3kw Carburetor Frosting

m38inmaine

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Maine USA
I just bought a 3kw MEP-026A genset and noticed when running it today the carburetor intake elbow is frosting up and the top of the carb is very cold as well. I leaned out the main jet as much as I could thinking it might be running rich and the extra fuel might be causing it but no change. Looking for any help on this. Genset runs great and the output seems right on. Only shows 60 hours on the meter. Dated 3/71.

Mark
 

Boatcarpenter

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Mark,
I can't remember the proper physics name for what is happening, but it has to do with a cooling of the carb as the air fuel mix is passing through. I don't know what the ambient temp was there when you were running it, but I'm guessing sort of cool and very humid or perhaps even raining. The frost was just the moisture in the air freezing on the carb when it got down to 32. I had an old Land Cruiser one time that would start stumbling and then quit at about 32-34 degrees when it was snowing. Let it sit for a minute and then would start right up again. Couldn't figure it out until one time it quit I jumped out real quick, popped the hood and saw the carb all frosted. The reason it would stumble is that the fuel wouldn't vaporize real well under those conditions for some reason. Was on a trip and all I could find to wrap it to insulate it was a sock. Wrapped it up and worked like a charm.
BC
 

maybefixit

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The scientific principle is Boyle's Law. It basically says temperature and pressure are related, lower pressure = colder and higher pressure = hotter. The air in the atmosphere is a higher pressure than the air in the intake (of course, engine vacuum), so its temperature drops with a decrease in pressure.

On Dodge K-cars, there was a water-heated space in the intake manifold to prevent the same issues from cropping up. Two small hoses connected the intake manifold heater plate to the coolant system, which helped keep the area frost-free.

The opposite effect is seen with the output tube on an air compressor, those get very hot due to increase in air temperature with the compression. This is why intercoolers are installed on some turbocharged engines, gets rid of some of that additional heat.

Airplane pilots have to watch out for the same problem, there are particular combinations of ambient temperature and humidity which can lead to the carburetor throat icing up. Usually it's when the dewpoint is within five degrees of the temperature.

If the engine RPM is high enough, the ice crystals can be drawn past the carb. throat before they get a chance to cling to the sides and cause trouble. If the system is running fine despite the frost on the outside, I wouldn't worry about it too much. I'd restore the mixture settings to original (or otherwise tune it for just-right), so you don't damage the engine. Since gensets always run at a constant high RPM, it should be fine, especially with Recovry4x4's note of personal experience.
 

m38inmaine

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Location
Maine USA
Thanks all, yes it does run fine and it was sort of misty out and about 46-50 degrees. I believe I have the high speed set at 3/4 to 1 turn as specified in the manual.

Thanks

Mark
 

m139h2otruck

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NH
There should be a hose connection between the air filter and the exhaust pipe and a little sheet metal valve inside the air cleaner housing. Under hot conditions, you use fresh cold air, and when cold/raining you suck hot air off the exhaust pipe. On our it works great, and with a hand held thermo unit you can see the difference in about 2-4 min. when running.
 

TheBuggyman

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It is the same reason that the air conditioning in your house works. You take any liquid or gas, pass it through an orifice or restriction to a lower pressure and the rapid expansion absorbs heat to expand and fill the low pressure area again. airplanes use air compressed and expanded to provide cooling, no refrigerant involved (to my knowledge).
 
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