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From: Dave Baker
Subject: Re: Condensation on Carb
Date: 04 Sep 1998

> In cold and/or damp
>weather the temperature drop inside the choke can be enough the freeze the
>and that's when the engine starts to misfire.


Before the flames start - it isn't the fuel itself that freezes, it's the water
vapour content in the air which the fuel mixes with in the emulsion tubes that
freezes. That is why damp weather makes carb icing so much worse. With a high
enough water vapour content and low enough temperatures the fuel nozzles
completely block with ice. In fact in bad cases you can find the entire outside
of the carb and sometimes parts of the inlet manifold covered with a layer of

In mild cases, by the time you get to a halt and open the bonnet all that is
left is water from the ice that has melted.

I once got a bad case of carb icing during a winter motorway trip on a car with
a twin choke downdraft weber and no air filter. The primary choke froze solid
but the secondary, which only opened at 3/4 throttle, was ok as no air was
going through it at normal cruise. The only way to proceed was to use full
throttle to open the secondary barrel and then coast back down again. I spent
an unhappy hour alternating between full acceleration up to 100mph and then
coasting back to 40 or so. The other drivers running somewhere in the middle at
a steady 70 must have thought I was several bricks short of a full load as I
zoomed out of sight and then let them catch up again and overtake. Fortunately
no feds were on the scene that day to stop me and enquire why I had such a bad
case of kangaroo petrol!

Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work.

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