Carburation on overrun

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Basil

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I have a Hirth F23 in my microlight. It runs well at throttle openings above about 1/4 and idles OK on the ground at about 1800 rpm. However when descending with the propeller being driven by the air stream and small throttle openings on cylinder cuts out. This is detectable from the low exhaust gas and cylinder head temps on one side.
My question is, at a throttle opening that would run satisfactorily at 1800 rpm on the ground but with the engine turning at 3200 rpm in the air, is the mixture weaker or richer due to the increased revs.
 

TFF

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Weaker. Because lubrication is attached to throttle opening not rpm like a four stroke, less oil is feeding the bearings. That is why sustained glides is not recommended. Best is to keep power in during descents if possible. The question come why you have one cutting out. That is important
 

Dana

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Weaker. Because lubrication is attached to throttle opening not rpm like a four stroke, less oil is feeding the bearings. That is why sustained glides is not recommended. Best is to keep power in during descents if possible. The question come why you have one cutting out. That is important
Less lubrication, yes, but that's not his issue here. If the airstream is driving the prop faster than the engine wants to run, the increased carburetor suction will pull in more fuel so the engine runs rich. However, 2-strokes don't like to be run that way, descending at partial throttle is asking for seizure, do your descents at idle.

The pumper carbs (Walbro and Tillotson) have adjustments for idle and full throttle but aren't as good in the midrange. Slide carburetors like the Bing and Mikuni have additional adjustments for midrange to make things smoother in mid range settings.
 

Basil

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My Hirth has oil injection so lubrication isn't a problem, I'll try weakening the idle mixture on the side that cuts out.
 

MadProfessor8138

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Less lubrication, yes, but that's not his issue here. If the airstream is driving the prop faster than the engine wants to run, the increased carburetor suction will pull in more fuel so the engine runs rich. However, 2-strokes don't like to be run that way, descending at partial throttle is asking for seizure, do your descents at idle.
Dana,please educate me on this one...I may learn something from your post.

Questions :
1. If the prop is being forced to rotate by the airflow driving it....how would the engine experience a rich condition ?
The amount of fuel that will be introduced into the system will be limited to the maximum amount that can be suplied from the idle jet and it has its limit.
To richen the mixture you would need to physically move the slide up so that the midrange and main jets would start to come into play.
The piston speed may be increased by the prop driving it but that doesn't mean it can draw more fuel from the carb...the fuel is not physically accessible until you get into the midrange & top end so it is only pulling air at that point....which means lean.
2. If you have a rich condition how can there be less lubrication if the fuel is premix ?
More fuel means more oil also.
3. How is descending at partial throttle asking for a seizure ?
One of the greatest 2-stroke guys to ever touch a Rotax explained the problems with a long approach at idle.....lean condition because the airflow if driving the prop more than the idle circuit can provide,too much air and not enough fuel and also the cooling of the cylinder which could cause a cold seizure if you need to panic slam the throttle for a go around.
Partial throttle helps control prop rpm from being driven by the airflow,provides lubrication to the cylinder with more fuel and keeps the cylinder warm so the aluminum piston doesn't seize in the steel cylinder due to tolerances being out of whack due to cooling.

Kevin
 

Dana

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All other things being equal, fuel flow is proportional to the amount of suction in the carburetor. If you restrict the airflow by closing the throttle but keep the rpm up by diving, suction increases, thus pulling in more fuel. It's why you'll sometimes see a 2-stroke "load up" and quit on short final or just after touchdown.

In the low midrange (just above idle), the.throttle is more open, so more air, less suction, less fuel, since the.midrange circuit isn't fully into play yet.
 

dino

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There is more vacuum (suction) in the manifold when the throttle is closed, but there is little airflow through the carb venturi which is what sucks the fuel. In addition to that the jet needles which serve the midrange are closed. The result is an overly lean mixture when the prop is being pulled by the airflow instead of the other way around. Some motor glider flight manuals actually warn against this condition like DG400 which advises avoiding long descents at low throttle position.
 

TFF

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More suction but not equal to flow as if it was throttled to that RPM so it’s lean for the RPM. The question comes, is the oil enough with no load.
 

