Modifications Done To
An Explorer Motorized Hang Glider Harness (FLPHG)

The purpose of this page is to document and share some modifications I performed to my FLPHG Explorer harness and its Radne Raket 120 engine, to discuss them and share feed back.

Intake Velocity Stack

"Intake velocity stack" is a fancy name for an air intake venturi. They speed up the incoming air while decreasing the turbulence of the air ingested by the carb. Presumable, velocity stacks improve fuel atomization in the carb=92s venturi resulting in complete combustion and increased power output. Air filters should still be fitted to prevent sucking debris in. On most engines, velocity stacks really only make a measurable improvement in the upper RPM range and wide open throttle (WOT), at low revs -seems to me- make no difference. There are softwares to correctly design a velocity stack with appropriate curvature and height; Lucky for us, Radne has already designed an aluminum intake velocity stack (they call it flange, #32161) for the Raket 120 that fits both the WG and most WB carb series:

Polished Ports

There are numerous reports from 4-stroke and 2-stroke engine mechanics that polishing the intake ports can increase torque. Since I was already decarbonizing the combustion chamber and piston, it was an extra step that by no means would hurt the engine. It was hard to reach deep inside but did my best. Care was taken to not enlarge the ports' diameter or scratch the cylinder's lining. While I was at it, I also polished the intake manifold and the exhaust port to slow down carbon accumulation. For this job, I started with a coat of "Sea Foam" decarbonizing spray, a good scraping with a metal drill brush and then switched to a Dremel rotary tool fitted with a right angle drill attachment # 575; I started with a light touch of the buffing wheels # 511DR then brush # 531 followed by rubber polishing bits # 461, 462, 463


An adjustable (or tunable) carb will NOT give your engine more horse power, but will allow you to easily tune-up both jets for optimal engine performance depending on your current weather conditions and elevation. The factors that will require you to go leaner are, higher altitude (climb 1,000 ft. or more), higher temperatures (changes of 20' F or more), and higher humidity (changes of 20% or more). Bear in mind that the engine (any engine) loses three percent of its power for every 1,000 feet (305 m) of altitude ascended. The Radne Raket 120 comes originally with the Walbro WG8 (formerly called WG6) carburetor and its high-speed jet is fixed for operating a 120cc chainsaw under a "standard atmosphere" at sea level. But I fly a powered harness and usually take off from 1,000 ft ASL and climb at altitudes of about 5,000 ft ASL. A tunable carb can be tuned in the air with a remote carb adjuster.

Most pilots know that carbs can't be set at the factory and stay correctly adjusted for the life of the engine at all altitudes. The fact is, some go-kart engine manufacturers think so, and have mounted a "fixed" high speed jet on their engines. These types of carburetor do not work well for aviation, the reason is that altitude, fuel type, pop-off pressure, temperature and a host of other factors cause the diaphragm carb to need periodic adjustment. I am sure you have noticed that the weather makes the biggest difference in engine performance. Indeed air temperature, humidity and altitude have a profound effect on carb jetting because of the changes in air density and oxygen. Changes that the WG8's fixed jet can't compensate for. This was the motivation behind my search for other compatible carburetors with both High & Low adjustable jets in order to optimize air/fuel mix depending on the current weather conditions. It had to have mounting holes' distance between centers of 46 mm and capable of correct air intake volume (CFM). The Tillotson HL series carbs fit these requirements and come in a variety of bore sizes, but they are somewhat heavier than the Walbro, so I took a closer look at the compatible Walbro carbs:
Walbro WB series
Compatible Walbro diaphragm carbs (Spreadsheet)

I tested the WB-32 but it seemed to run rich at midrange, provoking vibrations while 'four-stroking', also, it has no choke. I tried plugging its midrange jet aperture but that did not correct the problem. Then I tried the Walbro WB-42 because its smaller venturi aided with easier manual in-flight restarts. But then I adapted, tested and settled for the large-bore WB-39 ($104 USD). Its large bores still allow for a smooth idle and crisp throttle response.

*Adjustable Hi/Lo speed needle jets.
*The WB-39 hvave no choke but can adapt one using the throttle plate and shaft from your WG8.
*The venturi diameter of the WB-39 is the largest of all the WB series but it still provides a strong jet signal and unrestricted air intake at full power.

