Weak carb heat?

[bencpeters]

I recently installed a JPI 900 on my 64C. As part of this, I now have a carb temp gauge! I've read about using just enough carb heat to keep the temp out of the yellow arc (which on my gauge is somewhere in the ~10-40F range). However, I was out flying yesterday in pretty cold air (OAT ~-15C @ 12.5k) and even with full carb heat, I couldn't get the carb temp to climb above ~32-33F in cruise (from a no-heat temp in high single digits F). This was a bit concerning to me - I'm somewhat skeptical it would be very effective at melting ice at those temperatures. Has anyone else seen this? What kind of temperatures do others see when carb heat is added? CHTs were ~340-360F and EGTs mid 1400s at the time.

Not sure if it's related, but the cabin heat performance is also very underwhelming. I know can be an issue in vintage Mooneys, but when I put my hand in front of the vent with the heat on full, the air is barely warm - certainly warmer than ambient, but it almost feels cool on the skin.

I'm no expert on the airflow patterns we've got in these planes, but I was wondering if these are related and/or might be indications the "doghouse" needs to be rebuilt, or something else with the intake/exhaust manifolds being off?

Thanks!

[Hank]

Check the seals on the heat muff that surrounds your exhaust. If there are gaps and leaks, you will lose a lot of heat. Also check how well it operates--on my C, it slides sideways, others may have a rotating seal. Check that, too. Are the hoses in good shape--no holes or cracks, tight clamps at both ends?

The effectiveness of your carb heat will vary with OAT. I generally fly ~1450 EGT, and most days have no trouble getting carb temp > 5ºC with less than half carb heat. The only time I remember using the full stroke is during the After Start check to make sure that it works at all. Then again, I've never yet needed to melt carb ice, and haven't needed to add partial carb heat for IMC penetration very often because I'm a coward and avoid ice all the time.

[Jcmtl]

Somebody can correct me if I'm wrong, but you do not need to be using carb heat if you're cruising at -15c OAT.  I only use carb heat if OAT is between 0-10c, my carb temp gauge is indicating below freezing, and i'm flying through visible moisture because that moisture will ice up on the carb.   At -15 there is no (or not enough) moisture in the air to condense and freeze on the carb.  

regarding your cabin heat, i fixed my cabin heat by making sure the flapper on the heat box, attached to the firewall, co-pilot side, closes completely.  Then, look at the condition of the scat tubing in the co-pilot foot well.  if it's in rough shape, replace it.  Mine was leaking air all over the place.  Also, check the cabin air scat tube for leaks.   

[bencpeters]

I wouldn't use it prophylactically in clear air at those temps, but there actually were a number of clouds (e.g. visible moisture) in the area - tops were in the 12-14k range from a major winter storm below - that's what got me playing with the carb heat. I don't want to minimize the seriousness of ice formation anywhere in a non-equipped plane, but the idea of unmeltable carb ice from punching through a cold cloud with inadequate carb heat seems especially concerning.

Thanks for the ideas on the heat box flapper, I'll check it out! Is there anything on the carb heat system to check out specifically?

[WaynePierce]

I can't help but think, and smile a bit, about the use of technology. You can fly around fat, dumb, and happy all day on the way to the shop to have that fancy gadget installed, then worry about what it's saying all the way back home. What is that saying, not used in flying.... Ignorance is bliss?

Good luck with your heat and carb heat issues.

[David Lloyd]

Are you getting much rpm drop when checking the carb heat after startup? 

[carusoam]

The parts that wore in my M20C… 1965 version…

carb heat butterfly valve… check it’s operation visually…. Is it still there?  Does it close and open completely?

Annual heat and outside air valve lubrication challenge… slide gates freeze on an annual basis… simple Rusty slide gate…

 

Not there… there is a hose that belongs on the end of the carb heat valve… there are pics around here somewhere for the routing….

 

Now for the fun part… open the heat source… use your thermometer…. What temps are you seeing? Is it above 200°F?

Have you CO monitor handy… what does it say? (It should always be close to zero in cruise)

 

ROP/LOP/Peak/MP/FF matters… High power, near peak generates a lot of heat…. Low power at 10k’ LOP… doesn’t generate much excess heat…

Visible clouds, and warm weather… can be dangerous… the only real carb ice I had was on a trip to SnF… fully loaded, below the cloud deck…

Full power, with carb heat on… was not doing very well at maintaining altitude… carb heat off, carb icing would start, MP dropped slowly….

Without a carb temp gauge… it was an uneasy feeling … until we got past the clouds….

Remember the oddity of the throttle plate… when it is cocked, it creates a pressure drop… pressure drops decrease the temp of their passing by it…

Keep the carb temp on either side of the yellow band… don’t warm the air into the band…

If you can’t avoid the band… the car heat system needs work…

How good is the muff?

PP thoughts only, not a mechanic…

Best regards,

-a-