Run Up: Excessive Carb Heat Drop

[Raptor05121]

Today my engine dropped about 500rpms on carb heat application on run-up. The engine just about cut out. A quick search is showing a possible scat tube issue? Anyone have any pictures handy of what I should look for?

[RLCarter]

Sounds like you might have an exhaust leak

[Hank]

My Owners Manual doesn't include using carb heat at runup; it says to check it right after engine start, and that RPM should drop. For takeoff, all it says is Carb Heat should be off . . .

I did have a problem a couple of years ago, where the flapper hinge was broken and the carb heat would flap to full open all by itself. Made for interesting takeoff and climb out before I figured out what was wrong. 

[Guest]

Feel all of the Scat ducts by gently squeezing them, it's possible that one is collapsed.

Clarence

[Raptor05121]

Anyone ever have the muffler overhauled? I'm expecting worst-case scenario.

[carusoam]

Goofy PP thoughts that come to mind...

cold air comes in at one end, gets warmed up, then heads to the carb or gets expelled past the carb heat butterfly valve?

Check the muffler...See if anything rattles when you shake it?

Might be worth opening up the heat muff to see what is/was living inside of it...?

There are two types of heat muff constructions back in the day. A more robust design was added in the mid sixties, IIRC..

See which one you got.  Worst case reconstructive welding may be in it's future...

This is usually done during the annual.  Take pictures and post.  It would be interesting to see.

PP thoughts only.

Best regards,

-a-

Edited May 8, 2017 by carusoam

[RLCarter]

not too bad to remove and inspect the muffler, but i suspect if you have a bad muffler there would be tell tell signs at the heat muff and scat tube flanges. Just remember this also where your cabin heat comes from so any CO leak will end up in the cabin regardless of the heat being on or not.

[cliffy]

With that much drop on carb heat , as mentioned, you might have a collapsed scat tube being sucked closed and cutting off intake air (like closing the throttle). Check to see if they are all round in cross section and have a spiral wire wrapping from one end to the other that you can feel (like a cage around the tube but underneath the outer covering). One other item might be that you could have a double wall tube with an inner sleeve that is collapsing inside the tube. Look to see if all the curves and turns are full diameter and not kinked closed. 

Let us know how it looks. Post some pictures.

[Nokomis449]

19 hours ago, Raptor05121 said:

Anyone ever have the muffler overhauled? I'm expecting worst-case scenario.

A few years ago I replaced my worn-out muffler with a brand new Knisley Welding exhaust system, which replaced everything from the risers to the tail pipe with the only thing being re-used was the heater shroud around the muffler.  I'm thinking the parts were about $1600-$1700 plus sweat equity and the result was a noticeable performance improvement.  I still have the old core, risers, and tailpipe if you need them.  Risers and tailpipe are all in good shape and the IA said the core is definitely rebuild-able.

[Raptor05121]

Pulled the carb heat SCAT, problem goes away. Further proves the problem is in the muffler.

I managed to pull the heat shield off enough to see a crusty deposit about 1.5" across on the bottom with a noticeable heat ring in the center. Its obvious its blow-torching out of the muffler now.

A&P says he can't get to it for another week, but to go ahead and order a muffler. Looks like about $600, I'm pricing right now.

No flying for a few weeks :/
 

[RLCarter]

gotta love airplanes...I truly believe there would something that needed attention on a new bird right off the assembly line

[DXB]

Sucks for Alex for sure - Sorry!  But this is an interesting  thread for folks like me to learn something.  It's fortunate that this muffler leak showed up during run up rather than by causing unconsciousness upon turning on the cabin heat. I can't say that I've payed much attention to the exact amount of rpm drop with carb heat on, but this thread might be a good reason to start.  My POH says nothing about checking carb heat at runup, although that's when I do it because it seems to have the added advantage of clearing any ice accumulated during taxi.  In the POH, the check is only performed in the starting checklist  and doesn't specify an expected amount of rpm drop. My questions:

1. Should one check carb heat at start (per POH) or run-up or both?  I do it at runup at 1700rpm, and  see a drop around 100-200 but never paid close attention. How much of a drop is too much?  Every few runups, I also pull throttle to idle to make sure it doesn't doesn't quit with carb heat on - I guess this would be a more sensitive way to catch a problem.

