Carb Icing?

[Kerbs]

I think I had my first run in with carb ice today. I never had an issue with the A65 however I am still learning and breaking in the C85. I was flying at around 2,000 rpm and did an in flight mag check. Right mags great, left didn't sound good. I did the check a second time and when I switched back to both, I thought I was losing the engine. I then immediately pulled carb heat and RPM came up. I then added full power and flew with carb heat on. Everything seemed fine and subsequent mag check with carb heat on at full power showed no issues. It was a nice clear day, about 60' on the ground and 50% humidity. I initially thought it was a mag issue but the carb heat saving it makes me thing it was ice. I have heard the C85 is more prone to icing. That being said I am still a little surprised.

[Hank Jarrett]

Should have been an immediate slight RPM DROP with carb heat followed by a slow rise back to normal. Anyone else had a link mags and carb ice? Haven't ever heard of that before. I would still be suspicious of a Mag, plug lead or plug problem that just happened to clear at the same time as carb heat was added. Just to be totally honest, I would have pulled carb heat too with those symptoms. Couldn't hurt, right?

[Kerbs]

Thanks Hank. I don't want to rule out something else in the ignition system, especially after having just gone through a FWF overhaul.

[3Dreaming]

It could be a valve sticking issue that changed with the change in temperature. If you are not running any MMO I would start.

[N96337]

The 85 IS an ice making machine...at least mine was! A 60* day would be about perfect, as I think they say it's a 30* drop across the venturi?
John

[PA1195]

At 2000 rpm your throttle plate is more closed than at full power. Any ice there will have a greater effect on restricting intake flow when at low power.

Check first but I believe the right magneto fires the upper plugs and the left the lowers. If that's the case then it's not uncommon to have one or more bottom plugs fired by the left mag foul during times of higher oil consumption, such as break-in (?). Adding full power may have cleared the plugs as Hank suggests.

Have a look at your spark plugs, especially lowers, to see if they are getting oil and fuel contaminated. Also look at the bottom of the piston face through the spark plug holes. As the rings seat the lower piston face will become less covered with oil.

If you have room install a 2 1/4" manifold pressure gauge (taps into the induction spider or one cylinder). It's the first and best indication of carb ice when flying with fixed throttle at a steady altitude...ice and intake restriction causes the indicated pressure to drop before the tach does (which typically oscillates a bit). It also can be used to help diagnose cylinder/piston seal, intake leaks, valve sticking, and restriction to full power. I can explain if interested.

Marvel Oil is the elixir for valves.

Gary

[Hank Jarrett]

I think a post on using manifold pressure to troubleshoot and diagnose cylinder/piston seal, intake leaks, valve sticking, and restriction to full power would be great! Would you consider opening a new thread on it? If it is posted here it will be a BEAR to find in three or four years.

[PA1195]

I can start a new thread on the topic but in the meantime it's nothing remarkable or new...manifold pressure gauges (MP) indicate close to the reciprocal of the common vacuum gauges (VG) used in automotive engine diagnosis for decades. Edit: It's simply that High MP = low VG...low MP = high VG.

Read about VG and their applications. Here's some quick examples: https://www.motor.com/magazine-summa...-vacuum-gauge/ or https://www.gregsengine.com/using-a-vacuum-gauge.html

The MP reading we observe is calibrated air pressure (typically inches of Mercury - inHg but there's other values available) in the engine's air induction system prior to the cylinders. VG measure the same location, except they read the amount of vacuum not pressure in typically in similar units of measure. MP readings begin at the local ambient air pressure and decrease their values as the engine starts, runs, and climbs while VG's start reading at "0" vacuum and increase. Like MP they are also affected by local air pressure. That's true for both unless complicated when the engine's intake pressure is boosted by a super or turbo charger.

For example before startup a MP may read 30 inHg while a VG will show 0 inHg. When idling with a closed throttle and a tight engine the MP can read 10 while the VG may read 20. While flying in cruise with the throttle more open (which increases intake air pressure and lowers the vacuum value) the MPG may show 24 while the VG may show 6 units of vacuum. And at full throttle the MP may show 28 while the VG shows 2 for example.

In my experience MP gauge movements are more dampened, I assume by either a mechanical means or by restricting the flow of air in and out via an orifice restriction in their feed line. Vacuum gauges tend to respond more quickly to changes.

Gary

[PA1195]

Here's how I use them. Subject to review and any comments.

Calibrate the MP gauge by setting your altimeter to "0" feet (might be hard at a high elevation airport). If the altimeter is accurate the current air pressure will be reflected in the air pressure (Kollsman) window. The MP gauge should reflect that value - MP 30 inHg = Altimeter's 30.00 inHg. Note that static MP value for later. Most of what we do below only requires relative changes but in setting power we should expect accuracy.

