The two-piece cast-aluminum silencer
lets you position
the exhaust port as
The rear-mounted carburetor is a collaborative effort between O.S.
and Walbro. Note also the brass crankcase vent. This allows blow-by
recirculation for cleaner operation.
T-nuts and 10-32 socket-head cap
screws with lock washers did the
honors, as I secured the GF40 inverted
with a slight angle to use the existing
throttle pushrod installation and tuck
the muffler under the chin.
I drew and printed a mounting pattern with my computer
to locate the new hole locations on the firewall. Watch
clearances with your installation. I could not spin the full
silencer into place without fuselage interference. Instead of
removing the engine, I removed the rear silencer shell and
baffle. I also rotated the rear shell on reinstallation by 90° to
direct the exhaust farther from the Eindecker’s belly.
On the electronic ignition, I used self-adhesive Velcro and
a length of Vel-Tye’s Velcro One Wrap to secure it in place.
I powered it with a Thunder Power 2S 2, 100 mAh pack
mounted behind the firewall, again using self-adhesive Velcro.
The ignition draws 400 mA at 6,000 rpm, so the 2, 100 mAh
pack should be good for plenty of flying.
While I was connecting the throttle pushrod, I noticed that
the control arm was loose on its shaft. The throttle arm is
plastic and screws to the end of its brass rod without indexing.
Securing the arm at the right angle took some trial and error
because the throttle shaft can spin under the arm as you
Interestingly, the weight of the larger and more complex
GF40, when fully installed with standoffs and ignition, is
essentially the same as the original, smaller 26cc engine.
Confirm the appropriate carburetor settings before you
fire it up for the first time. The manual calls for 12/3 turns
open from fully closed for both needle valves. These settings
worked beautifully for my initial runs and, after some tuning
trials during the propeller testing, I have returned to these
The rear-mounted carburetor helps keep your fingers away
from the propeller, but that isn’t permission to adjust the
needles while the engine is running. It is always best to shut
down between needle changes.
I used regular unleaded fuel mixed 32: 1 with high-quality
oil. The manual recommends using an electric starter for safety
and I concur. Although the engine does start readily by hand,
why stick your precious fingers in harm’s way? The GF40
easily starts with a starter, usually with a bump when there is
some gasoline in the carburetor.
The recommended first propeller is an 18 x 8, with up to a
20 x 10 to best suit your model and flying style after break-in.
Following my first few flights, I ran the GF40 with four typical
propellers within the recommended range, monitoring rpm
with the telemetry capability of Hitec’s Aurora 9 transmitter,
backed up with a handheld tachometer.
The engine started easily and ran smoothly with all of them,
giving a dependable low idle and plenty of power on the top
end. I thought it might be slightly rich with the recommended
initial needle positions, but that wasn’t the case. The GF40
consistently transitions smoothly without sagging after it is
In the Air
What really matters is what happens in the air. I had
expected a performance jump with the O.S., but even I was
surprised. Previously, the Eindecker would accelerate and
rumble along the runway for nearly 100 feet before rising.
Although it flew well on its wing and would loop and roll
in a scalelike manner with appropriate coaxing, there wasn’t
much excess energy. I found that
pulling up for a stall turn
after a full throttle pass had
it running out
73 Model Aviation APRIL 2014