MadProfessor8138

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More suction but not equal to flow as if it was throttled to that RPM so it’s lean for the RPM. The question comes, is the oil enough with no load.
You are correct......
On an extended approach with the throttle pulled to idle.....the cylinders tend to go lean.
The airflow is driving the prop which is increasing rpm but only the pilot jet is supplying fuel because the throttle is closed.
The pilot jets can only physically supply so much fuel....any rpm beyond their fuel flow capability will lean the mixture to the point of seizure if the condition is allowed to continue for too long as the temps climb.
That's why you are suppose to blip the throttle occasionally while descending...to supply fresh fuel & oil to the cylinders for lubrication & cooling plus keep the cylinders temps stabilized to avoid cold seizure in the event of a panic slam of the throttle for a go around.

Dana......it has been my experience that if ANY engine "loads up" while at idle...it is because the carb/carbs is/are not jetted properly to begin with.
I do understand your viewpoint but let me see if I can explain this so that it will make sense.
The throttle only controls the carb/carbs that control fuel & air flow which in turn control rpm of the engine,correct ?
The throttle doesn't control the ignition system does it ?
So the ignition system is operating independently of fuel system and it is doing that by sensing the rpm of the engine....retarding & advancing the spark curve.
For an engine to "load up" while at idle 1 of 2 things has to happen.....either it is getting too much fuel or the ignition is being cut to stop the burn.
Even if the prop is being driven by the airflow....any mixture that is pulled into the cylinder is being burnt by the ignition firing...no matter what rpm.
The ignition system has no clue whether that particular engine rpm is due to the prop being driven or if it is due to the throttle being open and the carbs supplying fuel....the ignition just fires and burns the mixture with whatever spark advance that rpm calls for.
So an engine will actually be lean instead of rich if you drive the prop with airflow and increase the rpms without opening the throttle to supply the correct fuel mixture for that given rpm.
For the engine to go rich and "load up" means the pilot jets are too big and the prop being driven is pulling way too much fuel for the ignition to keep up with at that rpm.
Remember.....the whole principle is that the ignition system has no idea why the engine is at that particular rpm....prop driven or fueled...it only burns the mixture for that given rpm.....pilot jets too large to begin with and the cylinders "load up" because the ignition cant keep up.

I hope that made sense....

Kevin
 

radfordc

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I may be confused (it's happened before!) but all my experience and education on ultralight engines tells me that if you increase the prop speed by diving, the engine will run leaner at the same throttle position. The explanation is that the engine is basically an air pump....more rpm more air getting pumped into/through the cylinder. The amount of fuel is controlled by the position of the needle....if you don't move the needle the amount of fuel remains about constant. Perhaps an increase in suction may result in a small increase in fuel flow, but the dominant effect is the amount of air being pumped.

If I set my throttle for cruise conditions (level flight and normal cruise rpm of about 5800 rpm) the EGT will stabilize at around 1100-1150 F. If I now put the nose down so that the air speed increases I will see an increase in rpm and also an increase in EGT. Often the EGT will go above red line of 1200 F forcing me to close the throttle before seizing the engine.

I don't know what is causing your problem of the cylinder quitting but I'm sure that the mixture is leaner under the conditions described.
 

MadProfessor8138

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radfordc.......yes you are confused again...but only partially.

Yes, the engine is an air pump.
Yes, if you point the nose down and drive the prop the temps will climb because you did not increase the fuel being delivered to match the rpm...it goes lean.

No,the needle is not the only thing in the carb to dictate the fuel flow.
Fuel :
pilot jet provides idle....needle provides mid range adjustment...main jet provides top end....float position also effects fuel flow.
Remember....that all of these circuits in the carb overlap each other and they all have to play nice with one another for the engine to transition smoothly from idle to full throttle.
Air :
And the clip position dictates how far the slide will open....which allows the airflow through the carb.....low slide position is low airflow....high slide position is more airflow.

The jets provide the fuel.....
The slide allows a given amount of airflow through the carb according to clip position.
Float position will also effect fuel flow.

The carb is a very simple device that does one job......delivers fuel and air to the cylinders.
However,that very simple device can be quite complicated to tune properly.

Kevin
 

TiPi

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I have a Hirth F23 in my microlight. It runs well at throttle openings above about 1/4 and idles OK on the ground at about 1800 rpm. However when descending with the propeller being driven by the air stream and small throttle openings on cylinder cuts out. This is detectable from the low exhaust gas and cylinder head temps on one side.
My question is, at a throttle opening that would run satisfactorily at 1800 rpm on the ground but with the engine turning at 3200 rpm in the air, is the mixture weaker or richer due to the increased revs.
Hi Basil,
The Mikuni VM manual is in the files section on the SD Planes forum (groups.io), it should have all info that you need.
Critical is the proper balance at idle for air volume (vacuum or air flow) and mixture (hear/see/feel if you don't have a gas analyser or O2 sensor).
And then the same just off the stops (hanging on the cable).
If that doesn't fix it, there is a small blockage in one of the passages/jets/holes or a vacuum leak.
Do you have the dual ignition option?
 

