High CHT - SOLVED

[gsxrpilot]

I've been on here quite a bit looking for a solution to the high CHT problem with my C. A related problem was leaning. Sometimes I couldn't get below 11 gph and EGT's were all over the place. CHT's were also all over the place with the front cylinders up to 60 degrees cooler than the rear, or specifically #4.

• I've been told C's just run hot.
• I used up a tube of RTV plugging every hole, seam, etc in the baffling.
• It was suggested the the carburetor might be wrong or need overhauling.
• I even thought I should switch the O360 out for an IO360 to be rid of the carburetor.
• I've experimented with all types of power settings and mixture settings.
• The only thing that seemed to help at all, was to reduce prop RPM as early as possible and as low as possible.
• And I'd never been able to close the cowl flaps without raising the CHT's too high.

The solution I recently discovered is throttle position. At WOT and Full Rich mixture, everything is fine. But as the mixture is pulled back, the CHT's would go way too high, and I know the engine's not meant to be run full rich at say, 6000 ft.

Solution:  Prior to pulling the mixture back, pull the throttle back just until the MP needle moves 1/2", then push the throttle back in just enough to get the 1/2" back. Leave the throttle there and adjust mixture as required the rest of the flight. Now I can leave the Prop RPM high or at least around 2500 throughout the climb. CHT and EGT's are all much closer between cylinders now, and CHT's are lower across the board. I've tested this on four different 1.5 hour flights and two 4+ hour flights with excellent results. 

It's been said on this board before, that WOT with the O360 carburetor opens an additional port or jet dumping extra fuel. It seemed counter intuitive to me to reduce that extra fuel as a means of reducing CHT. And I'd tried pulling the throttle back just a bit, but it obviously wasn't enough. I have to pull it back enough to reduce MP and then put it back just enough to get the MP back.  So now I'm speculating that the extra fuel works fine at full rich mixture, but when leaning, it's somehow messing with the distribution of fuel, and putting some cylinders too rich and others too lean.  This picture is at 12,500 nicely LOP and running smooth as butter.  I neglected to get a picture of the CHT/EGT screen but will do that on the next flight.

[bonal]

Interesting, more information please. Will try on next flight but what you describe seems like how I've been doing it all along so will pay closer attention to my procedure.

[MB65E]

Yes Sir! It's similar on the IO360. About 1gal less when playing with the MP settings off of full throttle. Your Lop ops settings might/should work now too. 

Just wait for mp needle to move and you'll be in good shape. A old UAL tech taught me that in his 182. Works the same on a 470. 

-Matt

[cnoe]

Yes Sir! It's similar on the IO360. About 1gal less when playing with the MP settings off of full throttle. Your Lop ops settings might/should work now too. 

Just wait for mp needle to move and you'll be in good shape. A old UAL tech taught me that in his 182. Works the same on a 470. 

-Matt

Interesting. I've never heard of this with the IO. WOT has always worked well for me at altitude but my ears are open. I don't believe there's an enrichment circuit, and with individual injectors I don't see how causing turbulence across the throttle body would help atomize the fuel.

I'm not being a smart a$$; just wondering how partial throttle would change efficiency in an IO360.

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[Bob_Belville]

12 minutes ago, cnoe said:

Interesting. I've never heard of this with the IO. WOT has always worked well for me at altitude but my ears are open. I don't believe there's an enrichment circuit, and with individual injectors I don't see how causing turbulence across the throttle body would help atomize the fuel.

I'm not being a smart a$$; just wondering how partial throttle would change efficiency in an IO360.

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+1

[par]

I'll have to give this a try as well. #3 CHT is my biggest problem child on climb out. 

[bonal]

Number 3 is mine too because that's the one with the probe far as I Know the other 3 behave themselves.  But I understand 3 is typically the hot one and why the probe is there.

[gsxrpilot]

Just now, pmccand said:

I too have seen various throttle and prop settings that would somehow result in a well balanced distribution as well, but I haven't been able to identify the parameters, nor reproduced this effect on a consistent basis..  So let me get this procedure straight with a bit of clarification on your procedure..

1) Full (WOT) throttle, full prop on takeoff.

2) Pull throttle back till MP drops 1/2"

3) Push throttle back in to recapture the 1/2" ... Wouldn't this be the same position as WOT?

Assuming that the position set in step #3 is NOT WOT but something slightly less that full throttle, this is setting has got to be near full throttle power setting.  So if you used this throttle setting throughout the flight, how were you able to reduce mixture to LOP and remain outside of the "red box"?  I would assume you need to be at or below 65% power to have a safe margin for the cyl's.

