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Running Too Lean?

J

Jbon

Active Member
Well, I know my idle mixture is too lean, because I'm seeing no rise in RPM on shut down. Unscrewing the mixture screw doesn't help, so I suspect there's an issue with the mixture circuit. The the carb is headed to the shop to have this checked out.

The company who sells my electronic ignition system, Electroair, thinks I'm running too lean overall, this based on a large drop of 150-175 RPM at run up. The drop on the single left mag is only 50.

I've checked for an induction leak. . . no joy.
I've changed the plug wires. . . no joy.
I've swapped the upper/lower spark plugs. . . no joy.
I've checked the EIS timing. . . perfect.

So, I assumed there may be an issue with the EIS itself, perhaps a bad coil. They don't think so, especially since my plugs are so clean. They're saying that lean mixture roughness will be apparent on the EIS but not the mag because of the higher efficiency and longer spark duration of the EIS.

I Figure since I'm sending the carb to the shop to investigate the idle circuit leanness, I might as well wait and see if something else shows itself.

The carb shop tells me that a number of their customers have had their overall mixture richened by drilling out the main jet ten-thousands. I'm not sure mine needs this, but I'm pretty sure its not running overly rich either. My plugs appear pretty light gray. . not sure if it's light enough to indicate an over-lean condition, but I'm going to get the carb shop to look at it. I'm not really seeing excessively high EGT, but I've only got about 10 hours on the airplane, and I always aggressively lean in flight, so I can't say for sure. Oil temps could probably be higher. I run near the bottom of the green.

So what's the harm of having the carb richened a bit. Has anyone done this? I figure I can still use the mixture knob to lean where I need to be, but is there any downside to doing this?

Thanks!
 
Sounds like things are fine from your description. Perhaps the Electroair system needs some adjustment to their advance or some other tweak (not familiar with Electroair) . . . instead of calling the drop the fault of your induction system.

Lean is good. Lean is cool. Except when pulling 75% or more on takeoff and climb when the excess fuel is used . . . not for optimum combustion but for cooling. I think the reason for the drop lies with Electroair, not your engine as it is currently set up.

My two cents.
 
You need some holistic advice and support here. You are kind of all over the map. My comments in line below.

Jbon said:
Well, I know my idle mixture is too lean, because I'm seeing no rise in RPM on shut down. Unscrewing the mixture screw doesn't help, so I suspect there's an issue with the mixture circuit. The the carb is headed to the shop to have this checked out.

How do you know it's too lean. Does it idle rough and want to die? Do you get a stumble when accelerating from idle? If no to both, you are not too lean. The test you mention is a guideline. Most experts will tell you that you should only be rich enough to achieve a smooth idle and avoid acceleration stumble and lean enough to avoid plug fouling.

The company who sells my electronic ignition system, Electroair, thinks I'm running too lean overall, this based on a large drop of 150-175 RPM at run up. The drop on the single left mag is only 50.

This guidance makes no sense to me. RPM drop amount should change very little between rich and lean conditions and won't affect the two ignitions differently; Certainly not enough to explain your difference. Use the search here. Many have very similar numbers to those that you are posting. Most of the difference is likely due to the fact that the timing is different for the two systems at your runup speed/RPM. timing advance directly impacts RPM at any given throttle/mixture combination, especially noticeable at idle and lower RPMs. The electroair is changing the advance dynamically and is likely not the same 25* that your mag is set to at your runup RPM.

Further, above you talk about idle mixture being lean and here, at these RPMs, your talking about main metering. Carbs have completely different circuits for idle and main fuel metering.

I've checked for an induction leak. . . no joy.
I've changed the plug wires. . . no joy.
I've swapped the upper/lower spark plugs. . . no joy.
I've checked the EIS timing. . . perfect.

So, I assumed there may be an issue with the EIS itself, perhaps a bad coil. They don't think so, especially since my plugs are so clean. They're saying that lean mixture roughness will be apparent on the EIS but not the mag because of the higher efficiency and longer spark duration of the EIS.

This also doesn't make sense the more powerful spark should be MORE tolerant of the lean condition, not less. Very lean mixtures are harder to light and a stronger spark will allow a leaner mixture to fire off more consistently than a weak spark. However, as I mention below it is quite unlikely that you are running Lean at 1800 RPM, if the red knob is at full rich. These carbs are designed to run rich. This you can test quite easily by leaning at 1800 and observing your EGTs.

I Figure since I'm sending the carb to the shop to investigate the idle circuit leanness, I might as well wait and see if something else shows itself.