2.  I understand how a collapsed scat hose from the doghouse to the shroud could cause the symptom Alex describes, but I'm less sure I understand how a muffler leak did it.  Did Alex get the big rpm drop because the muffler leak makes the gas going through the shroud hotter than normal, thus leading to an even richer mixture?  Also I suppose the leaking exhaust gas is O2 depleted, thus making the fuel excess in the mixture leaving the carb richer still?

3.  If that is the case, then I don't understand why the problem goes away when Alex unhooks the scat hose to the shroud - wouldn't that suggest that the problem was with the hose itself?  Then the air source to the muffler shroud is directly from the cowl and not the back of the doghouse.  I don't get how this fixes the problem with exhaust gas entering the intake air stream with carb heat on.

Alex's problem also makes me recall this wonderful old Don Maxwell article that gave me a basic understanding of the carb heat system in my plane:

http://www.donmaxwell.com/publications/MAPA_TEXT/Carb Heat Maintenance/CARB_HEAT_CARE.HTM

He talks about how the bypass valve system for the carb heat box is prone to malfunction and potentially cause damage to the muffler, so it might be worth taking a close look at the system while replacing the muffler.  I am fortunate to have one of the first planes off the line with the newer system that lacks the valve on the carb heat box.

 

[N6758N]

9 minutes ago, DXB said:

 

3.  If that is the case, then I don't understand why the problem goes away when Alex unhooks the scat hose to the shroud - wouldn't that suggest that the problem was with the hose itself?  Then the air source to the muffler shroud is directly from the cowl and not the back of the doghouse.  I don't get how this fixes the problem with exhaust gas entering the intake air stream with carb heat on.

 

Dev,

With the scat tubing removed from the muffler shroud, you completely removed the exhaust system from the equation. So no matter what the condition of the muffler is, without the scat tubing connecting it to the air box, you won't see any symptoms. 

[DXB]

15 minutes ago, N6758N said:

Dev,

With the scat tubing removed from the muffler shroud, you completely removed the exhaust system from the equation. So no matter what the condition of the muffler is, without the scat tubing connecting it to the air box, you won't see any symptoms. 

Ah right- basic plumbing.  I was thinking of a different hose.

[Raptor05121]

DXB, and from what we are thinking, the hole is big enough that with the carb heat flap down, the air intake is sucking off the muffler shroud, and with the hole, is basically SUCKING exhaust into the intake, causing a loss of burnable air, extremely rich mixture.

Anywho, I cannot for the life of me find a new source for a new muffler. The two places I've had A&Ps recommend say they no longer stock it. Aircraft Spruce has one, at an extreme markup of $700. 

[N6758N]

Just now, Raptor05121 said:

DXB, and from what we are thinking, the hole is big enough that with the carb heat flap down, the air intake is sucking off the muffler shroud, and with the hole, is basically SUCKING exhaust into the intake, causing a loss of burnable air, extremely rich mixture.

Anywho, I cannot for the life of me find a new source for a new muffler. The two places I've had A&Ps recommend say they no longer stock it. Aircraft Spruce has one, at an extreme markup of $700. 

Did you try @Alan Fox? He might have a serviceable one lying around. 856-419-5209

[cliffy]

Do you want a "new" one or a rebuilt one for less money?

I used Dawley for mine to be rewelded when it needed an overhaul. Mine has been on for many years with no issues what so ever. There are others that rebuild them also. What they do is because the regs allow "repairing" stuff they keep just one piece of your old one (say an end plate) and then rebuild making every other piece out of new stock (just like dog houses). You usually get what for all practical purposes is a new muffler for a rebuild price.     

Now for the can of worms- :-) :-)

One big issue with all our mufflers (except the new Powerflow) is that we have what are called "flame cones" inside the can. These are perforated tapered cones welded on the ends of the exhaust tubing inside the can that disperse the hot exhaust gases more evenly around  inside. They contribute to more cabin heat availability and keep strong "blow torch" type flame from impinging on the wall of the can and bulging it out (annual inspection item for bulges, sign of imminent failure) (may be what the OP has). There may be some benefit to 100 ' rich of peak here as 1550 degrees might be much stronger and less detrimental to metal as say 1650 or higher at peak EGT. 