 

Yes, this is correct, but let me add some detail.

1. Full (WOT) throttle, full prop, full rich mixture - on takeoff.
2. I'll continue to climb in this configuration until I am high enough to require a mixture reduction, usually around 4000 or 5000 ft.
3. Sometimes I'll reduce Prop RPM early in the climb if temps are rising too high, but I'm still at full rich mixture.
4. At about the time I'm ready to reduce mixture...
   1. Pull throttle back till MP drops 1/2" (basically until the needle starts to move)
   2. Push throttle back in to recapture the 1/2" (basically put the needle back)... 

Isn't this the same as WOT?  No. I find that I can pull the throttle back what seems like quite a bit, before the MP needle moves. Then push it back in just a smidgen.  For example, (not to scale ;-) Pull the throttle back about an inch and push it back in an 1/8th inch.  I have a vernier throttle so it's many turns out and about half a turn back in.

Once I've established this throttle position, it stays there the remainder of the flight until it's time to reduce throttle descending into the pattern or on approach.

Once reaching my target cruising altitude, I'll pull the mixture back to LOP. If I'm at or above about 7500, I'm certainly low enough on power that the Red Box doesn't exist. Below that, just give the Mixture the "big pull" back to LOP.  I just pull the mixture back until I detect some engine roughness, then back in just enough to smooth it out.

Hope it works for you as well...

[N201MKTurbo]

 

Here is a discussion of the aircraft carburetor power enrichment system:

 

The power enrichment system automatically increases the richness of the mixture during high power operation. It makes possible the variation in fuel/air ratio necessary to fit different operating conditions. Remember that at cruising speeds, a lean mixture is desirable for economy reasons, while at high power output, the mixture must be rich to obtain maximum power and to aid in cooling the engine cylinders. The power enrichment system automatically brings about the necessary change in the fuel/air ratio. Essentially, it is a valve that is closed at cruising speeds and opened to supply extra fuel to the mixture during high power operation. Although it increases the fuel flow at high power, the power enrichment system is actually a fuel saving device. Without this system, it would be necessary to operate the engine on a rich mixture over the complete power range. The mixture would then be richer than necessary at cruising speed to ensure safe operation at maximum power. The power enrichment system is sometimes called an economizer or a power compensator.

[macosxuser]

The problem actually lies in the poor/uneven design of the O-360 updraft induction system. The intake tubes are NOT the same length and the carb is slightly offset to one side. The result is uneven mixture to the cylinders. Putting more fuel in does not make the problem better, the extra fuel isn't going into the right cylinders. Cracking the throttle plate back from WOT changes the airflow direction in the induction slightly, which helps make the fuel flow to the cylinders even out somewhat. We had this problem in the fixed gear Cardinals, and this was a solution.

[carusoam]

I've got some questions for you guys...

1) The induction tubes are different lengths on the O360? I may have mixed mine up, but they seamed to be identical parts... May want to check into that..?

2) the second fuel jet opens/closes with the throttle.  If you can get it to close while keeping the throttle at WOT, you have achieved something many of us are not familiar with...

If able, Please fill in some details...  People would like to follow your logic.

3) How can you tell the second fuel jet is open?  Are you using a FF gauge with that?

Just trying to learn and spread the the Mooney knowledge.

 

best regards,

-a-

[gsxrpilot]

I'm not a mechanic, just a PP, so this is all speculation.

1) I don't know anything about the induction tubes.

2) I've heard others talk of this "enrichment circuit" in the carb. I don't have first hand knowledge of it's operation. I just know what I see flying my C. 

There is a huge difference in the CHT spread when the throttle pushed all the way in vs. backed out as far as possible without reducing MP. I'm speculating that as long as I'm still getting full MP, more throttle has no benefit. 

I do have a FF meter with my GEM G2 engine monitor. There seems to be almost a gallon per hour difference between full throttle and reduced throttle at the same MP and leaned to just short of engine roughness. I'm not trying to save gas, I'm trying to reduce high CHT. With full throttle #1 is very cool and #4 is too high. With the reduced throttle, #1 is a little warmer and #4 is as much as 30 degrees cooler.

I'm speculating that there's got to be some problem with full throttle and a leaned mixture at the same time. And as far as I can tell, the fuel distribution is the problem, too much in some cylinders and too little in other cylinders. By backing off the throttle, but not so much that MP is reduced, allows for a much more even fuel distribution while leaning the mixture.