The carb shop tells me that a number of their customers have had their overall mixture richened by drilling out the main jet ten-thousands. I'm not sure mine needs this, but I'm pretty sure its not running overly rich either. My plugs appear pretty light gray. . not sure if it's light enough to indicate an over-lean condition, but I'm going to get the carb shop to look at it. I'm not really seeing excessively high EGT, but I've only got about 10 hours on the airplane, and I always aggressively lean in flight, so I can't say for sure. Oil temps could probably be higher. I run near the bottom of the green.

You may or may not need to drill your jet. Many RV'ers need to drill their jets to get proper fuel flow in WOT conditions. I did. However, you have not posted any data. Use the search. This subject has been addressed at great length. Be advised, drilling the jet will increase fuel across the range from just off transition to WOT and will waste fuel when you don't lean. However, the decision point is whether or not you have enough fuel at WOT to get into the 150-200 ROP range. This can only be adjusted by increasing the size of the jet. It is quite unlikely that you are lean at mid-throttle RPMs. These carbs have many flaws, but I have not seen anyone complain of lean mid-throttle conditions.

So what's the harm of having the carb richened a bit. Has anyone done this? I figure I can still use the mixture knob to lean where I need to be, but is there any downside to doing this?

The harm is in reduced power. Overly rich mixtures reduce power, in addition to wasting fuel. Better to make this decision to drill the jet based upon data, not hunches or a mixed bag of advice.

Thanks!
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MikeS said:
Sounds like things are fine from your description. Perhaps the Electroair system needs some adjustment to their advance or some other tweak (not familiar with Electroair) . . . instead of calling the drop the fault of your induction system.

Lean is good. Lean is cool. TRUE when LOP Except when pulling 75% or more on takeoff and climb when the excess fuel is used Again, when LOP it is fine, but takeoff and climb are generally better done ROP, full rich and a target EGT in the climb. . . . not for optimum combustion but for cooling. Completely false, the extra fuel is for optimum combustion otherwise the ICP's rise and that drives up the CHT. I think the reason for the drop lies with Electroair, not your engine as it is currently set up.

My two cents.
Click to expand...

Just thought I would fix it up a bit for you, I know you had the right intentions, but the delivery needed refining.
 
JBON,

What is your takeoff fuel flow MP and RPM, do you have an ems and can upload the data?
 
No engine expert here but I believe the idle mixture check you're concerned about is there just to insure that the engine is not running so lean or so rich at idle and full rich selected, that it could quit on you. Had that happen on base once when the idle mixture was set too lean. :-(

Drilling out jets to enrich the mixture is something else. If at cruise RPM and full rich, the RPM did not rise as you leaned from full rich but immediately begins to drop off, then you might suspect you were jetted too lean.
 
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Again, from your plug colors and other description of engine operation, I wouldn't change anything at this point. The "problem" is an excessive drop on the electronic ignition. It is there I would look. It's not firing at the proper time in the combustion cycle. Should be an easy adjustment, but I know nothing of that particular system. I'd leave your jets alone for now.
 
lr172 said:
You need some holistic advice and support here. You are kind of all over the map. My comments in line below.
Click to expand...

I agree that it doesn't make any sense that the EIS would be more sensitive to a lean condition. I've never been able to wrap my head around that concept.

I'm aware that the idle circuit is not a player at run up RPM, but it is pretty clear to me that there is an issue with the idle circuit. Turning the mixture screw out 5 or so turns should make a difference in idle mixture. With mine, it does not.

I think it's good advice not to do anything with the main jet until I have more data, so I'm putting that off. I have pulled the carb and sent it to the shop to have it looked over. Given that it hasn't been looked at for over 18 years, that seems prudent.
 
RV10inOz said:
JBON,

What is your takeoff fuel flow MP and RPM, do you have an ems and can upload the data?
Click to expand...
No fuel flow gauge (yet), and no EMS. Once I get this ignition issue resolved and I'm back in the air, I'm going to take a closer look at the engine's response to mixture. The airplane is new to me, so I don't have a lot of data.
 
Jbon said:
I agree that it doesn't make any sense that the EIS would be more sensitive to a lean condition. I've never been able to wrap my head around that concept.
Click to expand...

I offer that engines with electronic ignitions are more sensitive to lean conditions at idle than mags, and the more the timing is advanced at idle the more the issue will present itself (based on some personal fuel injector trouble shooting).

To the original issue, are you really sure you have the timing right? For Lightspeed and pMag I know the timing is set at Top Dead Center (TDC) - not 25 degrees before TDC like a mag. I don't know what Electroair tells you to do.

Is this the first run with the ElectroAir? If so, did the problem exist when running mags?

