Carb Ice - Continued

[Kerbs]

I had reported earlier that I was having carb ice issues with my new C-85. After confirming on a number of flights since, I am definitely getting ice and a lot of it. I flew today, 50' and low humidity, and I am getting ice at every power setting other than fire walled. Cruising today at 2400rpm, if I do not have carb heat all the way out, I will start to lose power until the engine stutters. Applying full carb heat, stuttering stops and power comes back as the ice clears

I know the C85's are known for making ice but this sees quite excessive. I talked to my IA and he wasn't sure how to remedy it. I am wondering if anyone else in the tribe has experienced something like this? My IA suggested I get a carb temp guage to see what is going on better. May not be a bad idea.

[PA1195]

In my experience the use of winter blend auto fuel will permit carb ice to form ... this is only a subjective observation ... more than 100% 100LL. How much I don't know as I use a 20/80 mix of auto/100LL. Depending on geographic location winter blend has a greater tendency to vaporize and therefore enable starting in cold climates, while 100LL is kept at a standard as planes can travel quickly between climate regimes. So try 100LL and see if it helps.

As I mentioned before I use a manifold pressure gauge to detect (as ice forms it slowly lowers the MP at constant power and altitude) and then try to prevent ice from forming. I've also used carb temp probes and gauges to keep the carb venturi area from running at temps conducive to icing. I suppose an external hot air source aimed at the carb venturi might reduce it similar to Lycoming's design that heats the carb via warm sump oil temp.

Edit: Carb temp probes can detect temperatures conducive to the formation of ice...but won't by themselves detect ice that's formed. It takes something else like a loss of power (rpm, manifold pressure, or both) to detect ice in my experience.

If the engine's about to quit even with carb heat applied open the throttle and lean the mixture until it regains power. Carb ice reduces intake air flow which creates an overly rich mixture which doesn't burn well.

Gary

[Kerbs]

Thanks Gary. I am running 100LL only right now. I may need to go the MP gauge route thinking it through because I do not have an electrical system and I believe a carb temp needs a 12v source.

What I am trying to figure out though is why my engine woudl be making ice so aggressively at high RPM. Would anything with baffling, etc cause such an issue? I know you can make ice at any RPM and any temp, however since the engine transplant, I basically make ice at every RPM setting other than full throttle.

[PA1195]

Hi Stu. You may have mentioned earlier but what carburetor do you have? I suppose it's possible some are more prone to icing than others but I'm not an A&P or expert on carb ice versus carb models. Is your carb heat box flapper valve tight as possible for a good seal when cold or hot air is needed? Port Townsend WA - 0S9 is close to the ocean. I suspect your humidity on a normal day is higher than you may realize (https://aviationweather.gov/adds/taf...AFs+and+METARs)

You have a new engine and it's likely tight and has good volumetric efficiency...it'll efficiently pull air through a carb and maybe better than an older tired one. More flow = more ice?

You could explore with your mechanic some way to direct or duct hot air off a cylinder to the upper carb and induction spider area. That may be a way or reducing the icing. Also make sure cold cooling air isn't blasting from the front of the cowling onto the carb area. Maybe that's confusing but there may be some way to warm up the carb externally.

Gary

[Scott]

Perhaps post some photos of your engine, cowling, baffling, air box etc.
Do you have the "chin scoop" open on the nose bowl? If so, how's your crankcase baffle configured?

[PA1195]

55 years ago when I was in auto tech school we'd routinely have carbs make ice. Some could actually form a thin layer externally. The auto manufacturers fix was preheated intake air off the exhaust manifold controlled by bi-metallic temperature sensitive springs, vacuum operated intake heat valves, but more likely intake manifolds heated internally by exhaust gas passages or hot engine coolant. It's been a problem since the inception of internal combustion engines.

I suspect carburetor design - venturi, throttle plate, fuel nozzle - may play a role. Does anyone here have knowledge or experience of Stromberg vs Marvel carbs and icing? I have limited experience with Strombergs. Marvel's will ice but it hasn't been a major problem for me in a relatively dry Interior Alaskan climate. The worse time is Fall when visible ice/rain/wet snow are present. Never had one quit so far as long as I keep an eye on the MP gauge and tach.

Gary

[PA1195]

Here's a helpful site that discusses Stromberg carbs. The Newsletters section is a good read especially about carb icing: http://thestrombergspecialist.com/index.php

Gary

[Robert Lees]

I'm following with interest, because it's all a bit strange!
We all know that the A- C-and O-series Continental engines are icemakers, but Kerb's situation seems excessive.

I have no suggestions (the ones posted so far seem very sensible) but there must be some other influence that is causing Kerb's problem.

Perhaps verify that there is no excess water in the fuel? (I know this would normally cause engine stoppage rather than ice formation). To do so, use water-finding paste on a wooden stick...your airport fuel checkers use the same every day to check for water in 100LL and JETA1 tanks.

Here's an example: (and yes, this is an Australian accent)

Rob

[n6346m]

When I lived in Western Washington, my 1962 Chevrolet Corvair Rampside pick-up would develop carb ice regularly. With no carb heat available I would simply pull off the road and let the residual engine heat melt the ice and then be on my way until it happened again. I finally fixed my problem by regularly adding a bottle of "HEET" to the gas during the damp season and had no more problems. I know that "HEET" is not an approved aircraft product and it is mostly isopropyl alcohol. Basically I was running de-icer fluid through the carb.

Bob Picard
Anchor Point, Alaska

PS: for those who are too young to know, the Corvair engine is also air-cooled.

