Question about carb air temperatures

[Jacques]

I've noticed that when flying at mid to high altitudes, I have a really hard time keeping Carb Air temps within the suggested range of 14 to 18 C. In fact I often see that at above 20,000 ft, the carb temp is moving into negative numbers, which probably isn't good.

The manual suggests that HOT air should not to be used above 12,000 ft due to a possibly adverse effect upon the altitude compensation feature of the carburetor.

I can keep the temperatures mostly within range except when at the limit of the "Low" blower, temps dropped to around 3 C between 12,000-15,000', this with the air source set to filtered.

My power settings for the climb are 46" MP and 2700 RPM
When "High" kicked in the carb temps quickly rose to 23 C.

At 19,000, still using ram air the temps were just getting to the high end of normal 19 C.
20,000, air source still at ram and carb temps now 13 C.
21,000, air source 35% filtered fluctuating between 13-14 C

I reached my desired altitude of 22,000' and reduced power to 43" MP
and 2300 RPM set full filtered air, 215 mph indicated and the carb air temperature has fallen to -1 C.

I feel like I'm missing a crucial step... anybody else experience this unrecoverable drop in carb temperature?

[robert41]

From a previous discussion, there is little if any need to use the carb heat or filtered air. I keep the heat off and ram open all the time without problems.

[Jacques]

Okay Robert, I admit I didn't search before I posted. I found four or five threads from the past discussing all I would ever want to know about the carburator, so thanks for encouraging me to seek the answers! Honestly, I have no idea why I was worried about the low temps when everything about the way the engine was running seemed perfectly fine.

By the way, I did take the P-51 up with all three internal fuel tanks full and...yes, I had some pretty wild rides, all recovered but only because I had some altitude! I like this aircraft more each day!