Hi res photos are courtesy of Rob van der Heijden, he took them while working on his ST20/25

Defining the problem

Ever since the start of model engine use, the problem of a steady fuel supply to the engine’s incoming air has been a major one. In normal operation and all other things being well, there are three major reasons why the once carefully adjusted  air-to-fuel ratio changes in flight.  

Fuel level variations in the tank.

When the tank is full, the fuel level is above the main needle level, causing the fuel to flow towards the carburettor. With almost empty tank, that situation is reversed, so the mixture tends to change from rich to lean as the fuel in the tank is used up.  

High g-forces in the flying model

Normal g-forces in model flight are extremely high, and can be, because there is no pilot to black out. During normal sports flight, 40g is not uncommon and has been recorded using in-flight measurements. That means, that as flight progresses, and the fuel level changes from one centimetre above the fuel bar to one centimetre below the fuel bar, the level change as the carburettor sees it is 80 cm (2cm x 40g).  

Model attitude in flight

In our models, the tank is situated way behind the engine, so with the nose up, there will be a much leaner mixture than with the nose down, also due to fuel level changes relative to the spray bar.  

In addition to these fuel level variations, the mixture can change due to changing engine revolutions.