[Hank]

I climb WOT/2700, but when I level off, I deuce throttle:

• Low level (ie, 3000 msl burger run), 23"/2300
• mid-level (~4500-6500/7000 msl), generally 22"/2400
• High cruise (where Owners Manual shows WOT <75%), I pull the throttle enough to make the MP needle move, and leave it there, then lean away.  It's surprising sometimes how much I have to move the throttle to make the MP needle twitch, I swear it's sometimes a third of the way back. I don't aim to reduce MP by a certain amount, I stop the throttle as soon as the needle starts to move.

This keeps me in the <75% power range, and I'm pretty much right on 9 gph. If I fly high at peak, I'll burn slightly less. The reasons to pull the throttle back are to shut off the enrichment circuit and to cock the throttle plate in the carb and create turbulence, which atomizes the fuel better and hopefully helps to even up the distribution between cylinders.

[jetdriven]

The IO-360 is the same in that regard, you have to pull the throttle back about a third at altitude to make the MP needle move.  It's not linear by any stretch. 

[cnoe]

The IO-360 is the same in that regard, you have to pull the throttle back about a third at altitude to make the MP needle move.  It's not linear by any stretch. 

Is that dead space? Or does moving the throttle without changing MP actually do something (in an IO)? Obviously it has an effect on enrichment with a carb, but how could it change much of anything in the injection circuit? Just curious.

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[mike20papa]

Economizer Jet - Marbel Scehbler shows the fabled Economizer Jet or what some pilots claim as an WOT enrichment jet/circuit only on the model MA4-5AA not the MA4-5 that is applicable to our 180hp O-360's.  I don't know if the Bendix fuel injection is set up with one, or not.  I've carefully watched fuel flows as I've back off WOT and never noticed any step in the fuel flow indicating I backed off some enrichment circuit or jet. 

[jetdriven]

31 minutes ago, cnoe said:

Is that dead space? Or does moving the throttle without changing MP actually do something (in an IO)? Obviously it has an effect on enrichment with a carb, but how could it change much of anything in the injection circuit? Just curious.

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Idk, I'd think an economizer circuit would have to be engineered and Built into the unit and so far, I haven't seen where the RSA-5AD1 has the economizer circuit. It takes about 15-20 degrees of throttle cock to affect MP. It's a foreign concept to me as I never once cracked the throttle off the forward stop in 4 years of ownership. And on that flight for 800+ nautical miles on 39 gallons of fuel, a LOP takeoff, cruise,  the whole bit..  

 

[macosxuser]

23 hours ago, carusoam said:

I've got some questions for you guys...

1) The induction tubes are different lengths on the O360? I may have mixed mine up, but they seamed to be identical parts... May want to check into that..?

2) the second fuel jet opens/closes with the throttle.  If you can get it to close while keeping the throttle at WOT, you have achieved something many of us are not familiar with...

If able, Please fill in some details...  People would like to follow your logic.

3) How can you tell the second fuel jet is open?  Are you using a FF gauge with that?

Just trying to learn and spread the the Mooney knowledge.

 

best regards,

-a-

1. The tubes from the sump to the cylinders are the same length, the pathway from the carb through the sump is not. This can be seen here:

 

2. It's not actually a different jet, rather it allows more fuel through the main jet. Because of this, you can mimic turning it off by leaning to specific fuel flow. I've run O-360's up to 13+GPH at 75% power/WOT with no additional cooling effect in the hotter cylinders. This is how this was empirically tested. These experiments were first done in the Van's RV community as far as I know, and I've duplicated much of their testing in the Cardinal.

3. Yes, total fuel flow is the only really accurate way of determining what is getting in. A reality check is done with how much lead fouling is going on in the plugs as well.

 

22 hours ago, gsxrpilot said:

I'm not a mechanic, just a PP, so this is all speculation.

1) I don't know anything about the induction tubes.

2) I've heard others talk of this "enrichment circuit" in the carb. I don't have first hand knowledge of it's operation. I just know what I see flying my C. 

There is a huge difference in the CHT spread when the throttle pushed all the way in vs. backed out as far as possible without reducing MP. I'm speculating that as long as I'm still getting full MP, more throttle has no benefit. 

I do have a FF meter with my GEM G2 engine monitor. There seems to be almost a gallon per hour difference between full throttle and reduced throttle at the same MP and leaned to just short of engine roughness. I'm not trying to save gas, I'm trying to reduce high CHT. With full throttle #1 is very cool and #4 is too high. With the reduced throttle, #1 is a little warmer and #4 is as much as 30 degrees cooler.

I'm speculating that there's got to be some problem with full throttle and a leaned mixture at the same time. And as far as I can tell, the fuel distribution is the problem, too much in some cylinders and too little in other cylinders. By backing off the throttle, but not so much that MP is reduced, allows for a much more even fuel distribution while leaning the mixture.

