The Stovebolt Site Forums The Shop Area Engine Shop Carburetor icing

I noticed this year some of my old trucks have a slight carb icing problem during a cold start , first mile or so of driving if it's below 50 degrees .All the engines have properly functioning manifold heat,nothing mechanical changed since last year.Not a big problem, clears up after a few minutes, never happened before.Must be the fuel formulas?I always use Sunoco gas.

The 235 I had in my '49 when I first got it used to do the same thing. If it was close to freezing and raining/snowing I could count on it trying to die at the first couple of stop signs.

As soon as I shut the motor off the first time (like parking to run into a store) then it would be fine. Used to find frost all over the insulator between the carb and manifold. Never did figure out a good explanation for it. It had the stock intake and exhaust setup and a working heat riser valve.

While carburetor icing is quite common under certain conditions (generally 25 - 40 degrees F.) and very high humidity and low heat; unexpected carburetor icing is the engine's suggestion to check the carburetor for being overly rich.

A float setting too high by only 1/32 inch can do it.

Jon.

Tony, the best way to stop carb icing is to move south. Course, they might not let you into Texas!

We used to ice up the manifolds on VW dune buggies running them hard in the Texas heat.

Carb icing happens when the temperature and the dew point are close. We get it all the time when it is raining or foggy and the truck is cold. Airplanes have carb heat controls, so I thought I would put one in the truck. I made a small box (stove) out of steel that attaches to the exhaust manifold. A zip tube connects the stove to a valve in the air cleaner snorkel. The valve is controlled by a choke control. When the engine runs rough, you pull the carb heat knob, warm air is drawn from the stove into the carb and it smooths out.

Carbking, Why would a over rich mixture cause a freeze up? I have never heard that before. Is it something in the fuel or the way the engine is trying to use it? Joe

You need a degree in chemistry or physics to understand the factors that could affect carburetor icing. Here is a little run-down about fuel, detonation, pre-ignition, etc . The following explanation is extracted from the link above:

Can I prevent carburetor icing?
Yes, carburetor icing is caused by the combination of highly volatile fuel, high humidity and low ambient temperature. The extent of cooling, caused by the latent heat of the vaporized gasoline in the carburetor, can be as much as 20C, perhaps dropping below the dew point of the charge. If this happens, water will condense on the cooler carburetor surfaces, and will freeze if the temperature is low enough. The fuel volatility can not always be reduced to eliminate icing, so anti icing additives are used. In the US, anti icing additives are seldom required because of the widespread use heated intake air and fuel injection.

Two types of additive are added to gasoline to inhibit icing:-
- surfactants that form a monomolecular layer over the metal parts that inhibits ice crystal formation. These are usually added at concentrations of 30-150 ppm.
- cryoscopic additives that depress the freezing point of the condensed water so that it does not turn to ice. Alcohols (methanol, iso-propyl alcohol, etc.) and glycols (hexylene glycol, dipropylene glycol) are used at concentrations of 0.03% - 1%.

If you have icing problems, the addition of 100-200mls of alcohols to a full tank of dry gasoline will prevent icing under moderately cold conditions. If you believe there is a small amount of water in the fuel tank, add 500mls of isopropyl alcohol as the first treatment, and isopropyl alcohol is also preferred for more severe conditions. Oxygenated gasolines using alcohols can also be used.

And, if you want to continue the lesson, read about a rich mixture and carburetor icing.

Curmudgeon

Interesting article. However, maybe a less technical explanation might be useful.

Remember 7th grade science class? We learned that to change the state of matter from a liquid state to a gaseous state required heat? The purpose of a carburetor is to mix gasoline (liquid state) with air, and convert the mixture to a gasous state. The heat required comes from the air, the carburetor metal, anything surrounding the gas/air mix. A richer mixture simply requires more heat.

The information about dewpoints, humidity, and temperature is correct. From a more practical standpoint, I have yet to see icing in Missouri at temperatures above 45 or so degrees F. (and we have about as much humidity as anyone!!!). Of course, since altitude also enters into the mix; what is true in Missouri is not necessarily true in Colorado.

How to solve the problem? Gord has the right idea. This solution may not be as difficult as one thinks. Take a look in your local boneyards for some of the early 1970's Japanese 4 cylinders. Many of them utilized an aircleaner with the heat tube and a physical switch, so that one could change the source of the input air (either from the heat stove on the exhaust manifold, or direct). Installation of this system will pretty well eliminate carburetor icing.

Jon.

Good thing my trucks aren't flying,lol.Piston engine planes have a pilot adjusted carb mixture control, my trucks don't besides the manual choke. Anyways, they don't run rich, I say it's the fuel, the formula was changed here in NY recently to comply with Clean Air Standards,it's got a small percentage of alcohol for oxygenating .The only thing difference between last year and this year is the fuel.

To ad to the corn-fusion, I've never had carb icing while at speed in my '52. It only affects the truck while idling, or thereabouts. Wonder why?

Never experienced this with any vehicle--one of the pleasures (I guess) of living here in what some term --Baja Oklahoma :rolleyes: . What's that carb icing like--frost? I mean ya don't have to chip it off with an ice pick do ya? Tony you ought to move out of that frozen place. :p

Bznhafdzn, when I was drag racing my Pontiac, I ran 100% methenal. On cold damp nights the intake would look like it was snow covered. The runners were frosted over. One real damp night, the car would shut off just at the finish line. By the time I got stopped and the hood open, the ice would be melting and it would fire back up. I had to wrap a towel around the distributor to keep the water off it from melting frost. Carburator do the same thing, its not a lot of ice, but it forms at the wrong spot and can shut off the fuel flow down in the throat, or block the throttle blade. Joe

Buzn,on my trucks, the carb icing gives no outward appearance other than moisture condensed on the carb throttle body.If the truck is driven from a cold start for a few miles at 40 mph, the intake heat does it's job , but if I drive a mile then down shift to make a turn,the engine may stall once, maybe,conditions have to just so.After the first stall, never happens again.During cold weather,never happens.I believe gas companies still use a different brew for cold weather that tends to lessen the problem.
Hmm, where I live in NY has no tornados,hurricanes,volcanos, floods fires or quakes,just cold winters.My trucks can live with that

So what do you do for excitement? Go around licking light poles?