Start the engine and while idling note the MP. It should eventually lower due to more vacuum as the cylinders warm though tighter ring seal if the rpm is held constant. It should remain steady but again system dampening may mask variations between cylinders. Typical reading from a tight low leak engine can be around 10 inHg +- and a minimal value can be found by slightly varying idle rpm. Write it down in your logs for later use as the engine wears out and/or leaks appear.

Note the MP at a normal runup like 1700 rpm. It should smoothly increase above idle and remain steady unless intake ice is already forming when it will show a decrease. Try carb heat to restore the higher reading and as part of the normal runup test.

Takeoff full throttle and safety note the MP. We want it to be as close to the pre-running static value as possible. It's normally lower by an inch or two due to air filter and inevitable intake restrictions. Any cylinder wear and leakage can lower the value as well. The higher the takeoff MP the more power produced at a given rpm. It will vary some depending on density altitude that day. Compute DA and convince yourself. As the engine wears out it will gradually decline under the same takeoff conditions.

In level flight with a fixed pitch prop set rpm and note MP. If intake ice forms it will decrease. Apply carb heat and hopefully will restore the MP when the heat is removed. If not and you're in visible icing, snow, or blowing dust the air filter may be contaminated. That may require carb heat or an alternate air door as a source of intake air if more power is needed.

The A-65, C-85, and O-200 Owner's Manual suggests a MP limit of 26.8", 24.5", and 24" respectively in cruise at sea level. Depending on what prop is installed (diameter and pitch) that may or may not be reached before a desired cruise rpm is set. If interested in engine performance at altitude the percent power from combined MP and RPM plus Density Altitude can be determined from Continental's Owner's Manual - own one.

Write down for future reference in your logs what's been noted above as normal...later if there's marked changes such as engine vibration, lower power, or increased oil consumption have a look at the MP gauge for visible changes or movement (see Post above) and share the info with maintenance. I avoided variable pitch/constant speed props as they are typically not Taylorcraft equipment and by themselves can influence MP readings at a given rpm.

Gary

[Scott]

Back to the OP, It sounds like the in-flight mag check was done with carb ice present (but perhaps not detected).

With both ignition systems on, you simply have a higher probability that combustion will occur. In a situation where the air/fuel ratio in the combustion chamber is approaching a non-combustible mixture (whether too lean or too rich), turning off one ignition system is likely to cause rough running. Nothing wrong with the ignition system, it's just that you've decreased the probability of combustion by halving the sources of ignition.

The only really effective way to detect early stage carb ice is to... wait for it... apply carb heat. This is because there are simply too many other variables in flight.

So unless you have a carb temp guage and you operators manual states to maintain a specific carb temp, you should as a matter of course, apply carb heat on a regular basis in cruise flight. If no icing is detected, decrease the frequency of application, if ice is detected increase the frequency.

[PA1195]

I had carb ice in my previous PA-11 with 90 Continental many times. Especially this time of year (Fall) with cooler temps and higher humidity/fog while still flying on floats. The MP gauge would drop from about 24 to 23 then 22" but never ran rough. Mag check in flight showed a normal drop on both of about 50-75 rpm. Mags were dual Bendix S4LN-21 and carb a M-S. Applying carb heat melted the ice and when off the MP returned to preset values. On a day conducive to icing the the MP would drop every few minutes, typically less than 5. In those circumstances continuous partial heat plus high cruise power and MP was required for safe flight. It's a compromise of carb heat and added throttle to maintain the desired MP and power/rpms.

If I had a bad mag drop on one mag in flight, especially the one firing the lower plugs, I'd have the mag-harness-plugs inspected at the earliest opportunity for proper function. In the case of spark plugs during break-in I'd keep an eye on them for oil contamination due to poor ring seal that typically improves with time.

Also Stromberg carbs may react differently to icing than the M-S type. I don't have enough experience with Strombergs in that situation to note any differences.

Gary

[astjp2]

Carb heat can also induce carb ice...if you use carb heat in very cold temps, it will put you right in the temp to induce the icing. I posted on this years ago...

[Jim Herpst]

I would check every thing EVERYONE suggested above. I once had a Cessna 140 with a C85 that it got to a point on EVERY flight you would get a rough engine...pull carb heat and it would correct withOUT getting initially worse as you would suspect it should as the water clears. In fact this would happen multiple times throughout the flights in all temps. Clearly another issue. One suggestion was a hairline crack in the venturie to wrong size items in the carb. The engine needed a top and I was done with it at that point any way. Cessna with low power engines suck in South Carolina summers to top it all off. Was glad to sell it. All that said my bet is when you get your issue resolved you will love the 85 in the T!

[PA1195]

I obviously use a MP gauge in these carbed engines - Lycoming and Continental - but have experience with carb temp probe and gauge in a DHC-2 Beaver I flew. The temp gauge can help especially if air pressure driving the MP gauge reading is changing via variable power and altitude or rpms. There's nothing wrong with more info.

Here's some examples of carb temp gauges: https://www.aircraftspruce.com/menus...torgauges.html

Gary