The Walbro's WB-39 bore diameters fall well within the Raket's air volume requirements (CFM), but the air flow is limited by the inlet manifold diameter in the cylinder wall. On one experiment, I bored the carb flange and the inlet located in the cylinder to match the throttle bore diameter of the WB-39; But on testing, no RPM increase was registered so the engine seems to take only as much air as it needs, as result, my Raket engine did NOT experience any additional power as compared with the original WG8 with the fixed High speed jet, even after boring the the cylinder air inlet, so, again: a WB tunable carb -even with the largest bore diameters- will NOT produce more power, but it will definitevly assist you to easily fine tune-up the jets before every flight to make 100% engine power available under all circumstances. For more details and calculations, click HERE and then look at Compatible Walbro diaphragm carbs (Spreadsheet).

Mounting the New Carburetor

If you decide to switch to a Walbro WB series carb (WB manual - pdf 800Kb), some simple carb preparations are needed before mounting:

- On the WB-42 The choke flap (butterfly) and shaft were temporarily removed and its "choke friction ball and spring" (Walbro diagram parts # 18 and 36 respectively) were discarded in order to allow the choke to flip freely between open/close position without resistance at all. - Next, regardless of the carb you are using, modify its throttle flap for a slow and smooth idle RPM as shown HERE. - Cut and bend a new mounting plate to hold the choke and throttle cable mounts at an appropriate angle and distance to the carb.

On the next photo, one can see the choke lever was coupled with a spring that PULLS it open when the choke control is released. The original spring that PUSHED it open on the WG8 carb was removed long ago before the swap, as its 'K' force was inadequate and on three ocasions entangled with the choke cable leaving it stuck at a partially closed position with subsequent loss of power at full throttle and also causing a flooded engine at start.

- Diaphragm carbs use the engine's internal oscillating pressures to propel its fuel pump integrated in the carb. In the original WG6 (WG8), this oscilanting pressure or "pulses" are transmitted to the carb through a special "pulse orifice" through the carb's plastic flange mount. Now, the WB carb series have an external inlet to channel the pulses, so I just created a bypass line from the mount flange to the carb's pump: I blocked the original "pulse orifice" at the top of the plastic flange and then drilled and placed in perpendicular a 1/8" barbed steel pipe and connected it to the fuel pump with with fuel-rated tubing.

Fuel Line Flushing & Carb Priming

Personal experience and a poll in the FLPHG list revealed that there have been several problems with sudden loss of power later traced down to air bubbles in the fuel line. To date, there are three theories:

The DoodleBug team traced the problem to a fault (leaky) design of the white plastic adapter of the carb's fuel inlet. The Wasp team identified the problem as defective diaphragm membrane material.

Replace the rear portion of the fuel line (from the fuel filter to the carb) with TRANSPARENT polyurethane fuel tubing to aid with visual inspections. In USA, consider this vendor part # 5108k45 at $0.32 per foot. Take this opportunity here to change the fuel filter too as it is underestimated by many pilots how much power is robbed from a dirty fuel filter. Since replacements can get expensive, I highly recommend this amazingly effective fuel filter with internal wire mesh, which can be dismantled within seconds for cleaning (If this link does not work, search part # SME702354 or # 7-02354).

In order to connect the new line (ID 3/16") to the carb, you must cut off its original white plastic adaptor. Route the line BELOW the carb and into its fuel inlet. It is important to note that the fuel line works under vacuum and not under pressure, so any loose connection will leak air INTO the fuel line. Place the fuel tank on the ground next to the harness to help flush out bubbles and prime the fuel system while you visually inspect the transparent fuel line for remaining large bubbles; After warm-up and jets' tune-up, I hook in the harness and fuel tank WITHOUT disconnecting the fuel line.


An average foam filter will flow 4.38 cfm/sq-in. A good paper filter will flow 4.95 cfm/sq-in. An oiled cotton gauze (K&N) will flow 6.03 cfm/sq-in. The use of appropriate air filters will prolong the life of your engine. Use them!


Irish engineer Gordon Blair is credited with innovations that not only prevented 2-strokers from extinction but also propelled them into the 21st century. One of his remarkable designs is the =93expansion chamber=94 (see animation) a.k.a: tuned exhaust, tuned pipes, resonance exhaust, etc; It is now recognized that tuned pipes are a major determining factor in an engine's performance. Tuned pipes will not extract horsepower from an engine that didn't have it in the first place, but they can reshape the engine's power-band to make more power at different engine speeds as well as helping get every bit of available power out of the motor. The Raket 120 will develop about 10% - 15% horse power increase by using a tuned exhaust, producing 14-15 hp tops.