Carl
 
Would an engine even start or run with a timing advance error of 50 degrees BTC?

My dual Pmags have always idled well, lean, one only or both.

Bevan
 
Carl Froehlich said:
I offer that engines with electronic ignitions are more sensitive to lean conditions at idle than mags, and the more the timing is advanced at idle the more the issue will present itself (based on some personal fuel injector trouble shooting).

To the original issue, are you really sure you have the timing right? For Lightspeed and pMag I know the timing is set at Top Dead Center (TDC) - not 25 degrees before TDC like a mag. I don't know what Electroair tells you to do.

Is this the first run with the ElectroAir? If so, did the problem exist when running mags?

Carl
Click to expand...

You're in agreement with Electroair regarding lean mixture. I have to admit I don't understand it. I get the concept of lean mixture misfire, I just don't understand why it only manifests itself when running on EIS but not a mag. Perhaps you can explain this to me.

Electroair has you jumper the module so that the there is no advance when checking the timing. I observed a perfect TDC reading w/ the timing light.

Yes, the engine has always had the EIS on the right side. I've only had the airplane for 10 hrs., and I didn't notice this large a drop early on.
 
Jbon said:
I'm aware that the idle circuit is not a player at run up RPM, but it is pretty clear to me that there is an issue with the idle circuit. Turning the mixture screw out 5 or so turns should make a difference in idle mixture. With mine, it does not.
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What makes you sure that there is an issue with your idle circuit? I have never seen an RPM rise when leaning at idle and mine is great. My idle is very smooth, I can get as low as the 500's in RPM and I have no stumble on acceleration (very smooth). I initially set it at 2.5 turns out. I progressively richened it maybe 1.5 turns or so through my phase I, as I was getting a stumble on acceleration. I also was trying to get an RPM drop and finally stopped when the engine was running well. Be sure to stretch the spring or add a washer to insure that you keep tension on the screw.

I would not get hung up on the RPM rise. I frankly don't believe that Dynon (my RPM indication source) reacts fast enough to see the RPM drop and I honestly didn't try very hard. There are many other ways to adjust carbs based upon sound, feel and performance.

Larry
 
Jbon said:
You're in agreement with Electroair regarding lean mixture. I have to admit I don't understand it. I get the concept of lean mixture misfire, I just don't understand why it only manifests itself when running on EIS but not a mag. Perhaps you can explain this to me.

Electroair has you jumper the module so that the there is no advance when checking the timing. I observed a perfect TDC reading w/ the timing light.

Yes, the engine has always had the EIS on the right side. I've only had the airplane for 10 hrs., and I didn't notice this large a drop early on.
Click to expand...

A spark is a spark is a spark. It is true that mags have weak spark energy at low RPM's and I suppose there is a possibility that a strong spark is more sensitive to lean conditions, but that defies the principals as I understand them. The spark timing is absolutely a factor.

I still believe that the issue you outline here (RPM drop differential) is all about timing and not mixture.

As I mentioned before, advancing the spark timing will increase RPM and vice versa. Let's look at how this affects RPM drop at run-up. If your mag is firing at 25 and your EI is firing at 35, you can estimate your effective timing at 30* (remember, two fires are lit and meet in the middle to form complete combustion). When you pull out the mag, your timing decreases from 30 to 25 (35-10) (a single fire must now go all way to the other side, not the middle and I am guessing at 10 here for the explanation) and the RPM goes down due to the reduced effective advance. You put the mag back in and timing goes back to 30 and RPM goes up. You now drop the EI and your timing goes down to 15 (25-10). Your RPM drop will be greater (lower RPM) than above because your advance is 10* more retarded than above.

As you can see from above, the timing of the EI at your mag drop RPM has a lot to do with the differential RPM drop. If your differential is larger than it used to be, that points to a timing drift in one or both or your ignition systems. On EI, that could be the overall timing reference, as set at calibration or related to the programmatic way in which it dynamically changes the timing (could be their system, but more likely a vacuum/MAP input that they use as a basis for adjusting the timing along with RPM).

I will add that RPM drop can be affected by several other ignition related problems, such as a failed or weak plug, failing wires, etc. Mag drop differentials can expose issue at an individual cylinder level in addition to the ignition "system," so you must be cautious in interpreting test results.

EDIT: I'll add that a more powerfull spark could ignite the mixture faster, especially when lean, providing a small effective increase in advance. However, it would be a small difference. I used to have two mag and my RPM differential was always in 10 RPM range. When I added my EI, that became 20-30 RPM. However, I built my own advance table and set the advance at 25* @ 1800 RPM to give me good comparison readings for my run-up.


Larry
 
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