--> Engines-List message posted by: Gary Casey <glcasey@adel...>

>

>  Nothing seems to correct the situation. I did read on the Lycoming
> web site last week that this is a common occurrence with Carb'ed  
> engines.
>   Iv I run the engine at different power settings, I get different  
> EGT's &
> CHT's. At 2350 RPM (fixed pitch prop) in cold air (less than 25*F)  
> # 4 CHT
> goes so low it will start skipping, especially if I have leaned at  
> all. All
> temps are fine at full throttle and at something like 2150 RPM. In  
> warm air
> the situation is much less pronounced....
>   So I've gotten to the point where I ether fly slow in cold air,
> (especially down low) or at full throttle, leaned, at high altitudes
> (11,000' or more). Sure makes me want to reconsider a fuel injection
> system....
>   I do have all EGT's and CHT's, and have talked to Bart at Aero  
> Sport Power
> extensively about this situation. He doesn't have too much to add....
>
>
> Fred Stucklen

One thing that we learned on car engines way back when was that the  
carb inlet condition makes a big difference.  I've seen some aircraft  
installations that were well done except for the carb inlet.  Ideally  
the inlet should be a bell-mouth (elliptical is a good approximation)  
or at least radiused with equal air flow from all directions.  Not  
easy to do with the tight space constraints that usually exist.  One  
way to do it that I have looked at (not done as I don't have a carb'd  
engine) is to bolt a radius - no duct - to the carb inlet and then  
surround the carb with inlet air by building a dam well above the  
inlet that extends outward from the carb to the lower cowl.  Air can  
then flow past the carb and enter the inlet from the rear, balancing  
the flow that comes from the front.  The lower restriction will give  
more power as well.  Remember, a sharp-edge inlet will have a Cd of  
about 0.6 compared to a radiused inlet that will have a Cd of almost  
1.0, meaning the pressure drop at the inlet will be 60% of what it is  
with a sharp edge.  Might be worth a try.

Another thing we tried was a "dip tube", which is a tube that extends  
down (oops, up) into the manifold slightly past where the individual  
runners come off the manifold.  This creates a sharp edge that  
prevents the fuel and/or air from clinging to one side and not the  
other.  Made a definite improvement and carefully done didn't impede  
the air flow too much.

Gary Casey



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