Martin

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radfordc.......yes you are confused again...but only partially.

Yes, the engine is an air pump.
Yes, if you point the nose down and drive the prop the temps will climb because you did not increase the fuel being delivered to match the rpm...it goes lean.

No,the needle is not the only thing in the carb to dictate the fuel flow.
Fuel :
pilot jet provides idle....needle provides mid range adjustment...main jet provides top end....float position also effects fuel flow.
Remember....that all of these circuits in the carb overlap each other and they all have to play nice with one another for the engine to transition smoothly from idle to full throttle.
Air :
And the clip position dictates how far the slide will open....which allows the airflow through the carb.....low slide position is low airflow....high slide position is more airflow.

The jets provide the fuel.....
The slide allows a given amount of airflow through the carb according to clip position.
Float position will also effect fuel flow.

The carb is a very simple device that does one job......delivers fuel and air to the cylinders.
However,that very simple device can be quite complicated to tune properly.

Kevin
Hi Kevin, I believe you are mistaken regarding the needle clip position. The needle can be raised or lowered using the clip, but this is in relation to the air slide and only affects fuel flow through the needle jet relative to the slide position. Moving the needle has no affect on airflow.
You are correct......
On an extended approach with the throttle pulled to idle.....the cylinders tend to go lean.
The airflow is driving the prop which is increasing rpm but only the pilot jet is supplying fuel because the throttle is closed.
The pilot jets can only physically supply so much fuel....any rpm beyond their fuel flow capability will lean the mixture to the point of seizure if the condition is allowed to continue for too long as the temps climb.
That's why you are suppose to blip the throttle occasionally while descending...to supply fresh fuel & oil to the cylinders for lubrication & cooling plus keep the cylinders temps stabilized to avoid cold seizure in the event of a panic slam of the throttle for a go around.

Dana......it has been my experience that if ANY engine "loads up" while at idle...it is because the carb/carbs is/are not jetted properly to begin with.
I do understand your viewpoint but let me see if I can explain this so that it will make sense.
The throttle only controls the carb/carbs that control fuel & air flow which in turn control rpm of the engine,correct ?
The throttle doesn't control the ignition system does it ?
So the ignition system is operating independently of fuel system and it is doing that by sensing the rpm of the engine....retarding & advancing the spark curve.
For an engine to "load up" while at idle 1 of 2 things has to happen.....either it is getting too much fuel or the ignition is being cut to stop the burn.
Even if the prop is being driven by the airflow....any mixture that is pulled into the cylinder is being burnt by the ignition firing...no matter what rpm.
The ignition system has no clue whether that particular engine rpm is due to the prop being driven or if it is due to the throttle being open and the carbs supplying fuel....the ignition just fires and burns the mixture with whatever spark advance that rpm calls for.
So an engine will actually be lean instead of rich if you drive the prop with airflow and increase the rpms without opening the throttle to supply the correct fuel mixture for that given rpm.
For the engine to go rich and "load up" means the pilot jets are too big and the prop being driven is pulling way too much fuel for the ignition to keep up with at that rpm.
Remember.....the whole principle is that the ignition system has no idea why the engine is at that particular rpm....prop driven or fueled...it only burns the mixture for that given rpm.....pilot jets too large to begin with and the cylinders "load up" because the ignition cant keep up.

I hope that made sense....

Kevin
I fail to see how the ignition system has any bearing on this subject. Turning the engine at high speed during a steep descent with throttle closed will cool the engine but lubrication will be marginal, and blipping the throttle will help keep the engine alive.
 

Martin

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I may be confused (it's happened before!) but all my experience and education on ultralight engines tells me that if you increase the prop speed by diving, the engine will run leaner at the same throttle position. The explanation is that the engine is basically an air pump....more rpm more air getting pumped into/through the cylinder. The amount of fuel is controlled by the position of the needle....if you don't move the needle the amount of fuel remains about constant. Perhaps an increase in suction may result in a small increase in fuel flow, but the dominant effect is the amount of air being pumped.

