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Old 19-11-2016, 09:07 AM   #12
Kato
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Join Date: Feb 2006
Location: East Molesey
Bike: Multiple Monsters
Posts: 2,219
Jeff
Your suspicion of Fuel icing, which is the result of water, held in suspension in the fuel, while probable generally happens only at altitude, a big problem for light aircraft possibly not for a M750 , what you experienced was probably a result of atmospheric conditions and not attributable to ethanol content

Basic fluid dynamics, says that a fluid's velocity must increase as it passes through a constriction in this case your carb’s venturi, causing a big temp drop

Seems that Duke’s suffer this problem a lot so it’s interesting that the Venturi effect is named after Giovanni Battista Venturi, an Italian physicist, however I digress

So the problem is caused by the sudden temperature drop due to fuel vaporisation and pressure reduction at the venturi, there is far more moisture suspended in cool air than in your fuel, the temperature drop may be as much as 30ºC and results in the atmospheric moisture forming ice, this initially blocks the smallest jets and affects the engine at low revs, the more mixture that passes through the carbs, the worse it gets, so riding fast (or revving the engine at a standstill) makes it worse, it’s not just carb’s either as Fi is not immune although nothing like as common

You of course know all this already but still might be useful to others

I agree totally that ethanol is a big problem thats not going away anytime soon, however on this occasion don't think it can be blamed.

This chart robbed from a aircraft forum is quite a nice graphical representation of the issue



In short

*Carburettored engines suffer most from icing because carburettors
cause evaporation, which cools the air.

*Icing can happen at temperatures of up to 38 degrees C.
Paradoxically, it is less likely at very cold temperatures.

*Icing is more likely at partial power settings because of the
cooling effect of a partly-closed throttle butterfly.
Right thats enough thinking for a hungover saturday morning
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Last edited by Kato; 19-11-2016 at 09:11 AM..
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