Engine trouble over NJ today

[Bruce Fenstermacher]

As the subject describes I had a little engine trouble over NJ today.

Flying in a formation of 4 aircraft of which one was another 170. At 3500 feet in the clear at probably about 0C (not sure) I noticed my engine starting to run ever so slightly ruff.

I went full rich and pulled on the carb heat and got the normal reduction in RPM and a slight stumble in the engine then it ran smooth so I pushed the carb heat off. Almost immediately the engine ran rougher than before and I had a reduction in RPM of at least 100 lower that that with the carb heat on. I of course immediately reapplied carb heat and the engine smoothed out. I repeated this 3 times with the same result.

I announced to my buddies I had a problem left the carb heat on and we descended into Sussex NJ with out further incident.

Once on the ground the engine seemed to run fine with or without cab heat with the exception that there was no RPM drop with carb heat applied. We inspected the carb and controls and found nothing wrong. We checked and had a clear gasolator with nothing but gas.

We decided Sussex was an excellent place for breakfast so we ate and returned to the plane. The plane started and ran fine and there was now a reduction in RPM when carb heat was applied but not the reduction I remembered to be normal.

I decided to play test pilot a took of for home. Ran full throttle no leaning or carb heat for the 30 minute flight and no problem. Once over my home base at 7500-ft I started to to play with the carb heat, mixture and throttle to try to duplicate the problem but could not. The only thing that seemed odd was I didn't get the same RPM drop with carb heat applied as I remember in the past.

Carb Ice? I can't believe it. I was flying 75 feet in trail of the first 170 so we were both in the same air and conditions. We both have the stock c-145 with stock induction box and carb heat. We both lean using the same method and don't lean aggressively.

The only differences in the planes are he has pancake mufflers and I have Hanlon-Wilsons and he has a climb prop operating about 2400 RPM and I had to be throttled back to around 2300-RPM. For this flight he happened to be running a mixture of about 50/50 car gas and 100LL and I was running straight good stuff (car gas:D). He had no problem with carb ice.

I should also mention that the last 4 times I flew the plane I had to fool with the carb heat to clear what I thought was carb ice. All of the previous flights where in the same temperature conditions but also in the clear except for once when there was some moisture. All of these times where uneventful after application of carb heat for a minute or two. I don't know if these flight have any relevance to my last flight.

Any thoughts.

[N1478D]

No expertise here Bruce, so it's not worth much - just stating what I would look at first. The mixture cable and what it controls. The logic, or illlogic, is because improper leaning, or the inability to properly lean can make an engine run as if there is carb ice.

[Jr.CubBuilder]

Could there be a leak or some problem with the scat tubing coming off the muffler?

I got a big slug of carb ice a while ago that scared the crap out of me. I had been foating along fiddling with the mixture because it seemed like the motor just didn't feel quite the same as usual. When I got in the pattern and pulled the heat my motor goughed and gaged while my heart quit beating, then it ran all nice and smooth again.

[GAHorn]

One of the puzzling things about carb ice is it's un-repeatability.
I'd suggest you thoroughly inspect your carb heat system. Check your scat/sceet hose for internal collapsing, and also your muffler shroud for blockage (insect nests, or evidence of recent infestation which went thru your engine.) If you don't get a drop during test of your carb heat system, then it's not working correctly. Either no heat, or no actual movement of the butterfly is occuring. (Possible slippage on the shaft in the air box.) Remove the air filter assy, and put your finger against the butterfly and physically apply resistance to determing the cockpit control actually moves the valve, ....AND....moves it fully and in the correct direction. Be certain no obstruction is within the muffler/shroud which may block the movement of air thru to the air box. (I've had CH test just fine on the ground and then got to the next inspection to find a huge dauber nest blocking the SCAT hose completely. No heated air ever got to the air box, but the restriction made it appear fine in the cockpit. Also make certain that any sceet hose that is double-lined is not collapsing the internal liner. (I never use sceet in that application for that reason.)
I believe it possible you've had an encounter with carb ice, and your CH system may also be malfunctioning.

[Bruce Fenstermacher]

The scat tubing is all new. A friend who's opinion I respect was riding in the other 170. He is now reporting that he believed we passed through some scud in it's early forming stages and that he thought the 170 he was riding in was running a little ruff but dismissed it to not having the aircraft leaned as well as it could be. In any case I did not see the scud and had I left my carb ice on for a much longer period of time, which I would have done had I seen the scud, we wouldn't be having this exchange.

Boy-oh-boy one thing I miss about my first airplane which was a Cherokee with a Lycoming 0-320 is the fact that in 800 hours of flying it in all sorts of weather I NEVER have an indiaction of carb ice

In fact I had just about wiped it from my brain matter when I bought my Cub . That is when a friend told me in no uncertian terms I was now flying a Continental engien and I'd better figure out what the carb heat was for and when to use it.

[Bruce Fenstermacher]

George I believe you are correct in that it was carb ice. We did visually inspect the workings of the carb heat system as well as we could at he airport before I left for home and the CH system will get a much closer look after the snow melts that feel today.

One puzzle I need to figure out is this. On takeoff the carb heat check developed the normal RPM loss yet at the airport after the incident carb heat had no effect on the RPM till after breakfast and then not what I'd expect. The CH cable is working and the lever and shaft of the butterfly is moving and we can hear the butterfly moving inside. Perhaps the butterfly is not moving as it should as you suggest. Further inspection will tell.

[Bruce Fenstermacher]

Boy I sometimes I wish I had a turbine like my work helicopter.

