Carb Heat - Gimme a number
I'm looking for info on what is "normal" RPM Drop for turning on Carb Heat.
What is your RPM drop ?
Background:
C-172E, Continental O-300D, carb temp gauge, MA-3 carb
Engine MM shows "see A/C MM for guidance
A/C MM says "RPM Drop, then rise"
No numbers or number range that I can find in any manual.
Mixture is correct, Idle RPM correct.
I'm getting 200 RPM drop and rough running engine with carb heat on, which I believe is excessive. Carb temp goes up quick. RPM drop is about the same on both run-up and flying.
I'm thinking something (rigging,scat hose problem) is restricting hot air flow, but I need a "normal" go go by.
Thanks, A.J.
Comments
Aj, well it sounds like you getting massive quantities of heat without any issue. When you get the roughness try doing a little bit of leaning you really shouldn't have roughness at all and 150 drop might be a little more normal. Obviously with a temp gauge you can make sure you're not actually getting into the icing range by adding carburetor heat. Sounds like you just exaggerating a rich mixture condition to begin with. I'm not sure it's a problem especially on a continental with a very interesting runner system. On my 172n I only see about a hundred if I'm lucky.
Just as a side make sure the muffler that is supplying the carburetor heat does not have a leakage issue like carbon monoxide. On my N model I'm surprised I get any heat at all as it just takes it from a little cuff around an exhaust pipe. You have an entire muffler system supply and heat to that carburetor under engine
Carl
IA 47yr A&P DAL A/C Inspector 172n
Thanks for the response Carl. I have an electronic CO detector and everything seems ok when I turn on the cabin heat. (I believe its the same source). I'll check the muffler properly later. As far as the carb temp gage, when I turn on carb heat, it goes right up to about 25c (way out of icing range). I will be doing more diagnosis this weekend so I'll know more.
Just a little background. I'm a wannabe A&P. At 70 years old, I went back to college and I'm in last semester (powerplant) will be taking my O&Ps in December. Bought the 172 two years ago. Low time Commercial pilot (about 1370hrs) mostly helo. I'm gonna check with my instructor on his thoughts when we get back from fall break Monday. Just wanted to do as much research as possible before then. I was looking for more of a baseline but you are the only response so far. Thanks again. A.J.
Hi AJ
I would be a little more worried if you were not getting any rise, take a look and see when the last time the carburetor was looked at. you seem to be running really rich.
Good luck in December where are you located? Carl
IA 47yr A&P DAL A/C Inspector 172n
I'm pretty confident the idle mixture is correct. RPM set to 1000, I'm getting about 25 rpm rise as I go from full rich to idle cutoff on the mixture. Regarding the last time the carb was "really" looked at? The annual was 8/25 but who knows. I've invested almost 2 years at Somerset Community College (Kentucky) and learned a ton ! And yes, I'm well aware its just the tip of the iceberg. A&P will be my license to learn. I am a retired Chief Deputy Sheriff and after 36 years of service, I have trouble trusting anyone to do quality work these days (if you can get them to work at all). That's the main reason I went back to school at my age. We'll see tomorrow when my wife lets me go to the airport. LOLOL
It's excellent to hear that you're going to the a&p school. That really is the only way to do it and you'll have a lot more knowledge than people who just do it by experience alone. My hat's off to you.
Don't get me started on quality look at my bio.
Your idle RPM should be no more than 750 it'll make a big difference on how you're able to land. The rich range I'm talking about is more in the mag check range which is above 11 or 1200 RPM. That's more a function of float level.
Congrats on being able to retire. I did the same last year after 36 years in the airline work. Carl
IA 47yr A&P DAL A/C Inspector 172n
Hi Carl. I may have been unclear in my last post. My idle RPM is about 550-600 like the maintenance manual (MM) says. I was saying the idle MIXTURE was set using the procedure in the MM which says: "warm up to operating temp,run up to 1000 RPM then pull mixture slowly from full rich to Idle cutoff. There should be about a 25 RPM rise before cutoff. more rise=too rich, less rise=too lean." Mine is right at 25 rise. (I wanted to try to be clear for other readers). I'm headed to the airport now to diagnose the problem further. I'm still "leaning" toward (pun intended) a rigging problem in the air box. Acording to the MM, I may have a 2 cable arrangement with 2 flappers that control the airflow. I'm betting one of these is out of rig but we'll see. Side note, here in KY, if you are over 65, you can go to college free except for books. How cool is that ! Cheers, A.J.
