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Hi res photos are courtesy of Rob van der Heijden, he took them while working on his ST20/25
Work on the Bench
The system I chose, makes use of a Walbro or Tillotson floatless chain saw carburettor, combined with the standard methanol carburettor as fitted to the engine. Of the floatless carburettor, only the pump and the regulator are used, and the needles for idle and full throttle are shut completely. Butterfly choke valves may be removed to save weight.
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The Walbro is known for it's sensitive pump membranes; (Go-cart racers change them every race), so it is possible to saw the carb in half, and use the regulating part only, and provide the fuel pressure using a Perry fuel pump, or an automotive electric pump. Fitting level of the regulator compares to float level in motorcycle carbs (click the picture to enlarge) |
| First close the original crankcase pressure connection (see foto)The carb is provided with a new nipple for crankcase pressure, fitted on the pump membrane cover, and one nipple for the regulated fuel outlet to the motor carb, as well as a soldered exhaust pressure line on the steel cover of the metering diaphragm. Drill all holes at least 2.5mm to ensure adequate fuel flow. See the pictures for all locations. I used M5 pneumatic nipples for all connections, but it is perfectly ok to press/bond in brass tubing, using locktite sealant. | ||
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The ST 23cc has a pump pressure nipple added to the back plate for the pressure line to the Walbro pump. In this pressure line, a T-connection is inserted to add an extra line for priming the engine directly into the crankcase. All lines used, should be a firm, thick-walled quality with ample 2.5mm bores.The pressure line from the brass tube soldered to the atmospheric vent of the metering diapgragm is connected to the exhaust. This metering diaphragm must be at about the same horizontal level as the spray bar.(this is very important, or the system will not work as it should)
Below is a section through a Walbro carb system to gain insight in the working of the pump section, and the metering part.
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The scetch of the Walbro is shown 180° rotated as compared to the installation photo. |
Why should it work?
In the Walbro carb, the fuel delivery to the fuel jets is regulated by the metering diaphragm. The regulated fuel pressure is equal to the air pressure in the atmospheric vent. This can be used to advantage to solve the problems of fuel delivery in model engines. i.e. the lack of fuel level control.
In model engines, exhaust pressure to the tank is a great means to provide the required fuel pressure depending on the engine load
situation in a stationary system. Drawback of this system is, that due to
the the tank's level variations and g-forces in flight, the fuel pressure as the spray bar sees it, is
far from controlled and differs between unacceptably wide limits.This
drawback can be eliminated by using the Walbro carby as a metering system.
At idle, the exhaust pressure is about
equal to, or slightly above atmospheric, while at full throttle the pressure is
considerably higher.In a dive the pressure will rise such, that a
slightly richer mixture results compared to the stationary settings, so the
engine never will go lean. The Walbro carb can be fitted alogside the
engine in such a way that the horizontal level of the metering membrane is positioned at the same height as the spray bar, so that high g-forces have no influence
on the mixture strength any more. In side-slipping, very low g-forces are encountered, with which the system easily can cope. By placing the regulator
as far forward as possible, i.e. beside the engine, model attitude problems are
also eliminated, although a small influence is still present. This is
advantageous, becaus at slightly rich settings in level flight, the engine will
lean out a bit in a steep climb and then delivers maximum power.