Scudrunners

Wondering if you guys could help me out...please be gentle


I fly a club 172.  1969 (K?) with 180 hp normally aspirated engine


Many of pilots are reporting abnormal carb heat issues.  Mechanic (also club member) says we are nuts.


Here is what happened to me tonight.
This was done on the ground after the flight.
1- 1700 rpm, mixture leaned for max power (DA about 3500 feet, 25c)
2 - Applied carb heat, 200 rpm drop and engine returned to 1700 rpm when carb heat off
3 - Mixture full rich
4 - Carb heat on - 600 rpm drop
5 - Full throttle gives 1500 rpm, then throttle set to 1200 rpm
6 - Carb Heat off
7 - Engine goes back to 1700 rpm


We had similar reports over the winter when the temp and DA would not have been an issue.  In flight pilots have reported they would apply carb heat and get a abnormal large drop, recycle the carb heat and get a 200 rpm drop.


The question is.  Is this normal, and if not what are possible problems/solutions


Thanks in advance

From the description I wonder if there is something in the ducting of the carb heat circuit. In the winter I would expect a slightly bigger drop because the temperature difference of the heated air should be more. The drop should be consistent. I hope it works out for you, but trouble shooting on the Internet can be difficult. Has anyone tried to show the problem to an AME? You might want to ask on AvCanada forums, they have a maintenance sub forum with lots of AMEs posting. A couple of them are flaky but most seem solid. Then again, "Online nobody knows you are actually a dog."


http://www.avcanada.ca/forums2/viewforu ... 65057514a9


I'm not bashing the place but if you go, make sure your virus protection is up to date, they are the big city and have had some issues in the past. I think they are all cleared up right now. I wish I could be more help, good luck.

Thanks..I will ask there also.  Hopefully Col Sanders will pop in on this.