The octane quality of a gasoline is its ability to resist detonation, a form of abnormal spontaneous combustion under high temperature and pressure. In other words, the octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gasoline ignites by compression rather than because of the spark from the spark-plug, it causes unwanted "detonation". Note that octane rating is unrelated to potential power or horse power.
Every engine design is different and requires of specific octane rating. For the Radne Raket 120, Radne recommends automotive fuel with 95 - 98 octane (Europe's rating) and since where I live I can only obtain 91 octane fuel, I was initially sent on an octane-booster search across town. But then, Gregg Dewenter explained in the FLPHG list:
"There are 2 values measured in the USA which are, Research Octane Number (RON) and Motor Octane Number (MON). The value posted on the pump (by law) is the Anti Knock Index (AKI), AKI is (RON + MON)/2
In Europe, they only report the RON so the Raket 120 requirement of 98 (Europe) is equivalent to 93 (USA)octane. Octane boost should not be needed."
AVGAS - Some blends of avgas (100 -110 octane) will quickly separate from some oils used in premix situations so I do not recommend its use.


Radne recommends to mix 4% Castrol TTS 2-stroke oil with the fuel. It can be difficult to find in North America but Proformance USA will ship a 12-pack case to your home with free shipping anywhere within the USA.

My engine experience some carbonization while using 4% oil, so I have been reducing the concentration while watching the EGT and CHT and the sweet spot seems to be 2.5%; for continuous engine (2-3h) use, I mix 3% oil with the fuel. MISCELANEOUS

I have a manual pull-start engine but since I had never been able to do an in-flight re-start, I copied the kick-start idea from veteran Hang 5 and FLPHG pilot Gerry Uchytil.

Kick-starter update (Summer 2006): I found it inconvinient having to unclip the landing gear and open the harness' ziper in order to use the kick-starter stirrup, which furthermore, I had to place in my foot with one hand while I control the aircraft with the other. So I removed the stirrup and shortened the starter rope and its housing so that the starter handle rests next to my right thigh, at arm's length. This modification allows for easy access without unziping and provides best ergonomic position and best pulling leverage with my arm.


Having read of two reports where the harness got hooked to a rear side wire, I inspected my harness for any possible snag and decided to re-orient the skids' rope attachments in a vertical manner:


I like to take an RPM readout just before starting my takeoff run to ensure the engine is producing maximum power. Wishing to place the digital Tiny Tach tachometer in a place where I can see during flight, I spliced and elongated (I am told Tiny Tach service can supply you with a new unit with custon wire length) its small coaxial cable and mounted it with velcro at my chest just between the parachute container and the harness:
I also mounted a few small velcro ties to hold the cable steady along the harness and a second velcro mount (loops) at the feet of the harness to momentarily place the Tiny Tac at a visual spot to facilitate tune up of both carb jets.

Mouth throttle

NRG's small mouth throttle combined with the Explorer's base tube clip for thumb control.

Foot Throttle

I found this home-made foot throttle too sensitive to hold on intermediary settings, so I mostly use it when I am transitioning the mouth throttle on to the base tube or when taking photos, using electronic instruments, pissing, etc. The clearance witht he clutch is minimal so you will also need to install the NRG's clutch guard.


Wills Wing winglets inhibit the formation of wing tip vortices. As a result, they reduce induced drag and increase glide ratio and stability. A pleasant surprise is that when mounted on my Wills Wing Ultra Sport 147, they also provide enhanced control with full VG on; I can now fly on thermal days with full VG while turning with or without power and the glider is significantly easier to control. Also, they may have widened the ground effect & landing flare window as the combined result of the two phenomena described above is an overall increase of the ratio between the lift and the drag (L/D). The lift increases when the ground is approached.

Steve Pearson discussed some aspects of the polar curves displayed on the Wills Wing web site: "[...] As I'm sure you know, choice of VG setting for best effective performance is more often determined by controllability than best raw performance. For climbing in smooth air and light thermals, I typically seem to get best performance with VG 3/4. For gliding flight, VG tight. I recommend that you avoid VG settings of less than 1/4 unless necessary for control."
For FLPHGs, this means that you benefit of using VG to climb IF it does not compromise control - and the WW winglets have been an invaluable addition to my HG maneuverability under full VG. Although they are not being fabricated anymore, it is well worth the effort to find a pair in the used-market.


REDHEAD high-compression cylinder head.
Double divergent expansion chamber exhaust for the REDHEAD.
Static thrust results.




X1 Harness







REDHEAD Cylinder head for the Raket 120 engine


Gerry Farell Counter
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