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MG MGB GT V8 Factory Originals Technical - Carb iceing/engine temp.

Every winter I have trouble with ice forming in the primary venturis of my Edelbrock 500cfm carb. Seems to happen when the outside temp is between 2 and 7 deg. especially when damp.
The engine seems to always run cool with the guage pointing to 7.00oclock.
Wondering if my thermostat is too cool for winter, its rated at 82 degrees C. Maybe a warmer thermostat would warm things up a bit in the engine bay and maybe help with the problem.
Engine is 4.6, RV8 headers, Bonnet vents.
How hot is your thermostat?




Yes its the water vapour content in the air which the fuel mixes with 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.

I use a 88 winter with 0W40 and 82 Summer. 4.6 RV8 ex but no vents.



Thanks Paul Ill try an 88 and see if it helps.
Are you using the Edelbrock carb? if so do you know which jets/metering rods U are using?
Mine are 083/060x052. still got a slight flat spot just off idle.



Mark, try changing the accelerator pump cam. You can buy a multi-pack of different shapes to give you the properly timed "squirt."
Jeff Schlemmer

Thanks Jeff.
Ill see if the cams are available over here.


If it is carb icing, the only way you will get rid of this is to increase (drastically) either the temperature or moisture content of the intake air. The factory v8s with their lobster claw intakes didnt suffer overly with carb icing... their intake air was heated by the exhaust manifolds, and once air is over 30 deg c carb icing is no longer a problem. If you have a circular filter on top of a single vertical downdraught carb the only solutions are not very pretty. Try putting some sort of fabricated duct between the filter and the area above each exhaust manifold, or blanking off the area behind the rad, so the engine bay temp rises a little. Once the ambient temp in the engine bay rises to above 30 deg c, the problem will go away, as it will if the moisture saturation of the air decreases. It is a recurring problem in both aircooled vw motors and flat 4 aircooled carbureted aircraft motors.
Nick Bentley

Please forgive me for describing the process of carburetor icing in some detail. I hope it may help people understand why icing can occur at quite high ambient temperatures.

Ice is formed in carburetors when the ambient air temperature is between about -10 °C to 30 °C if refrigeration and adiabatic cooling are sufficient to lower the air/fuel mixture temperature, and consequently the metal of the carburetor, below freezing point. Note this is equivalent to 14 °F to 86°F, so given the right conditions carburetor icing can take place on a very warm day.

There also must be moisture in the air, although you may not be particularly aware of it.

“Adiabatic cooling” - mentioned above is caused by the constrictions at the throttle valve and choke venturi creating a local increase in air velocity, with consequent increase in dynamic pressure and decrease in static pressure.

Density remains constant so the temperature instantly decreases in line with the decrease in static pressure. This adiabatic cooling is more noticeable when the throttle is closed, or partly closed, for extended periods, but it is unlikely to cause more than a 5 °C drop at the maximum and overall, probably much less, say 2 to 3 °C .

“Refrigeration cooling” - is caused by the evaporation of the fuel. The latent heat to facilitate the fuel evaporation is taken from the surrounding air and metal, which is already being cooled adiabatically.

The temperature drop caused by refrigeration may be as much as 15 °C , thus giving a total drop within the carburetor as high as 20 °C . If this temperature reduction reduces the metal of the carburetor to a temperature of or below freezing super cooled water droplets will freeze on contact with the carburetor components causing what is referred to as carburetor icing.

I have never suffered icing on my factory B GT V8 with the original lobster claw air intake; although similar B V8s travelling with me have suffered if they have had shorter, and thus cooler, air intakes. As Nick suggests this may be a solution, at least for those of you with SU carburetors.

I don’t believe that a hotter thermostat will help Mark much, he may have to consider some form of shutter on his bonnet vents so that they can be closed off in icing conditions.

Nigel J S Steward

Thanks Nigel

I have tried blanking off a section of the grill which seems to work on the open road but the car tends to get too hot in traffic.
Think Ill try the hotter thermostat as any raise in temp can only help.
Recently when having a problem I removed the airfilter to find that both primaries were completely blocked by ice.



Im using a C bonnet which lets cold air over the top of the rad straight into the filter. This is good for cool air induction in warmer weather. Think Ill try to blank off the area between the rad and under side of bonnet to see if it helps.

Just a thought.

Thanks for all help and suggestions



I agree and use a plywood behind grille as temperature lowers before fitting higher thermostat, but temps rise if in jam using blanking. I had this problem with the 3.5 but found when changed to 4.6 that the problem disappeared, so it does not take much additional heat to change matters. Also heater is warmer and oil at a good flowing temperature. Porsche do use flaps to aid warm up as a speedy warm up is important for oil, so adjustable flaps would be a good addition all year round.


This thread was discussed between 06/03/2005 and 08/03/2005

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