[Kerbs]

Thanks for the input guys. I have a freshly overhauled marvel carb. Everything is overhauled or new firewall forward. I will take some pictures of the baffling etc this afternoon when I go out to the hanger.

We are right by the sound so moisture in the air is common, however this week we have nice high pressure and lower moisture than most days I fly. All that said, I can't help but believe something is making my ice prone engine really make excessive ice.

I don't believe I am getting excessive blow by air into the lower compartment but will take a look, that could make sense. The engine is tight and runs really strong so I am sure it's pulling a ton of air through the carb. For the baffling, I reused the baffling from my a65 but modified it where needed to fit. I went through the carb heat box when it was all apart and it was closing tight. I am getting pretty good carb heat off the luscombe exhaust thankfully. There is not a ton there for a muff but it's working to clear the ice.

The idea on routing some warmer air to blow on the carb was one of the ideas my IA had after talking to him yesterday, but he was also pretty unsure why I was having the issue. (That said he was elbow deep installing a new oil heater in his hanger so our conversation was limited.)

Scott - I do think I have the chin scoop if that's where the air filter is. I'll take some photos.

[PA1195]

Thanks Stu for the feedback and dialog. What is your source of fuel? A personal bulk tank or commercial vendor? If interested put some fuel from your plane and commercial source into jars (like canning jars) and place them in a freezer. When cooled below freezing is there later visible precipitation of ice crystals in either or both?

It's not uncommon to have a plane's fuel vent system ingest and condense/accumulate water from vapor during daily swings in temperature. The commercial fuel source should have quality fuel but any abnormal moisture absorbed into the plane's tankage or commercial sources can freeze out and cause icing problems in the plane's fuel system. Autofuel is especially susceptible to water contamination.

Common fiber matrix fuel filters can pass small frozen ice crystals for example, and unless water block filter (typically with a cornstarch based filter element that expands and stops flow) is used, there can be dissolved water passage.

This is a wild card comment but worth eliminating as a potential source of carb icing per Robert's post above.

Gary

[Robert Lees]

Kerbs, may I suggest you check your intake rubbers for leaks (I'm sure this has been mentioned before)

[Kerbs]

I went and flew this afternoon, Conditions were the same as my last flight. I climbed out and after one touch and go in pattern and leveled at about 2k feet. I was able to run at 2,400rpm with no carb heat for a good 10 mins or so. I flew to the end of the valley, turned around and was heading back when I heard the engine start to stumble. Immediately pulled carb heat and stumble cleared and I got power back. When I tried to turn carb heat off, the engine saw a decline in power almost immediately and then started to stutter. I flew another 15 mins and was never able to take the carb heat off regardless of RPM unless i started to climb at full throttle. Talked to my IA and he is baffled, told me to double check part numbers on the carb etc to make sure something isn't out of whack.

Attached are some pictures of my baffling, etc. I can get some more tomorrow when I am out there if there are specific things to look at. I did notice that there is no baffling from the cowl around the intake filter. I can see light from the inside and in one picture you can see my finger looking from the inside. I'm not sure if enough air would make it around to make a difference.

Carb ice aside, climb performance with the C85 and a CM7144 prop continues to blow me away.

I attached some pictures i took of the baffling, etc. I am running 100LL from the FBO's commercial tank. I haven't talked to anyone else who is having issues with water in it and nothing is showing when I sump.

Robert - Not sure what you mean by intake rubbers? Do you mean around the filter because I got nada in that case.

Thanks for all fo the input guys, really appreciate it.

[PA1195]

Hi Stu. I wonder why you don't have baffling from the top of the cylinders/valve cover area that goes up to meet the top of the cowling. Wouldn't cooling air spill sideways and down around the valve covers instead mainly flowing down through the head and cylinder fins? Nothing to do with carb ice just an observation. Every plane I've owned or flown has had that cylinder head area sealed with baffling.

When you fly what happens when you lean the engine when it starts to run rough (you assume the roughness is from carb icing correct)? Does it get worse or better when leaned with the carb heat off? Robert's question pertains to the sealing rubbers for the induction tubes I assume. If there's carb ice causing the roughness onset it may improve with a leaner mixture as intake air flow is reduced by ice and there may be too much fuel for the air through the carb.

If there's a leak in the induction system (maybe around the rubber tubes or elsewhere) or not enough fuel for the air flow it will run lean and stumble...further leaning by the mixture control can make that even worse. If so (there's induction air leaks) then applying carb heat reduces the incoming air density which richens the air/fuel mixture and can make it run better. Is it possible you are running lean when it's rough and carb ice is secondary?

The front air leaks are minimal but some do seal around the air filter with rubber baffling.

Others here with A&P/IA know more about these matters.

Gary

[Kerbs]

Gary - First I don't know how I missed the area around the cylinders. I reused the baffling from my a65; it was never there to begin with and it had not occurred to me. I see exactly what you are pointing out and am guessing the air would go right over as well. I am going to enclose it as it should be. Thank you for pointing it out.

What you are explaining with the mixture makes sense. I understand what Robert is saying as well now, I had not considered a leak in the induction tubes. I will check everything when I am back out there tomorrow or sunday. I did have to drop the intake manifold to put the baffling so there is a possibility that I did not get one of the tubes sealed back up correctly.

I have not tried leaning it when it started to stutter, mostly because i have been afraid to lose the engine and just pulling carb heat, but it sounds like it is worth exploring to see if it makes it better or worse to narrow things down.