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Lean of Peak stumble

rvanstory

Well Known Member
I have a standard compression IO-540 that I run LOP in cruise. My injectors are balanced to .4 gph of each other. While cruising in smooth air, every once in a while, I feel a very small (but perceptible) "stumble" in the engine. I would not call it a "misfire" and you can't hear it at all (engine purrs like a kitten), only feel it. If I'm in smooth air and go to ROP, I don't feel the slight stumble at all.

In a webinar, Mike Busch once said that some pilots don't like LOP because their engine isn't "as smooth" as ROP. He agreed. He said he'd trade a little extra smoothness for lower fuel burn anyday. The suggestion was that you would experience a slightly rougher engine in LOP ops.

Here's my questions for brain trust:
1) Is a slight stumble once in a while normal while running LOP?
2) Could I possible have a spark issue? (weak plug or wire)
3) Would fine wire plugs help engine run smoother at LOP?

I'm not looking to debate LOP ops, just seeing if others have same experience.
 
I’m not saying this is the root cause, but it is something I have noticed with the way Van’s sets up the fuel system in the RV-7. If you fill the tanks near full then taxi on rough ground, some fuel will slosh into the vent line. This will drain to the low point in the vent line and cause a temporary blockage of the fuel system. As the engine runs and draws fuel out of the tank, a small vacuum will develop in the tank. As the vacuum gets large enough, it will eventually pull air past the fuel that has got trapped in the vent line. This is less noticeable at ROP because the engine will simply become leaner and approach peak power without you likely noticing any difference in tone (you may see it in the EGT rising). When LOP and especially with a GAMI spread as great as yours (mine is 0.1GPH) this vacuum issue may be enough to stop one cylinder from firing properly because it was already on the leaner limit of successful combustion. This issue can be made worse on hot days, high altitude or when running mogas with a higher vapor pressure. I’m not saying this is definitely the cause, it is just something that I have noticed. You will also see the issue reflected in fluctuations of your fuel pressure, assuming you have a reading for that and your sensor is located between the mechanical pump and fuel servo.
Tom
RV-7, IO-360 M1B
 
I’m not saying this is the root cause, but it is something I have noticed with the way Van’s sets up the fuel system in the RV-7. If you fill the tanks near full then taxi on rough ground, some fuel will slosh into the vent line. This will drain to the low point in the vent line and cause a temporary blockage of the fuel system. As the engine runs and draws fuel out of the tank, a small vacuum will develop in the tank. As the vacuum gets large enough, it will eventually pull air past the fuel that has got trapped in the vent line. This is less noticeable at ROP because the engine will simply become leaner and approach peak power without you likely noticing any difference in tone (you may see it in the EGT rising). When LOP and especially with a GAMI spread as great as yours (mine is 0.1GPH) this vacuum issue may be enough to stop one cylinder from firing properly because it was already on the leaner limit of successful combustion. This issue can be made worse on hot days, high altitude or when running mogas with a higher vapor pressure. I’m not saying this is definitely the cause, it is just something that I have noticed. You will also see the issue reflected in fluctuations of your fuel pressure, assuming you have a reading for that and your sensor is located between the mechanical pump and fuel servo.
Tom
RV-7, IO-360 M1B

Tom I recently had some fuel pressure fluctuations (-1 or -2 psi) but only after topping off the tanks and only on the first 2 tank switches. Long story short this is what you describe, fuel in the vent line. However, I don't think it will matter to the engine enough to cause a LOP miss. The only thing it does to me is drop the fuel pressure a tiny bit until air is drawn in, the engine won't ever know as there was still 27ish PSI of fuel to the servo.

Randy, not an IO-540 but my engine does not do this. Perhaps next time it happens do an inflight mag check. I'm betting it's nothing to worry about.
 
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I have a standard compression IO-540 that I run LOP in cruise. My injectors are balanced to .4 gph of each other. While cruising in smooth air, every once in a while, I feel a very small (but perceptible) "stumble" in the engine.....just seeing if others have same experience.

