ORIGINAL: combatpigg
Manifold pressure [vacuum] decreases with demand. The spring loaded metering rods are held in the lean position by high manifold pressure when demand is low. The main jets are enabled when the manifold pressure is low, then this circuit is allowed to flow as much fuel as the venturi can siphon from it.
The OS 120 has a simple barrel carb
I ran one once in a 8 lb pattern plane I designed specifically for the engine - worked very well
The only thing really required is to keep the fuel tank centre on the centreline of the carb - which is a trick because the carb sets quite low.
With a nice oily high nitro fuel - the engine is dependable and very smooth tho not very powerful
the ram air -is not worth mentioning on this setup.
Brer Noolie laws -well the effective low pressure in the carb/manifold tract is pretty meager on these old four cycle setups
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