Carb icing
- Thread starter MarkO
- Start date
Robert Kenney
Super Moderator
I would really like to have some vehicle info but I'll give you some ideas.
The oem's use a heat muff or fitting to allow the engine to draw warm air from around the exhaust manifold area during cold starts and weather. If you dont have the factory air filter housing instaled you May want to find one and rig the heated air tube. The housing also has an air door to block the hot air once the engine is up to temp preventing exessively high intake air temperatures. The door opening and closing is controled by a thermo switch in the fillter housing.
The oem's use a heat muff or fitting to allow the engine to draw warm air from around the exhaust manifold area during cold starts and weather. If you dont have the factory air filter housing instaled you May want to find one and rig the heated air tube. The housing also has an air door to block the hot air once the engine is up to temp preventing exessively high intake air temperatures. The door opening and closing is controled by a thermo switch in the fillter housing.
I have only experienced carb icing twice -- on the m1 in great britain in a Ford fiesta (rental car).
Basically, I think you need to raise the "temperature" of the carb and the air flowing into (and over) the carb.
As robertk states -- though I thought the stock air cleaner "hot air intake" only worked on initial start up for emissions / "drivability" purposes.
The only cheap way to do this imo -- is to block some / all the air flow thru the radiator.
In my case, I did not think of "carb icing" until later.
The car "stalled out" climbing hills when I pushed the accelerator pedal down to maintain speed -- creating a low vacuum situation. I pulled over to the side of the freeway wondering what had happened and after 5 minutes or so the engine heat "warmed" the carb (my assumption) and I was able to restart the car and continue.
Does your carb icing occur "pulling a hill"? You might try using a higher gear though I have no idea if that will really work.
Basically, I think you need to raise the "temperature" of the carb and the air flowing into (and over) the carb.
As robertk states -- though I thought the stock air cleaner "hot air intake" only worked on initial start up for emissions / "drivability" purposes.
The only cheap way to do this imo -- is to block some / all the air flow thru the radiator.
In my case, I did not think of "carb icing" until later.
The car "stalled out" climbing hills when I pushed the accelerator pedal down to maintain speed -- creating a low vacuum situation. I pulled over to the side of the freeway wondering what had happened and after 5 minutes or so the engine heat "warmed" the carb (my assumption) and I was able to restart the car and continue.
Does your carb icing occur "pulling a hill"? You might try using a higher gear though I have no idea if that will really work.
Robert Kenney
Super Moderator
Carb ice is a form of drivability really. The warming of the intake can be almost continuous in very cold weather and not just at start up. If the air entering the af housing is below say 60f the damper will remain open.
Carb ice is most prevalent when manifold vacuum in high like at idle. As the moves past the throttle plate the pressure drops and causes a reduction in air temperature (adiabatic expansion).
In most cases carb ice will occur at outside air temperatures way above freezing when the air holds more moisture.
While it will occur at higher throttle settings it is much less likely. It May be impact ice on booster venturis and other items that hang into the air flow of the carby.
Another aircraft reference here, sorry.
all recip engined aircraft have manually actuated carb heat and alternate air paths. In known or actual icing conditions air can be rerouted from the intake usually on the front on the engine cowling where ice can form to a air source drawn across the mufflers where it is heated. This is the same place cabin heat comes from.
My take on carb icing has little to do with heated or not air intakes/filter housing flappers gizmos. Some of my pre 70's rigs over the years since I started driving have had an occasional icing issue
the main causes I have come to learn are in 2 main areas.
One is weather conditions and fuel blends. It's quite rightly known that at the right temperature/humidity conditions; the velocity increase of the air charge through carb venturi's decreases the temperature enough to freeze the air's moisture on the carburetor's parts. What isn't widely known is the fuel's blend can influence that as well. Winter fuel blends have a higher percentage of lighter volatiles,(hexanes, propanes, etc.) to assist in cold weather vaporization of the fuel for mixing and starting/drivability. It's normally not an issue, but an engine with marginal induction/intake heating ability or has a carburetor with air leaks, these blends can affect icing because their vaporization takes heat from an already low temperature fuel/air charge. Fuel vaporization is related to reid vapor pressure, and one of the specs for gasoline that has a seasonal stat. Reid vapor pressure takes 3 chapters in the crc manual to explain and not pertinent here.
Two, icing probability can be related to the condition of the engine's induction. I've seen worn carburetors with say loose throttle shaft bosses more prone to icing. The extra leakage, as mentioned previously, May be tuned for and not a drivability issue except it's extra moisture into the carb on those certain weather conditions. Also heating the manifold as soon as possible will prevent it. A lot of v8's had exhaust crossovers, and like i6's some had flappers in the exhaust manifold to divert the exhaust gas around or under the carburetor's base. After 10 or so years, there's hardly any that still work unless they've been maintained with lube and a new bi-metallic. Thermostats that leak even a little can cause long warm up times and contribute to carb icing if the intake doesn't have an exhaust crossover, or one that's plugged. Vw & corvairs also had thermostats that controlled flappers to throttle cooling air across the engine as well as some had an exhaust riser siamesed to the intake runner for charge pre-heat. Over time the riser plugged, people threw away the thermostats in the interest of "performance", and the icing sitch came.
If you can't repair all the mechanicals; such as a "tight" carburetor or an exhaust riser, preheating the charge by drawing air over the exhaust manifold such as done in later rigs should prevent it. It's only a matter of a couple of btu's to condition the air charge to keep it above "freeze point".
the main causes I have come to learn are in 2 main areas.
One is weather conditions and fuel blends. It's quite rightly known that at the right temperature/humidity conditions; the velocity increase of the air charge through carb venturi's decreases the temperature enough to freeze the air's moisture on the carburetor's parts. What isn't widely known is the fuel's blend can influence that as well. Winter fuel blends have a higher percentage of lighter volatiles,(hexanes, propanes, etc.) to assist in cold weather vaporization of the fuel for mixing and starting/drivability. It's normally not an issue, but an engine with marginal induction/intake heating ability or has a carburetor with air leaks, these blends can affect icing because their vaporization takes heat from an already low temperature fuel/air charge. Fuel vaporization is related to reid vapor pressure, and one of the specs for gasoline that has a seasonal stat. Reid vapor pressure takes 3 chapters in the crc manual to explain and not pertinent here.
Two, icing probability can be related to the condition of the engine's induction. I've seen worn carburetors with say loose throttle shaft bosses more prone to icing. The extra leakage, as mentioned previously, May be tuned for and not a drivability issue except it's extra moisture into the carb on those certain weather conditions. Also heating the manifold as soon as possible will prevent it. A lot of v8's had exhaust crossovers, and like i6's some had flappers in the exhaust manifold to divert the exhaust gas around or under the carburetor's base. After 10 or so years, there's hardly any that still work unless they've been maintained with lube and a new bi-metallic. Thermostats that leak even a little can cause long warm up times and contribute to carb icing if the intake doesn't have an exhaust crossover, or one that's plugged. Vw & corvairs also had thermostats that controlled flappers to throttle cooling air across the engine as well as some had an exhaust riser siamesed to the intake runner for charge pre-heat. Over time the riser plugged, people threw away the thermostats in the interest of "performance", and the icing sitch came.
If you can't repair all the mechanicals; such as a "tight" carburetor or an exhaust riser, preheating the charge by drawing air over the exhaust manifold such as done in later rigs should prevent it. It's only a matter of a couple of btu's to condition the air charge to keep it above "freeze point".
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