I like that college idea AJ. That's great you're able to take advantage of it.
Yeah with your explanation it sounds like you're idol and mixture are okay. Carl
IA 47yr A&P DAL A/C Inspector 172n
Note that the O-300 has 2 mufflers. the right side [ co=pilot] is cabin heat. the left muffler provides carb heat.
As Planewrench noted if the muffler is leaking exhaust gas, it's very hot and can cause the engine to run poorly.
Very good possibility guys. Was only at the airport for a little while today. Got the cowls off and took a quick look at the cables and levers. All good there. Will have to wait until Monday to continue. The airbox has the old two flapper system.
It appears that both left and right muffler shrouds supply CABIN heat (when carb heat is off) and right side only supplies CABIN heat when CARB heat is ON. CARB heat is only supplied by the left side.
Correct?
Yes
IA 47yr A&P DAL A/C Inspector 172n
Try leaning the mixture when the carburetor heat is on. You should be able to get most of the RPM back by leaning it. By “back” I mean that if you’re getting a 200 RPM drop it means you have a very rich mixture (not a bad thing) and when you lean the engine with the mixture control you will be getting closer to a “normal” mixture and the RPM will come back towards the 1700 RPM that you started with. If by doing this the engine quits stumbling and the RPM comes back towards 1700 RPM that you started with, then the system is operating normally. If in flight and the engine stumbles in the pattern after turning on carburetor heat for landing it is acceptable to lean a little bit to get smooth operation.
David
The only times I’m at full rich is at start and takeoff.
Thanks for the suggestions. I tried leaning with carb heat on, while testing on the ground and I can get it right back to normal, meaning carb heat on, is making it super rich either by an air blockage or as planewrench and jimh suggest, a possible muffer leak sending super hot air to the carb. Since I get a really fast rise on the carb temp gage, I'm thinking the muffler leak is very possible. I will be checking both of those for sure. Especially a muffler leak which could be very dangerous this winter.
I don't feel comfortable leaning in the pattern at lower rpm and in the case of a go-around, I want to be full rich. I do make it a habit to turn off the carb heat on very short final so I don't have to think about it on a go-around. But I do appreciate the input.
No air blockage. Moving on to muffler leak...
Update: Muffler pressure checks good. Running out of ideas.
Aj, you may not actually have a problem as I said before you are getting good carburetor heat which is better than no carburetor heat. As long as the engine is making takeoff power RPM accelerates fine and your mag drops are reasonable., then it sounds like the engine is working fine.
Without getting too involved other things that could affect the mag drop are magneto timing and condition the spark plugs as well as individual cylinder compressions. Also some engines are set up where one magneto runs all the top and one may need to run all the bottom plugs instead of splitting top and bottom. That definitely can affect what you see for a mag drop when you do a mag check..
When you're cruising around the area pull the mixture back and see how far you can go before your RPM starts to drop. Will give you a good idea of the settings in the carb.
Carl
IA 47yr A&P DAL A/C Inspector 172n
Everything else is good, mag drop, takeoff power, compressions (mid to high 70's with proper orface),cruise leaning etc... all well within spec. Great news for an engine thats 144 over TBO..LOL
The ONLY issue I'm having was/is what I believe is excessive RPM drop and roughness when using the carb heat. Even my instructors haven't got it figured out yet (so I don't feel so bad). I've got one more "hail mary" to try this weekend. Then I will be out of Ideas. P.S. pressure testing my muffler was good experience for my class. We we supposed to do that next month but my powerplant instructor did it now to help me. My instructors (most IA's) have been pretty amazing and bring a ton of practical/real world experience to the table.
Good to hear AJ yeah it may not be an issue at all I'm not overly concerned about it as long as everything else is normal.
Are your instructors anywhere close to my 50 years? Lol Carl
Show them a couple of the articles I've written in the last past year.
IA 47yr A&P DAL A/C Inspector 172n