[GAHorn]

Yeah....you'd REALLY like the fuel burn, too!
Carb ice is a funny thing. You can fly for years and not experience it. And you can then get a good case of it in clear air! (Scud doesn't necessarily make carb ice. Carb ice, in my experience, occurs most likely on a clear day.) In the last 4 years I've ever had any in my 170, and yet in another's 170 it happened twice it two days. It's really unpredictable, and that's the way I teach avoidance. Pull full heat before any power reduction, and watch the tach....then remove the carb heat till the next pwr reduction. Keep an eye on the tach and MP if you have one. Full heat again at the beginning of the approach and check rpm drop/rise again. Then off for the approach. (But many instructors make their less experienced students leave it on fully any time pwr is below green arc. I've got no problem with that, but it's a less-than-full-understanding approach to the problem.)
True, LYcs have fewer carb ice problems due to the nature of routing of their intake induction systems in a warmer environ.

[Roesbery]

Not likely a factor in your current experience. But something to keep in mind is the chance of sucking a leaf into the carb throat when using carb heat, if a leaf gets into the muffler shroud while parked or???? Know of two people having that problem, one a Citabria and the other a super cub. Ran fine at low power settings but not at higher settings in each case.

[Robert Eilers]

I am currently flying a Champ 7BCM with an auto gas STC. I have found at higher engine temps the engine would act like it was experiencing carb ice. Use of carb heat cleared up the problem. However, I suspected the fuel was vaporizing just prior to the carb (as the fuel line passes by the left side muffler). After installing a heat shield on the fuel line the problem disappeared. Just two cents worth.

[HA]

I know many will not like to hear me say this, but car gas act differently that avgas - there are reasons they invented avgas. different vapor pressures, BUT output, blah blah.

so sometimes things just act funny, which probably meant that you were getting some carb ice - like you figure - and car gas is more prone to producing that problem. I like the thought about checking your fuel line routing, and I would definately check your carb heat flapper UNDER A LOAD to make sure something isn't slipping and that it gets full travel.

also, what kind of air filter do you use? I have a Brackett, but those are more susceptible to moisture problems too, and could be part of the reason for your not getting different RPMs during your checks. try it without the air filter element once.

[GAHorn]

I know many will not like to hear me say this, but car gas act differently that avgas - there are reasons they invented avgas. different vapor pressures, BUT output, blah blah.

so sometimes things just act funny, which probably meant that you were getting some carb ice - like you figure - and car gas is more prone to producing that problem. I like the thought about checking your fuel line routing, and I would definately check your carb heat flapper UNDER A LOAD to make sure something isn't slipping and that it gets full travel.

also, what kind of air filter do you use? I have a Brackett, but those are more susceptible to moisture problems too, and could be part of the reason for your not getting different RPMs during your checks. try it without the air filter element once.

I disagree with the technique of operating without a filter. It subjects the induction to a very real risk of FOD/ingestion.
How does the Brackett differ from other filters? It's less troublesome in visible moisture than a paper element (from a reduced airflow point of view. The chemical treatement inherent to a Brackett waterproofs it. I've got personal experience of engine failures in rain using paper elements, and that's why I'll never have one (paper) on my airplane.) It's more effective than a flocked screen, IMHO.

[Bruce Fenstermacher]

I run a Brackett filter. At this point I'll try a ground run to see to see if the filter may have anything to do with it but I doubt it.

After checking my carb heat system today I could find nothing wrong with it except I don't get the amount of RPM drop I remember. On the ground after a 5 minute warm up with the OAT about 38 degrees at 1600 rpm I get about 100 RPM drop.

Now I know there is 3 different muffler/shroud/carb heat settups for our stock airplanes depending on your aircraft year.

I'm currently running 2 Hanlon-Wilson mufflers with both muffler shrouds plumbed with 2" scat (single wall) to the carb heat union (0550119 170A IPC).

I changed to this setup from having just 1 Hanlon-Wilson muffler shroud plumbed to the carb heat box with part #0450314 (170B IPC) in hopes of getting more heat in the cabin.

I think I remember getting nearly a 300 rpm drop with my current setup untill now. But maybe I'm dreaming.

So what kind of RPM drop should I get? What is normal?

BTW the 3rd carb heat setup would be to the pancake mufflers instead of the Hanlon-Wilsons with my current (early) setup.

One more thing HA. It's not the car gas but I do appreciate your ideas.

[GAHorn]

There's no specification for amount of rpm drop to test carb heat that I know of. My airplane gets about 150 rpm drop typically....I think.
You'll get less drop if your air filter is dirty and needs replaced (or if your carb heat butterfly does not seal well.) It's not critical that a certain amount of drop be observed....only that one IS observed ...which confirms the cable/butterfly is working. (And that's all it actually confirms.)

[Indopilot]

Bruce Thought I would pass on a couple of things from my experiances in case they might help. One thing I always check is the flame tubes in the mufflers in case some thing has broken loose and partially blocked the outlet , Probably not in this case but lets me check off one more thing checked out.
About 18 years ago I had some roughness in the O-300 which I initially thought to be carb ice since I couldn't duplicate it. After landing everything checked out ok with mag checks, full power checks and whatever else I could think to check. Loaded the motorcycle I went to get, did another complete set of checks,Ok. Started T.O roll and got rough again. Turned out to be #5 piston, top ring groove went south and spit a chunk of ring into the combustion chamber, lodging in one of the sparkplugs.
Finally I have seen several Carbs which have gotten water into the bowls for various reasons. As a result a gelatous substance forms( dries to a whiteish powder) which eventually chokes out the carb. By the way if you are to lazy to push the plane 3/4 of a mile back to the hanger , you can taxi a piper Pacer using the primer .
Hopefully it was carb ice, but if not maybe some other things to think about. It sounded like carb heat richend the mixture but going back to normal (leaner) caused rougher operation. Brian