These cones fail with regularity. Usually they break up in small pieces and go out the exhaust pipe harmlessly BUT if one breaks off complete it can block the exit of exhaust gases out of the muffler and hinder engine horsepower to a big extent. Always look up inside the exhaust pipe at annual to see if it is blocked. You can even do it when you change oil or just look around the engine compartment. I use a small boroscope to look around inside the muffler at annual time.

One interesting item- if the cones break away and leave the muffler there is no blockage of the exhaust so you'll not really notice anything other then maybe less cabin heat. Now there are IAs that will ding the muffler if these are missing. Why? Because (and many FAA types will support this conclusion) the muffler doesn't then meet its as designed approval therefore the airplane is not airworthy with them missing. Just be aware that some IAs and Repair Stations may take this position and some FAA types have even done ramp checks looking for this very item. 

 

[Raptor05121]

I called 9 separate places. 7 of them will rebuild them, cheapest I've found is $800 and lead time STARTING at 3 weeks, most places are much higher (another Mooney guy has his exhaust currently at Dawley and they just told him another EIGHT weeks).

My governor issue had me down for about three weeks. I forsee this having me on the ground for well over a month. I'm likely going to go close up the plane, fill the tanks, oil the blades, and pickle the engine. I just need time to save for this.

[N6758N]

35 minutes ago, Raptor05121 said:

My governor issue had me down for about three weeks. I forsee this having me on the ground for well over a month. I'm likely going to go close up the plane, fill the tanks, oil the blades, and pickle the engine. I just need time to save for this.

Some may argue this, but if you're going to be down for less than 2-3 months, pickling an engine is a waste of time. You have it in a hangar, and you fly it a lot, 2-3 months isn't going to hurt anything. Keep at it and you will find something. 

[Guest]

22 hours ago, Raptor05121 said:

Pulled the carb heat SCAT, problem goes away. Further proves the problem is in the muffler.

I managed to pull the heat shield off enough to see a crusty deposit about 1.5" across on the bottom with a noticeable heat ring in the center. Its obvious its blow-torching out of the muffler now.

A&P says he can't get to it for another week, but to go ahead and order a muffler. Looks like about $600, I'm pricing right now.

No flying for a few weeks :/
 

If the muffler has a hole in it, thankfully you're alive to tell us about it.  The post of the C model up north that crashed comes to mind.

Clarence

[cliffy]

You might find a good used one and then save up to have your current one repaired in case the used one takes a dump. Might get you flying sooner. 

Would like to see pictures of the one you have when it comes off. 

[170driver]

Anyone ever have the muffler overhauled? I'm expecting worst-case scenario.

I had mine "rebuilt" two annuals ago by a company in WA - about $1600, if memory serves. As long as they can re-use a flange or some other small portion of the old system the work they do is considered a repair. A new muffler was around double the repair cost. I'd have to ask the shop that did the annual the name of the company - will do if you want to investigate.


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[Raptor05121]

2 hours ago, M20Doc said:

If the muffler has a hole in it, thankfully you're alive to tell us about it.  The post of the C model up north that crashed comes to mind.

Clarence

 

Luckily I don't run my heat much. I think I used it like 5 times over the dead winter.

[Hank]

My muffler was replaced at annual in Jan 2014 for ~$1600 parts. It was found at the end of the work, the hole was on the bottom, big enough to put my fist in. I had made a trip to W. NC at Thanksgiving, and other flights in Dec & Jan with the heat on . . .

[cliffy]

Another item to remember about CO-

It combines with hemoglobin hundreds of times easier than O2 does therefore if you have a CO leak in your heater the CO binds faster than O2 to your blood. It gets worse with altitude as the partial pressure of the O2 lessens up high.

BIGGEST POINT TO REMEMBER?

If you use a pulse/oxymeter to measure your O2 SAT if you have a CO leak your Oxymeter will LIE to you and read perfect O2 SATs when in fact your are going under due to CO and hypoxia. 

Your pulse meter can't tell the difference between O2 or CO combining on your blood vessels.