C150 Rough running..Advice please!!

[Vone Rotate]

Hi all, I'll try to keep it to the point!......

I passed my PPL in sep and trained in a PA28 which I loved and had total confidence in. As i'm hour building towards my ATPL I decided to get checked out on my schools C150. Its been sat for a while as no body uses it.

They got it serviced etc and I'm now the only one who flys it. (it also has a new engine with only 90 odd hrs).

What my concern is when I apply carb heat it always drops 200 rpm and runs like dog s£*t. I'm aware they are prone to ice so I use plenty of carb heat. probably a 20 sec burst every 5 mins or so.

What I'm trying to ask is do theses little machines just run poorly with carb heat applied or is it carb ice?? If it helps it always does it. Not now and then, if carb heat is on its running rough!

I ask as everytime i apply it im kind of looking for a feild into wind!!!
The piper PA28 hardly dropped 10 rpm and still ran smoothly.

Is it just a case of me being spoilt on the PA28 or am I right to be looking for that field??

Thanks people and safe flying....
Ben.

[Comanche250]

I know people reckon C150's are prone to carb ice but blimey!! probably a 20 sec burst every 5 mins or so. thats ever so slightly excessive imo and your fuel consumption will be higher as a result of using that much carb heat all the time!

If you have any carb ice you should get a drop in rpm, and when you remove the carb heat you will get a rise in rpm over what you had selected before you applied it.

I would say a drop of 200 rpm is a bit much but its no reason to start looking for a field, all the C150's ive flown have run rough with carb heat applied.

C250

[jamestkirk]

Just a stab in the dark: They generally do not run 'really' rough with carb heat applied. This sound like the mixture is going very rich when the heat is applied. But again this is a guess.

C150 are known for thier carb ice antics. Don't worry too much about over doing it a bit. I used to teach 'on for 10-15 seconds and monitor RPM'. 20 seconds?, whats that between friends.

If you are in known Carb ice conditions, High humidity, temp/dew point close together in the right range, base of cloud etc. , then every 5 minutes is OK.
Just adjust the time intervals for the meteorlogical conditions of the day.

Thease machimes are prone so if you have not already, take a look at the Carb ice guide in lasors. Also, winter'ish time of year is great for picking it up on the ground while taxi/waiting to take off so check again. Especially the
C150.

Even with the best intentions, I have had a couple of rough runs myself in 150/152's due to ice so air on the side of safety. Better be a bit over cautious than look for that field.

I have never heard of a C150/152 having an engine failure due to 'a fouled plug' due to using the carb heat too much . Or any CFI measuring fuel after someone has landed and saying 'your using carb heat too much'.

I have however read many AAIB reports about C150/152, PA28 forced landings due to suspected carb ice.

I have ranted on a bit but hope that helps

[sternone]

it also has a new engine

Do you know the type ? I train on a C152 with a Lycoming 110bhp (O-235-L2C) when i apply carb heat i get a drop of 75rpm, if you got a drop of 200 maybe you can check the engine compartiment if nothing is blocking the air intake before it get's air warmed ?

Also what is rough running ? do you feel it's missing some ignition strokes ?

You are right when you say your 150 is weak for the carb icing, better use it to much than to less, if you get engine faillure or REAL rough running i hope one of the first things you do (after flying the plane and selecting a landing spot) is apply carb heat OFF :O

[TheOddOne]

200 RPM drop? Good! It shows the carb heat actually works! Not like the woosy 75RPM drop on a PA28.

Well, now, all that the above does is to instil confidence that it's working, it doesn't REALLY mean that the carb heat on the PA28 won't melt your ice away, it just feels like it.

TOO

[JUST-local]

200 rpm drop is outside the limits in the flight manual/ poh!!!
I would say the aircraft needs looking at asap, sounds like exhaust gases are leaking into the carb heat collector not just hot air, if this is the case you are putting exhaust gases back through the engine - not good

Get it looked at before it flies again.

JL.........

[smarthawke]

Forward of the firewall the C150 and C152 are completely different aircraft so anyone comparing the 2 should think again.

The Continental O-200 (or sometimes O-240) in the 150 takes it carb hot air from inside a shroud around one of the exhaust mufflers (there are two, one for the left hand cylinders and one for the right), the other muffler shroud feeds the cabin heat.

The result of this set up is VERY hot air to the carb because of the large contact area inside the shroud.

On the Lycoming O-235 in the 152, the carb hot air is taken from a shroud around just one exhaust down pipe (#4 cylinder) and as a result isn't as hot as on the 150.

Very hot air will cause the engine to effectively run very rich and the rpm to drop a lot. Possibly a muffler leak but that is easy enough for the engineers to check and I doubt the engine would be running or sounding sweet with carb air in cold.

On a Piper Warrior, the hot air is taken from around the #4 down pipe like the 152, others from larger shrouds around 2 down pipes (Archer) or a long bit of exhaust heading towards the muffler (PA28-140).

If I fly a carb fed engine I check for ice very often and for at least 15 seconds (how long does it take a hot air gun to melt an ice cube?!) - do the rest of the FREDA checks with it in hot and give it a good burst of heat if you are held at the hold for long pre-take off.

Remember, low rpm equals a relatively cold exhaust and not as much heat to the carb.

Anyone that thinks it can't be that much of a problem, get hold of the monthly AAIB bulletins!

[flybymike]

As has already been intimated, hot air is less dense than cold air, which will result in a richer mixture. Set up the mixture control first using the usual technique and then apply carb heat. If the rough running disappears or improves then hey presto. I remember this technique worked very well for me in my dim and distant past C150 days.