If I set my throttle for cruise conditions (level flight and normal cruise rpm of about 5800 rpm) the EGT will stabilize at around 1100-1150 F. If I now put the nose down so that the air speed increases I will see an increase in rpm and also an increase in EGT. Often the EGT will go above red line of 1200 F forcing me to close the throttle before seizing the engine.

I don't know what is causing your problem of the cylinder quitting but I'm sure that the mixture is leaner under the conditions described.
If you are worried about seizing, then it makes more sense to open the throttle rather than close it
 

Dana

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If by “lean” you mean insufficient fuel/oil to adequately lubricate and cool the engine at a particular rpm, then OK, that’s one way to look at it. But by “lean” and “rich” I was speaking of only the air/fuel ratio and what affects it. But I looked back at what I wrote above and it’s not quite correct, or at least mixed up. With the throttle closed, airflow through the carburetor is relatively slow so it’s not generating much vacuum in the venturi. However, the engine-as-air-pump is still sucking so you have low pressure downstream of the mostly closed slide, where the idle circuit is. That low pressure draws fuel through the idle circuit. At wider slide openings, Bernoulli effects in the venturi start to predominate and the situation changes.

With high rpm pulling against a closed slide, the increased suction pulls in more fuel but the mass airflow doesn’t increase as much due to the choked off slide so it may be “rich” according to the actual air/fuel ratio but still too “lean” according to the engine’s needs. Of course, that will depend on the particular carburetor’s idle circuit design, which like everything else in carburetor design is a bit of a black art and not my area of expertise.

I think we all agree that a long descent at partial throttle will certainly let the engine go lean, and in the OP’s case—if I understand him correctly—descending fast enough that the engine is turning 3200 with the throttle closed is probably not a good thing. But (I just looked), that engine has dual carburetors, no? Sounds like one just may not be adjusted quite right.

Ignition doesn’t affect any of this except whether the engine keeps running. I never thought of “loading up” in terms of a failure of the ignition system to light the non-optimum mixture, but I guess that’s what it comes down to, though the fault is not with the ignition if the carburetor doesn’t supply the correct mixture.
 

MadProfessor8138

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Martin.......you are absolutely correct about the clip position on the needle.
Totally my mistake...
It was late and I was multitasking when I wrote that.....I was thinking of a particular "very weird" carb that I have been dealing with that has nothing to do with the carbs we are discussing.
Good catch and thank you for pointing that mistake out......

Kevin
 

MadProfessor8138

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I fail to see how the ignition system has any bearing on this subject. Turning the engine at high speed during a steep descent with throttle closed will cool the engine but lubrication will be marginal, and blipping the throttle will help keep the engine alive.

The point that I was trying to describe was that the engine is already running lean due to the prop being driven by airflow so for the engine to "load up" by having too much fuel in the cylinders then you either need incorrect jetting or a faulty ignition system to cause that situation.


Turning the engine at high speed during a steep descent with throttle closed will cool the engine but lubrication will be marginal, and blipping the throttle will help keep the engine alive.
We will probably agree to disagree on this one.....somewhat.
Closing the throttle and dropping the nose will cool the engine for a bit.....but continue that descent with the prop being driven by airflow,rpm increased and a lean mixture....temps will start to climb again.
That's why blipping the throttle is recommended.....by doing so,you take control of the rpm,fuel & oil supply and control temps.

Kevin
 

radfordc

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If you are worried about seizing, then it makes more sense to open the throttle rather than close it
If the throttle is completely closed to idle it doesn't matter what the EGT is, the engine won't seize; and the EGT normally won't go up anyway with the throttle closed...not enough fuel I guess. Descending at partial to full throttle is the danger zone where the EGTs can get out of hand. While "blipping" the throttle might provide some help I've found it better to just pull back to idle power. If you are concerned about the engine loading up and quitting at idle there is another technique that works. Slow the plane into a very nose high attitude....this puts you on the back side of the power curve where you need to add more power to go slower. Now you can run the engine at mid throttle and still be descending.
 

radfordc

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I think we all agree that a long descent at partial throttle will certainly let the engine go lean, and in the OP’s case—if I understand him correctly—descending fast enough that the engine is turning 3200 with the throttle closed is probably not a good thing.
It's not necessarily a bad thing. As long as the throttle is closed the engine won't seize unless it doesn't get enough lubrication to the bearings. At 3200 it doesn't take very much lube to keep the engine happy. If the carbs are properly adjusted so that the engine idles well there should be enough oil/fuel mixture present.
 
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