Carb Heat Advice

[kevind]

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

[photofly]

Mechanic (also club member) says we are nuts.

Get your iPhone out and take a video, then show it to the maintenance personnel. Nothing will get their attention faster or better.

[praveen4143]

What indicated or actual altitude is your aerodrome?

Regardless of anything. An Cessna 172 / Lycoming engine that is leaned out properly shouldn't really drop 200 RPM unless there is Carb ice present. If you can confirm that there was indeed no carb ice, then there's definitely a problem. With the right setting, carb ice can easily form even at temps in the mid 20s.

I'd definitely get this looked into. Also, PF has a good idea on the video bit.

[kevind]

Video was taken and sent to mechanic.

We are confident that carb ice was not present.

[photofly]

Carb heat on... Full throttle gives 1500 rpm

That's rather concerning. Have you checked for a blockage in the carb heat air intake?

[cgzro]

Its likely a collapsing hose between the carb and the heat shroud. When you apply heat there is a vacuum in the tube and it can partially or completely collapse. This can and has caused engine stoppages and off field landings. Im flabbergasted a mechanic would laugh at this.

[J31]

Beat me to it! Check the inside of the hose also as some hoses are double walled and the inner hose collapses.

[cgzro]

I have two friends that have experienced this so it cannot be that uncommon. Sometimes the hose stays collapsed sometimes it flexes back. Anyway write something to ground it in the journey log ASAP.
As pilot this is your responsibility.

[anofly]

take a look for an exhaust leak inside the carb heat shroud
or case, that always makes the rpm drop way worse as the exhaust has little oxygen in it. I have seen that happen. and stuff in the unfiltered hose I have seen that. In fact a mouse once! made things run poorly with carb heat on.