Carb Heat Operation

aarvig · #1 02-17-2010, 02:24 PM

Ok, this is not an RV installation, it is a 1969 Cessna 172K with an O-320 150hp engine. I am looking for some good advice. My question is this. About a month ago when I would pull the carb heat I would get a 100 rpm drop. 2 weeks ago I pulled it and got nothing. The plane is a club plane. We put a squawk in and immediately the plane was grounded. Our mechanics pulled the carb heat mechanism and repaired a broken butterfly valve (it had fallen off) by riveting it back on. I went out to fly it and lo and behold, pulled the carb heat on the runup and nothing. Called the mechanics out and they concurred that it was unusual. They pulled it all apart and found a broken cable, rechecked operation and put it back together. I went out to test it again and again got no drop. They called two mechanics who told them it is normal for a lycoming engine 150hp model O-320 NOT to have any drop when you pull the heat because the carb is down by the sump and this keeps the air around the carb warm. There was no explanation why it used to drop. They just told us that maybe something was wrong previously and that is why it dropped in the past. So the club is riding on this advice that it is normal to NOT have a drop in RPM when the carb heat is pulled. To me this doesn't make any physical sense. When you add warm, less dense air to the intake mixture it should enrich and run slower. What am I missing? I am not comfortable flying this plane until a solid answer is achieved. The POH has no listing of what acceptable RPM drop is when the carb heat is pulled. It just states to "check operation." By the way, the plane has a new exhaust system and it heats poorly (cabin air-I don't know if this is normal or not). I guess what concerns me about this situation is that the mechanics are duely confused as to what is the proper operation and are riding on a conversation with a mechanic over the phone. Also, one of the mechanics mentioned to me that the butterfly is positioned so it doesn't close all the way. He said this was normal. To me it should be closed ALL the way when properly adjusted otherwise the engine is breathing unfiltered air all the time. Please chime in as to what you think is normal and what you feel I ought to do about this. Thanks in advance.

Don Jones · #2 02-17-2010, 02:56 PM

I fly a 172 with the o-320 for my instructing, it drops 100 to 150 RPM
when you pull carb heat. Something is not working correctly on yours.
Some of them get so worn out they have carb heat all the time due to leakage
around the valve and show no RPM drop when tested. I bet that is the case here
as well.

Kevin Horton · #3 02-17-2010, 06:15 PM

Quote:

Originally Posted by aarvig

Please chime in as to what you think is normal and what you feel I ought to do about this.

These guys sound like they have no idea what they are talking about. There should be a very clear rpm drop when the carb heat is selected on. Something is wrong somewhere. It is very troubling that these guys just want to gloss over a problem with the engine. I wonder what else they are not properly maintaining.

I want the aircraft I fly to be very well maintained. Flying is risky enough without adding new risks from poor maintenance. I'd be looking for another flying club.

aerhed · #4 02-17-2010, 07:00 PM

Kevin be right. Somebody makin smoke. Piece 'O' **** airbox, linkage, cable. Probably everything and nobody wanna pay/take time because of what it is. Yeah rubber flappers in door tips should contact top & bottom of box. Shafts & bearings are often shot. Stupid bent 2 rivet standoff tab that holds the cable housing before lever often loose (both ends). Airbox itself often so worn, cracked, lamed out no one wants to fix just one piece. Re-riveting door to shaft often dangerous FOD wannabe or obstruction. Put your foot down. Don't fly the beer can until you personally know that airbox won't put you in a box.

LifeofReiley · #5 02-17-2010, 07:50 PM

Run like **** I say... if the carb heat is functioning properly you will have an RPM drop. My drop is about 60-70 RPM. Your problem is not repaired properly.
I would not fly the plane until it is.

aarvig · #6 02-17-2010, 10:03 PM

Ok, thanks for the imput. I was certain the carb heat being pulled should create a drop in RPM. The club owner and AP feel the same way as well. What was confusing to all of us was that a reputable engine shop opined that it was normal not to get a carb heat drop. The POH is non-specific on the amount of the drop so it left us wondering what is right...carb heat=RPM drop or not? I knew some of you gents would get a bit hot under the collar but let me reassure you the club is doing everything they can to rectify the situation immediately. They have always been on top of repairs and maintenance. They are going to run some more checks tomorrow and fetch a second opinion from another AP so hopefully we get the answer.

mahlon_r · #7 02-18-2010, 05:15 AM

If the airbox has been repaired properly and the carb heat door is rigged correctly and functioning, maybe the duct from the heat source to the airbox is installed incorrectly or obstructed. I can't believe a mechanic would do that type of major work and not run the engine to check it's operation, after the work was completed! Especially, after the first time showed the problem wasn't fixed and you had to have him back for another go at it.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

pierre smith · #8 02-18-2010, 05:25 AM

....fairly recently while I was taking my -6A to its new owner.

I had flown the airplane for 4 years and 500 hours and don't recall ever having carb ice, but when I flew through a cold shower at 45 OAT, my RPM dropped by 100. I hadn't moved the throttle, so I figured 'ice' and pulled the heat. It immediately went back up the 100 RPM.

Good advice to not fly until it's fixed, whatever it takes.

Best,

CESSNADON · #9 02-18-2010, 05:34 AM

I too have a 172L. The rpm drop with carb heat on is minimal. The heat muff for the carb heat is little more than a semi enclosed rectangular box attached to one of the exhaust stacks. So a very small amount of heated air is collected at best. The fact that the carb is mounted directly to the HOT oil sump goes a long way toward mitigating carb ice. My POH does not require carb heat for the landing checklist, unlike the Continental engines which require carb heat on landing. Of course their carb is hanging out in the airflow with no heat flow from the engine.

Having been used to the Continental engine carb heat drop of about 100 rpm I was concerned when I started to fly the 172 and saw little carb heat rpm drop(barely noticable) I checked the heat box, the scat tubing , the cable linkage and the butterfly valve. All are fine....Minimal carb heat rpm drop is normal at least for an 0 320 E2D with the Cessna heat collector box.

LifeofReiley · #10 02-18-2010, 06:20 AM

Quote:

Originally Posted by CESSNADON

I too have a 172L. The rpm drop with carb heat on is minimal. The heat muff for the carb heat is little more than a semi enclosed rectangular box attached to one of the exhaust stacks. So a very small amount of heated air is collected at best. The fact that the carb is mounted directly to the HOT oil sump goes a long way toward mitigating carb ice. My POH does not require carb heat for the landing checklist, unlike the Continental engines which require carb heat on landing. Of course their carb is hanging out in the airflow with no heat flow from the engine.

Having been used to the Continental engine carb heat drop of about 100 rpm I was concerned when I started to fly the 172 and saw little carb heat rpm drop(barely noticable) I checked the heat box, the scat tubing , the cable linkage and the butterfly valve. All are fine....Minimal carb heat rpm drop is normal at least for an 0 320 E2D with the Cessna heat collector box.

Aaron states before the mechanism malfunctioned he had 100 RPM drop in his Cessna 172K, so I think repaired correctly he should be able to get close to the same. Zero change is unexceptable. I'm sure they'll get it fixed.