Exactly, the theory as to why is that the airflow pattern changes, which allows more of the atomized fuel to remain in the air streams to each cylinder. Another technique has been to add Carb Heat to Vaporize the fuel droplets, which keeps them in the air stream more linearly as well. This obviously WILL result in some power loss, but can help achieve LOP operations in SOME aircraft.

19 hours ago, mike20papa said:

Economizer Jet - Marbel Scehbler shows the fabled Economizer Jet or what some pilots claim as an WOT enrichment jet/circuit only on the model MA4-5AA not the MA4-5 that is applicable to our 180hp O-360's.  I don't know if the Bendix fuel injection is set up with one, or not.  I've carefully watched fuel flows as I've back off WOT and never noticed any step in the fuel flow indicating I backed off some enrichment circuit or jet. 

What 10- Number is your carb? The testing has all be done with the 10-3878, which is the standard O-360-A1A carb. I know there is a richer one available for the M20C, but I don't know the 10- number. Either way, the AA is an altitude compensation system, not power enrichment/economizer. I'm not trying to start an argument here, this is just right down an alley I've been down before. The powerflow exhaust also helps even out the fuel distribution in Cessna's. I don't know how the Mooney exhaust is, so I don't know if it helps much there. The downside is, there is extra power available, which amounts to extra heat that needs to be dissipated.

 

 

[DXB]

24 minutes ago, macosxuser said:

Exactly, the theory as to why is that the airflow pattern changes, which allows more of the atomized fuel to remain in the air streams to each cylinder. Another technique has been to add Carb Heat to Vaporize the fuel droplets, which keeps them in the air stream more linearly as well. This obviously WILL result in some power loss, but can help achieve LOP operations in SOME aircraft.

Thank you for the excellent commentary.  Is there a minimum or optimal carb temp to benefit mixture distribution?  In cruise I generally set partial carb heat to about 50F in carb temp.  I assume this is enough to provide a buffer for carb ice, and I notice no decrease in power at this low setting.   I am wondering if this is enough to help the mixture distribution.

[carusoam]

OSXer,

That is a very descriptive photo there...

the fuel and air mixture enters vertically from the bottom and splits four ways.  Sort of an unfriendly flow situation in a short distance...

the tubes are sitting in the oil bath of the sump.

the advantages of this arrangement include:

1) fuel/air cools the oil some.

2) hot oil heats and evaporates the fuel some more.

3) carb temp will give the starting point where the fuel evaporation begins.

4) carb temp is where there is evaporation and a significant drop in pressure going on, which both add to the thermodynamic effect of lowering the temperature of the mixture.  Lower pressure and lower temperature are two reasons for moisture to fall out of solution and freeze.

5) evaporating and minimizing the size of fuel droplets is best for evenly spreading them around prior to the four way split.  After the split the evenness of fuel distribution can no longer be improved.  

Below 20°F 100LL evaporates very slowly and preheat was required to start my old M20C.  Not enough evaporation and too much fuel draining on the nose wheel....

30°F a lot of excess fuel is used to get enough evaporated fuel in the system to light.

40°F some choke is needed.

70°F little choke is needed.  More an assurance of fuel getting to the carb....

Above 70° Mooney pilots have all kinds of procedures they follow to prevent too much fuel from evaporating and entering the engine.

where the fuel evaporates,  there will be traces of blue goo.  The vertical intake tubes of my 65C had a thick coating of the goo.  They got washed and cleaned once while the seals were being replaced.

wondering if having your mechanic wash all the tubes and do the same for the carb may work wonders if it hasn't been done since an OH decades ago...  Taking one off for inspection is pretty easy compared to the exhaust tubes

This is from some pretty aged PP memories. I am Not a mechanic.

Best regards,

-a-

[Hank]

I've never noticed fuel dripping on my nose wheel, even when I had trouble starting; that's why we overhauled the carb, my mech thought it needed to drip out when I tried to flood it on purpose.

When temps are near freezing, I preheat. When I travel in cold weather, I have an extension cord that I carry with me, 100' long, specially made to weigh about a third less than normal. Generally takes about an hour, but I use it overnight whenever possible. Makes those cold winter starts almost effortless.

[carusoam]

10 throttle pumps should get some drips.  Repeating it will coat the tire and kill the battery in a single not well thought out staring attempt.  

I liked the days before MS, they were adventurous.  OWTs were the norm.  My flight instructors I could afford barely had more hours than I had.  

Best regards,

-a-