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Author Topic: Carburettor Icing  (Read 387 times)
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betabuoy
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« on: May 07, 2020, 09:20:53 AM »

I've had a look through the site but can't find much information or experience of Carburettor Icing in the Beta when running twin-Weber.  Is this likely to be a problem?  I've mentioned the concern on my GCRE project page but it seems appropriate for a new thread over here for easy future reference.

I'll shortly be running twin-45 DCOE Webers with 38mm chokes on a GCRE inlet manifold.  Guy has build the engine for me and throughout our conversations he was adamant that I needed to ensure the carbs received a good cold air feed to optimise performance.  As work now continues in the engine bay with a local preparer, the team have significant concerns about carb icing (when using the Beta 'normally') because, as we know, there is no water heated jacket around the GC manifold.   

What if any experience does anyone have on this topic?

Chris
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rossocorsa
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« Reply #1 on: May 07, 2020, 03:34:26 PM »

Not a beta and not twin carb but I recall on an early (not reversed head) carb 1.6 Prisma that had its warm air pipe missing having this issue but it was only on cold days around freezing.
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mangocrazy
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« Reply #2 on: May 07, 2020, 07:02:50 PM »

No experience with carb icing on a Beta, but remember it being a 'thing' on unfaired motorbikes in the 70s and 80s. Faired motorcycles, where engine heat is contained within the bodywork seemed to be far less affected. As I recall, it only manifests itself when ambient temperatures are above freezing and the air is moist or misty. The trick for bikes was to use an additive in the petrol, I believe.
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Neil-yaj396
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« Reply #3 on: May 09, 2020, 03:33:23 PM »

Won't the very close proximity to the radiator prevent this? Certainly once the car has warmed up. My engine bay always seems nice and warm, and it's generating half the horsepower you are projecting.
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betabuoy
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« Reply #4 on: May 09, 2020, 03:53:28 PM »

Won't the very close proximity to the radiator prevent this? Certainly once the car has warmed up. My engine bay always seems nice and warm, and it's generating half the horsepower you are projecting.

Hi Neil - that's exactly what I'm hoping but my preparer is not sure that's enough.  And I get that carb icing in general is very important because the temp drop in the venturi is actually huge.

From Wiki:  Carburettor icing itself is caused by the temperature drop in the carburettor, as an effect of fuel vaporisation, and the temperature drop associated with the pressure drop in the venturi. If the temperature drops below freezing, water vapor will freeze onto the throttle valve, and other internal surfaces of the carburetor. The venturi effect can drop the ambient air temperature by 70 absolute degrees Fahrenheit (F), or 38.89 absolute degrees Celsius (C). In other words, air at an outside temperature of 100 degree F, can drop to 30 degrees F in the carburetor. Carburettor icing most often occurs when the outside air temperature is below 70 degrees F (21 degrees C) and the relative humidity is above 80 percent.

Carb icing is/was a big problem in the UK because our air temp/humidity is usually right in this zone - this is why some manufacturers designed manifolds heated with water jackets.  For example, see Lancia Fulvia manifold.  But as you say, the Beta engine - being transverse - puts the manifold right behind the radiator and this is what I'm hoping will negate the problem.

I'm still very interested to hear from other twin-45 users as I'm still in a position to modify intakes.

Chris
« Last Edit: May 09, 2020, 03:55:16 PM by betabuoy » Logged

1983 Beta Coupe S2
1967 Morris Minor Traveller
1925 Austin 7 Chummy
and the mighty... 2007 Panda!
rossocorsa
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« Reply #5 on: May 09, 2020, 05:17:12 PM »

Won't the very close proximity to the radiator prevent this? Certainly once the car has warmed up. My engine bay always seems nice and warm, and it's generating half the horsepower you are projecting.

Hi Neil - that's exactly what I'm hoping but my preparer is not sure that's enough.  And I get that carb icing in general is very important because the temp drop in the venturi is actually huge.

From Wiki:  Carburettor icing itself is caused by the temperature drop in the carburettor, as an effect of fuel vaporisation, and the temperature drop associated with the pressure drop in the venturi. If the temperature drops below freezing, water vapor will freeze onto the throttle valve, and other internal surfaces of the carburetor. The venturi effect can drop the ambient air temperature by 70 absolute degrees Fahrenheit (F), or 38.89 absolute degrees Celsius (C). In other words, air at an outside temperature of 100 degree F, can drop to 30 degrees F in the carburetor. Carburettor icing most often occurs when the outside air temperature is below 70 degrees F (21 degrees C) and the relative humidity is above 80 percent.

Carb icing is/was a big problem in the UK because our air temp/humidity is usually right in this zone - this is why some manufacturers designed manifolds heated with water jackets.  For example, see Lancia Fulvia manifold.  But as you say, the Beta engine - being transverse - puts the manifold right behind the radiator and this is what I'm hoping will negate the problem.

I'm still very interested to hear from other twin-45 users as I'm still in a position to modify intakes.

Chris

If it's a warm day/summer use car I don't think it will suffer
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WestonE
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« Reply #6 on: May 09, 2020, 08:15:14 PM »

I ran twin 45s for years in a Beta and had no carb icing issues. It was not a daily driver or use much in winter however.

Eric
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