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Need help to solve Mogas problem in Pawnee Glider Tug Finland


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Hi Eddie


We do need some help.


We have changed over to mogas with our two Pawnees in Räyskälä Finland last spring.


Both are working fine at lover temperatures but the other had some hair raising events when the temperature got to 30C.


It cuts the power totally at say 10-20m height when towing a heavy twoseater (eg Arcus)


This happened mostly when the towplane had been waiting for the next tow with stopped propeller.


The second Pawnee has not yet shown any tendency to this. We have been studying the differences but have not found anything.


The only modification on both is the red insulation hose on the fuel line.


We think that it has something to do with the higher vapour pressure of mogas (compared to Avgas) causing the easier boiling of the fuel in the carburettor, fuel line or fuel cock while waiting the next tow. Main heater could be the oil pan.


I am contacting if you happen to know somebody who have worked with same kind of problems with Pawnee in Australia and perhaps could help us to find the right medicine for the next summer.


We ourselves only can try to find the crucial difference between out two towplanes.


with best regards Hannu



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Vapour lock is the main fault with mogas. You either keep the whole fuel system cool and/or pressurise it with a pump in the tank. If the carb body gets hot the fuel in the bowl may boil and the float sinks in the bubbly liquid as the density of it drops, and the carb floods and vents fuel either into the throat or out of an external vent. Your pump won't stop this happening, and either way it won't make your day. Another reason to have fuel injection. Nev



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