[Whirlybird]

I had a share in a C150 for several years and flew it a lot, including over to France at least twice.

The C150's Continental engine is VERY prone to carb icing. I had obvious symptoms of icing (rpm getting gradually lower etc) at some time in every phase of flight except with full power for the climb. I used to apply carb heat for 30 seconds every five minutes, regularly. With high humidity, rain, or over water, I kept the carb heat on all the time. I checked the fuel consumption quite frequently and it never seemed to make any difference.

When I applied carb heat the rpm did drop quite a bit, though I can't quite remember how much. That seems to be normal. However, the one flight when it did that but also ran rough, we got it checked out and there was indeed something wrong...sorry, but I can't remember exactly what! So it might be worth getting the engineers to look at it if that's happening all the time.

When and how often to use carb heat is something that pilots will rarely agree on....ten pilots, elevent opinions on carb heat. :( But too much will rarely do any harm, and too little easily could...especially in a C150.

But please don't be put off flying it. It's a great little aircraft. We took ours everywhere, and in and out of some short fields that you'd never dare attempt in a PA28...and for a lot less cost. :ok:

[Mark1234]

Maybe the rough running is down to an excessively rich mixture..

One question nobody has asked - where do you fly?

If it's hot or high (high density alt), you may be into a regime where you should probably be leaning some; assuming you're full rich, you are ALREADY very rich without carb heat. ADD carb heat and you're so far rich of correct mixture it causes rough running. Maybe. You might find leaning the mixture some will resolve.

IIRC the POH suggests leaning pre-takeoff / at full power above a DA of 5000ft... but my memory ain't that good.

If in doubt, get it looked at..

[Vone Rotate]

Thanks for all the reply's folks..

I think from what people have been saying its fairly normal to get this drop. Think it just spooked me a bit as the PA28 you hardly notice any change with carb heat applied.
I'm going to keep giving it plenty of heat as like others said its unlikely to do any damage with too much....

I'll try leaning the mixture as a always fly with it rich. I fly around south england by the way.

I should be flying again this week i'll post any changes.
Thanks again everone.:ok:

[ericferret]

If this is a mixture fault then you will be masking it by leaning in flight.

I would suggest getting the basic carburettor setting checked.

[flyme273]

I concur with the rich mixture theory (though it could be a fault)

Next time flying as part of the diagnostics try carb heat on and lean it aggressively - see if it runs smoothly.

Of course you should lean in the cruise.

[jabberwok]

The C150's Continental engine is VERY prone to carb icing. I had obvious symptoms of icing (rpm getting gradually lower etc) at some time in every phase of flight except with full power for the climb.

I flew your C150 many years ago at Cambridge and had classic carb icing in a full power climb one morning. Carb heat was applied but it took a while to get rid of the stuff.

There was no extended time on ground before departure - no icing was noted during the engine run - and full power was giving healthy rpm on take off. As we climbed through 1500ft rpm started to fall off very quickly indeed and carb heat applied. At 2500ft the icing stopped and carb heat was cancelled. I recall it was a damp November morning but no different (to my eyes) from the previous few days when flying was unaffected.

It is the only time in 35 years that I experienced icing on take off. It's obviously rare - but it can happen.

[rtl_flyer]

I had a C150 until recently. Applying carb heat would drop revs and sound rough - common on the O-240 engine I understand.
When the engine was leaned applying carb heat would result in rpm drop but not anywhere near as rough running.

[spernkey]

Smarthawkes post has it QED'd. At some point in the future (when we all have better more efficient power plants) all the old O-200's removed will be recycled and used as ice makers. This will be a role for which they will at last be unsurpassed in their efficiency!

[ericferret]

Our 150 had exactly the same problem. On its annual we discovered that the fuel priming pipe was damaged and leaking. The pipe was replaced. We flew it for the first time today and the rough running has disappeared. Smooth as a babies bum!!

In flight the leak would have allowed extra air to be drawn into the manifold giving a weak mixture. .

[Croqueteer]

:)Check if your carb has a one-piece choke tube, If so, then a pepper-pot jet might cure your problems.

[poss]

Our 150 had exactly the same problem. On its annual we discovered that the fuel priming pipe was damaged and leaking. The pipe was replaced. We flew it for the first time today and the rough running has disappeared. Smooth as a babies bum!!

In flight the leak would have allowed extra air to be drawn into the manifold giving a weak mixture. .


I would have said the primer as well having experienced this myself.

[timzsta]

We had just such a problem with our C150 at my club last year and it took some time to get to the bottom of it. Symptons were as you described. It was finally resolved as overly rich mixture setting and an adjustment has solved the problem. The engine still runs rougher then in a C152 with carb heat applied but not excessively so (so as to make the whole instrument panel vibrate) as it did prior to the adjustment.

[S205-18F]

I was going to ask if you had leaned the mixture and then tried the carb heat was the RPM drop less if so it over rich! Due to to the warm air being less dense therefor changing the mixture to a rich one! Be careful if you get an over rich mixture with carb heat and then put on fuel pump on the approach you risk a rich cut which will be almost impossible to restart from, and fouled plugs are also a great risk too! I would suggest you get the engineers to look at the mixture sooner rather than later!!

[wigglyamp]

The chap flies a Cessna 150 - gravity fuel feed with no electric pump, so rich cut on approach with pump on is a non-starter!

As for suggestions of leaning before applying carb heat, certainly Lycoming suggest leaning only above 5000 feet - I would have thought something similar woukld be appropriate for a normally aspirated Continental.