Carb Icing

ajmac

ajmac

Member
I ran my flathead today for the first time on the road into 4th gear, it ran great but when we pulled up to turn around it stalled and wouldn’t start. I noticed the base of the carb was covered in frost. We waited for a couple of minutes, the ice melted, the engine started again and ran fine from then on. Is carb icing a thing? Ford 94 carb on a std 1942 flathead. Electric fuel pump.

https://youtu.be/1fp7zA_Klw4?si=N4RQt0Ax4BHptFCI
 
Yes, carb icing can be a very normal thing depending on your climate or weather conditions at the time. And the procedure you used to solve the problem was right on. I've had to do it once or twice on a Holley 2 barrel on an International truck v8. High humidity and a cold engine are a combination that causes it. It is the reason we don't block off the heat riser passages in the intake manifolds here.
 
The carburetor icing up will have been caused by a malfunction of the heat riser. As you haven’t identified your intake manifold, I can only give generalized advice. Later models had a bimetal operated valve that would get rusted up and never open, and early models could have had the heat risers intentionally blocked with a wheat stalk penny, which today would function better with a ⅛” hole drilled through to allow a small percentage of heat to pass to the carb rather than too much heat or none at all.
 
Here is a photo of my engine just after coming back from dipping. Are the two circular holes in the middle of the inlet manifold gasket face water or exhaust gas ports to heat the inlet manifold? This photo was 10 years ago and I never checked! The engine is an 81A, came over to England as lend lease in 1942.
 

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ajmac said:
Here is a photo of my engine just after coming back from dipping. Are the two circular holes in the middle of the inlet manifold gasket face water or exhaust gas ports to heat the inlet manifold? This photo was 10 years ago and I never checked! The engine is an 81A, came over to England as lend lease in 1942.
Click to expand...
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Yes, the two circular holes are exhaust and match up with holes in the intake manifold to supply heat to the carburetor base area. The holes in the intake are very often plugged with carbon. Even without a heat riser valve in the exhaust manifold a lot of heat passes through those holes if they're clear.
 
ajmac said:
Here is a photo of my engine just after coming back from dipping. Are the two circular holes in the middle of the inlet manifold gasket face water or exhaust gas ports to heat the inlet manifold? This photo was 10 years ago and I never checked! The engine is an 81A, came over to England as lend lease in 1942.
Click to expand...

Correct, those are exhaust crossovers that go up through the intake manifold, then down to the opposite side. As cylinders fire, exhaust is pushed back and forth in those runs, which heats the fuel charge, and also the carburetor above. Earlier manifolds for Stromberg carburetors had a hole directly to the base of the carb, but manifolds intended for use with 94 carbs crossed over below, not having direct access to the base of the carb.
I live in sunny California, never having to deal with extreme cold weather, so the icing never happened to me. Right now the weather here is sunny and mild, while much of the country is below zero Brrr! so I guess the heat risers are needed in those places!
 
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