aarvig
Well Known Member
Carb Heat Operation:UPDATE
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.
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.
Last edited: