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.
1958 MGA with an attitude