the frost on intake.

[rustred]

i started the mccormick today to split some wood. it was sitting since april not started, fired up pretty good with the 6 volt battery .it was 20 f here so i was watching this icing thing some people are concerned with. this is after running it for 15 minutes. i split wood for 1/2 an hr. and the ice was pretty much all dried up. this is about the same i have always saw on carbs around here.

 

[Hemmjo]

i started the mccormick today to split some wood. it was sitting since april not started, fired up pretty good with the 6 volt battery .it was 20 f here so i was watching this icing thing some people are concerned with. this is after running it for 15 minutes. i split wood for 1/2 an hr. and the ice was pretty much all dried up. this is about the same i have always saw on carbs around here.

Carburetors are actually evaporative cooling devices. Gas needs heat to vaporize. When it is hot out side you don’t notice it. But when it is already cold, dropping the temperature just a couple of degrees makes a big difference.

 

[UncleTom]

I have had vw air cooled engines running on the shop floor in 40 degree temps. Just the velocity and wind chill at 40 will frost up the carb and manifold and then it doesnt vaporize fuel and runs rich putting black smoke out the exhaust just like the choke was on.

 

[Tgrasher]

My Case 300 does the same frost up.

 

[Hemmjo]

Wind chill is a perceived low temperature not an actual temperature. If it is 40 degrees outside with 0 wind speed the reading will be 40 degrees. If the wind jumps up to 100 mph the temp reading will still be 40 it may even rise a little due to friction interaction.

But if you add some gasoline to that air stream, like in a carb the temperature will drop significantly. This is what created the frost. IF it is cold enough, all of the fuel may not vaporize, which makes the smokey exhaust.

If is also why most intake manifolds are physically connected to the hot exhaust manifold. There is so much cooling from fuel vaporization, the intake needs an external heat source.

A cold engine cannot heat the intake enough to completely vaporize the fuel.

 

[Dave G9N]

Wind chill is a perceived low temperature not an actual temperature. If it is 40 degrees outside with 0 wind speed the reading will be 40 degrees. If the wind jumps up to 100 mph the temp reading will still be 40 it may even rise a little due to friction interaction.

But if you add some gasoline to that air stream, like in a carb the temperature will drop significantly. This is what created the frost. IF it is cold enough, all of the fuel may not vaporize, which makes the smokey exhaust.

If is also why most intake manifolds are physically connected to the hot exhaust manifold. There is so much cooling from fuel vaporization, the intake needs an external heat source.

A cold engine cannot heat the intake enough to completely vaporize the fuel.

If we are getting technical here, it's the boiling point of the fuel under vacuum that is causing the frost on the carburetor and intake manifold. The place you need to worry about the frost is inside the carburetor. It can build up and block the airflow. My 78 Honda Civic used to do that sometimes. I looked inside the carb when I had to pull over and found a build up of ice in the venturi. Had to wait for it to thaw before I could get back on the road.

All liquids have a vapor pressure that changes with temperature. When you expose a liquid to a pressure lower than it's vapor pressure, it will boil. Boiling (phase change from liquid to gas) absorbs a lot of heat, so the liquid will cool until the vapor pressure of the liquid at that temperature drops to the pressure inside the chamber. That's why there has to be a way to heat the carburetor, usually by locating it close enough to the engine.

The VW carb was too far from the engine to keep it warm enough passively, so they had either an oil line or vented some hot air off the engine to warm it.

 

[ShakerPrairie]

Does relative humidity play into this?

 

[Hemmjo]

Does relative humidity play into this?

Yes, it plays into it but in a more complex way. If the humidity is 0 it is not possible for frost to form as there is no water vapor to condense. If the humidity is 100%, so much water could condense that it could warm the carb above the freeze point. You would just have water dripping off the carb.

I have no idea what the optimum % might be.

 

[Janicholson]

A dramatic factor is the adiabatic cooling. from the venturi and throttle plate restriction. evaporation is a contributor, but not much. For the same reason air compressors need to have fins and intercoolers when they compress air, that compression is making the air hot. Also diesel engines are massively associated with heat through compression. When air or other gasses expand, they cool, the opposite of the compression heating. The cure has always been heat stoves that raise the temperature of the incoming air. from the early sixties most autos had heat stoves on their exhaust manifolds, or heat risers in the intake manifold to do that same thing (often both were applied). This is compounded by the presence of moisture in both the air as humidity, and fuel as water suspended in gasoline. Jim

Wiki

 

[rustred]

Yes, it plays into it but in a more complex way. If the humidity is 0 it is not possible for frost to form as there is no water vapor to condense. If the humidity is 100%, so much water could condense that it could warm the carb above the freeze point. You would just have water dripping off the carb.

I have no idea what the optimum % might be.

its all in the venturi which is causing the drop in temperature. it dont matter winter or summer. i have felt the intake on these tractors in summer at 80 degree's and they turn cold . the thing is high humidity is actually causing the frost. yes relative humidity plays in this but in the summer you just dont see the frost, the intake is still way cooler than the outside air. that is also why an airplanes wings ice up due to venturi effect. ever hear of a diesel engine ice up in the intake?... no, it has no venturi. its the nature of the beast and as i said before you will not stop the air from cooling off passing through a carb, updraft or down draft. they work the same

 

[super99]

Way over my paygrade!! All I know is when the intake on my Oliver gas tractors ice up, the Oliver guys tell me that I have the carb set just right.

 

[rusty6]

Its mostly the humidity that causes the icing on the outside, and inside of the carb venturi. Simple fix is to shut down the engine after it has ran a few minutes so that the heat from the exhaust and block can melt the ice. Once I do that I can re-start and continue to run with no further icing.

 

[plattman]

i started the mccormick today to split some wood. it was sitting since april not started, fired up pretty good with the 6 volt battery .it was 20 f here so i was watching this icing thing some people are concerned with. this is after running it for 15 minutes. i split wood for 1/2 an hr. and the ice was pretty much all dried up. this is about the same i have always saw on carbs around here.

Just evaporative cooling saw it often on the Case DC. Doesn't hurt a thing. Carbureted airplanes have a carb heat function to keep it from getting out of control.

 

[SkipperII]

i started the mccormick today to split some wood. it was sitting since april not started, fired up pretty good with the 6 volt battery .it was 20 f here so i was watching this icing thing some people are concerned with. this is after running it for 15 minutes. i split wood for 1/2 an hr. and the ice was pretty much all dried up. this is about the same i have always saw on carbs around here.

I used to drive a little 1960 Ford pick up, 6 cyl. in the late 70s. Worked swing shift, and drove 90 miles each way. On cool fall nights, there was one stretch that was always foggy, and it would loose power and die. By the time I could get the hood up, it would start, and run, and keep running. The third time it happened, it dawned on me that it was icing, so I pulled the air cleaner top, drilled a hole with a hole saw, and brazed a 2" exhaust elbow on it that drew right off the exhaust manifold, cured the problem.