping with what is usually required.

One thing you may be doing is pressurizing the cowl and that will throw off the diaphragm carb. You need roughly twice the exit opening as you do intake to avoid this. You can check this by running it without the cowl and if you get it running well then everything changes when you put the cowl back on that may be the problem. Also sometimes a velocity stack helps as it will keep the spit back from the piston ported engine inside the intake stream instead of letting it blow away.

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Old 05-31-2005, 06:45 AM   #4
Kevin McGrath
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What he said!
But did he say that the low range needle affects the whole rpm range,while the high end needle affects high speed only,so that if you change the low needle you will have to re-adjust the high one.?
Lotsa folks are not "engine" types so dont let Dennis,s really excellent work here intimidate you.
The diaphragm pump type carb is a little marvel of engineering and 99.99% of the time is easily adjusted and trouble free,and once set seldom needs adjustment or fussing.....
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Old 05-31-2005, 10:50 AM   #5
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I would like to take credit for the info but it is a section on carbs from the book called the Quatra Bible.

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Old 05-31-2005, 11:46 AM   #6
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If in doubt, return the carb to the factory presets. They will be a good baseline from which to work from. Second, make sure the coil/pickup thing is adjusted as close as you can get without contacting the flywheel. I used 3 or 4 thou brass shimstock to set mine up after I had the G26 stripped down for inspection.

Starting at the factory settings, the engine should run decently enough - mine had excellent transitions up and down, and hand started with ease. I remember only adjusting the nv's very little (lean) and not untill all my bench running was finished.
Andrew Coholic -MAAC #26287L

1/2A to giant scale, IMAC, SAM, R/C sport, turbine jets, Heli's...
if its got a wing or two and an engine - I like it!
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Old 05-31-2005, 09:03 PM   #7
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Boy Dennis that must take the prize for the longest post??? Good info tho. And thanks guys for the sugestions and I will take them all into consideration next chance I get to play mechanic. Thanks again
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