[Top] [All Lists]

Re: general carb icing question

To: "Phil Bates" <>,
Subject: Re: general carb icing question
From: Barney Gaylord <>
Date: Fri, 22 Dec 2006 11:51:23 -0600
At 08:36 AM 12/22/2006 -0700, Phil Bates wrote:
>.... my car has started to run crappy in the snow. .... there was
>frost on the outside of carbs. .... what exactly causes icing??

Evaporation of fuel cools the intake air which in turn cools the 
throttle body.  Icing happens on humid days with ambient temperature 
just above freezing when moisture will condense out of the air and 
freeze in and on the carburetor.

>.... is there a lean or rich condition that also increases the 
>likelyhood of icing??


>.... I have a car with downdraft IDA weber carbs - one per cylinder 
>- no chokes. .... What is the effect of carb icing ....

The ice will not obstuuct fuel flow unless there is moisture in the 
fuel (which can be fixed with a can of "HEET" in the fuel tank).  It 
is possible (but not likely) that ice could cover the outlet holes in 
the main fuel jet to make it run lean.  More likely ice will build up 
around the throttle plate to obstruct air flow, acting like a choke 
plate to enrich fuel mixture making it run rough.

With or without the presence of ice, the cold carburetors will 
inhibit fuel vaporization causing a lean run condition.  That calls 
for fuel enrichment to make it run right, same as is needed for cold 
start.  An automatic choke might take care of that 
automatically.  Manual choke can adjust for this condition also.  If 
your setup has no provision for cold start enrichment, your sort of 
screwed on this issue.

Running cold with constant fuel enrichment is bad for the engine with 
the excess liquid fuel washing oil off the cylinder walls and 
diluting the crankcase oil.

The cure for icing is heat.  Olser MGs have the carburetor near the 
exhaust manifold, which helps a lot in cold weather.  Late model 
single carb MGBs have a water heated automatic choke for cold start, 
but that does not help an icing condition.  They also have a one 
piece intake/exhaust manifold that transfers exhaust heat to the 
carburetor.  The same late Bs also draw warm air from around the 
exhaust manifold to warm the intake air during cold conditinos, as is 
done with most modern cars.

Most modern cars run hot water through the intake manifold to 
stabilize intake air temperature. Slightly older carbureted cars 
often run hot water through a thermal block around the base of the 
carburetor to stabilize the carburetor temperature.  This last method 
can be used with almost any type of carburetor.  You need a thin 
hollow block under the carb with a pair of carburetor base gaskets 
and two external ports for water passage.  Connect these in series 
with the heater hose, and you have a nice carburetor heater.  You 
might find these parts on another car which uses the same type 
carburetor.  Otherwise you get to fabricate your own.

Barney Gaylord
1958 MGA with an attitude

<Prev in Thread] Current Thread [Next in Thread>