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I have been flying my bc12d-85 for 3 winters now in the sierra Nevada mountains and I recently experienced what I believe to be carburetor ice. It has happened twice now not sure of the temperature on the first flight how ever it happened after a decent , on a second flight a week or so latter (testing in the pattern) temp was -1 to -2 Celsius and due-point was - 11 Celsius. After applying carb heat the stumbling / hesitation that was happening when opening the throttle goes away (with reduced rpm of coarse). After a little research I found the attached continental service bulletin it describes the symptoms accurately and offers a solution. My question to the tribe is 2 part, 1st has anyone tried this and did it work? and 2nd is there other methods others are using successfully?
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I believe the Service Bulletin was developed mainly for Stromberg carb equipped engines. That model carb doesn't have an accelerator pump to inject fuel when the throttle is quickly opened and the mixture can go briefly too lean. Marvel carbs (typically MA3-SPA for C-85) usually have a pump but not all MA3s do (MA3-PA's don't I believe-may be wrong).
A local Taylorcrafter used that baffle but had better running after the idle mixture was readjusted richer for cold weather to see 50rpm rise at idle mixture cutoff. I don't know if he's still using it as it can rob some airflow and maybe power.
The best pre-indicator of carb ice is a small manifold pressure gauge tapped into the induction system. As ice forms at constant power and altitude the MP indication will slowly drop due to reducing the carb air flow from ice on the throttle plate or venturi.
Carb temp probes monitor temps conducive to ice formation but not the actual ice formation.
GaryN36007 1941 BF12-65 STC'd as BC12D-4-85 -
I think the SB came out because of the early Cessna 150's, and they would have used a Marvel carb. From all I have heard it helps the stumbling issue. I don't see any down side from trying it.Comment
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I've yet to see but one plane (the Taylorcraft I mentioned above) with the mod installed in my 45 years of flying in Arctic Alaska. But still try it. If power is needed then evaluate that as well. Just make sure the idle mixture is correct for the OAT flown and accelerator pump functioning properly before and during the test. The accelerator pump has adjustments for volume so think about that.
The real secret if there is one is to not jamb the throttle open in one second (like instructors sometimes teach) but rather slowly open it in a couple or more. The fellow that tried the baffle no longer needs it after he adjusted his mixture ant takes his time with the throttle.
The three planes I've owned with small Continentals and several C-150's have never stumbled down to -30F my limit with that engine. But I do what I do and others may not.
GaryN36007 1941 BF12-65 STC'd as BC12D-4-85Comment
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It doesn't really do anything for carb ice. Back in the 60's it was usually described as a way to keep snow from blocking the intake. They are not uncommon on Cubs, but aren't really needed if you don't fly in snow or freezing rain.
The acceleration stumble is related to mixture, not ice. Solution is to adjust the mixture in cold weather and don't cob the throttle, ever.
Re carb ice, you are as likely to get it at 90°F as at 0°.
In a J3, I apply carb heat anytime the rpm is below 1900.Comment
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Applying the carb heat effectively enrichens the fuel mixture. Carb ice does not show in that manner. You will no you have carb ice when you are flying along level flight steady rpm, and you start losing rpm and engine starts running rough. It can happen at any temperature hot or cold. Small continentals are bad about it. Your issue I think relates to the Marvel carb not having a accelerator pump. I don't think they was used until the C90. Try slowing down throttle advancementComment
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that temperature and dew point spread represent a relatively low probability of carb ice.
the most likely reason things improved with carb heat is because the heat results in a richer mixture.
Look for something that can cause lean running, like an induction leak or mixture control problem or dirt in the carb. If you're running mogas, make sure it's ethonol-free.Scott
CF-CLR Blog: http://c-fclr.blogspot.ca/Comment
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Of course it could be the opposite . A restriction from the air filter could be causing rich mixture in which case carb heat might be effectively an alternate air intakeScott
CF-CLR Blog: http://c-fclr.blogspot.ca/Comment
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Probably not related, but my experience with my Stinson Franklin 150 in winter (it had a Marvel MA-3 with accelerator pump) was that I got stumbling and even in flight, all the signs of carb ice ONLY WHEN FORCED TO RUN ON 100LL!
If operating on 80 back when it was available and then with the STC approved mogas, I never had that experience. Not easily explained except perhaps the 100LL has a different vaporization characteristic. I have had others with the same setup that have observed the same.
My Taylorcraft was wrecked by its former owner due to engine stopage (65) on takeoff after a long idle in cold, humid weather. It was running on 100LL and the FAA seemed to think it was carb ice. It had a legally installed MA3 but don't remember if it had an accelerator pump. I don't think it did.
As an aside, I found that with the Stinson and the MA-3 carb with accelerator pump, if it was left set for a year or two with mogas in it, the little ball valve in the accelerator pump circuit would gum up and then not take throttle without stumbling because the accelerator pump was not functioning. Once higher rpm was reached they ran just fine, just wouldn't take throttle rapidly.
Larry Wheelock, BC12-D still under repair; Stinson 108 now with Lyc 180; and Mooney M20c with Lyc 180Comment
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Fuel vaporization can cause a chill. Seasonal auto gas can be worse than aviation. Carb heat can help if the temp rise on application is sufficient.
We dealt with the plugged accelerator pump on Supercub.Org. Throttle would stall during application and pump passages were plugged. Fixed via removing the accelerator pump check valve and cleaning the passages between the pump seal and outlet nozzle.
GaryN36007 1941 BF12-65 STC'd as BC12D-4-85Comment
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Ding Ding ding...does anyone know what the temperature rise needs to be for carb heat at the base of the carburetor in a certified system?N29787
'41 BC12-65Comment
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It's in the CAR's and depends under which the plane was certified...
For CAR Part 04 planes pre-WWII: 4.6291 says an adequate means shall be provided for preventing the formation of ice in the engine carburetors. I haven't discovered a specific temp rise but ???
Post WWII CAR 3 planes 3.606 (a)...for sea level engines with a venturi carb +90F rise at +30F air temp w/o visible moisture @ 75% of max continuous power. It's a bit clearer here for those planes that apply.
Now the question. Does changing design of exhaust or intake affect that temp rise? How about EGT adjustments? Do STC's or Field Approvals for engine accessory changes consider the requirements? How do we deal with CAR's for the run of Taylorcrafts and the requirements have changed?
GaryN36007 1941 BF12-65 STC'd as BC12D-4-85Comment
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Ok then are planes that fail to comply unairworthy? Who's going to determine compliance after manufacture and possible change? There's lots of anecdotal comments that indicate poor carb heat performance can happen. Luscombe exhausts barely comply according to http://luscombe.org/technicaldocs/ba...t%20system.pdf
GaryN36007 1941 BF12-65 STC'd as BC12D-4-85Comment
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useful chart. Note the low probability of carb ice with temp and dew point from original post
Scott
CF-CLR Blog: http://c-fclr.blogspot.ca/Comment
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