Yes. My observation is the tendency toward roughness increases with altitude. It is smooth while far LOP down low, with the stumble you describe appearing richer (but still LOP) with increasing altitude. I typically cruise at peak EGT in the 9.5-11.5 altitude range. Where are you cruising?

Always found it interesting to see Deakin's red box illustrations show "optimum" mixture switching from LOP to an ROP setting above 9000 ft. Busch's copycat red fin stuff more or less ignores it (see below) showing only best power (ROP) cruise at the sort of NA power available above 9K. Deakin liked smooth. Busch (as you relate) accepts a stumble to save a buck.

Personally I can't help but think a stumble is hard on pendulum bushings, cranks, and prop hubs.

When LOP and especially with a GAMI spread as great as yours (mine is 0.1GPH) this vacuum issue may be enough to stop one cylinder from firing properly because it was already on the leaner limit of successful combustion.

Unlikely, as the ball valve diaphragm signal is based on deltaP across a fuel restriction. Within the practical flow range, pressure on both sides of the diaphragm changes equally. It's why a Bendix-type control is largely insensitive to fuel pressure.
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When I get to around 80 LOP above 8K, I get a VERY slight miss every 1-2 minutes; Definately not enough to characterize as a stumble. Lean mixtures are harder to ignite and even more so at higher altitudes, so not odd that you get a misfire every few thousand cycles when operating near the ragged edge. When that happens, I pull the throttle back a hair and richen it up a hair to the same FF rate and the mis disappears with no loss in airspeed. I typically cruise at 165 KTAS. When I am at 14K, closer to peak, this does not happen.

My injectors are balanced to about .3 GPH and have dual EI. I don't feel this is a sign of any problem beyond being too lean for the atmospheric conditions and the engine configuration. I speculate this is happening only on the leanest cyl, but cannot get my spread to 0 with available injector sizes.

Larry
 
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I typically cruise at peak EGT in the 9.5-11.5 altitude range. Where are you cruising?

I'm typically between 6.5 and 9.5. Feel it at all altitudes. Actually seems more pronounced at lower altitudes with higher MP.
 
On a 4 banger with 8 plugs it only takes one to do this. It is a misfire, just not both plugs at once in the same jug. My 7 seems to wander a touch and only go LOP at 9000 or less. Higher (12-16), I push towards peak EGT to maintain cruise speed and still have lower than 8 GPH, much more steady. If you record or look closely you will see the offending jug(s) have excursions of EGT upward for the retarded cylinder. If all are balanced, it can skip to other cylinders too. In doing LOP limit testing it was easy to find and hard to understand why I was getting two EGT peaks progressing to lean limit. It was the misfires bumping the EGT!! The first peak is best torque, the second is due to random retarded firings due to misfires. SAE papers from decades ago showed the realities of combustion initiation from plus firing. The studies resulted in better understanding of the whole process and focus on some of the variables contributing. One is the random nature of the kernels in the combustion chamber from preflames in compression before the spark lights them off. Some SAE and other papers in Combustion & Flame publications. The latter being much more scientific.

For some reason, at cruise I have a wander of FF. Maybe some build up of vapor, or thermal cycling of parts of the fuel delivery, so there is always some wander of A/F that can go in and out of a missing/roughness cycle. One day I'll figure out precisely what the wander cause really is, but not yet.

My recent west coast trip found lots of time to re-experience these phenomenon.

Two-Mags
 
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On a 4 banger with 8 plugs it only takes one to do this. It is a misfire, just not both plugs at once in the same jug. My 7 seems to wander a touch and only go LOP at 9000 or less. Higher (12-16), I push towards peak EGT to maintain cruise speed and still have lower than 8 GPH, much more steady. If you record or look closely you will see the offending jug(s) have excursions of EGT upward for the retarded cylinder. If all are balanced, it can skip to other cylinders too. In doing LOP limit testing it was easy to find and hard to understand why I was getting two EGT peaks progressing to lean limit. It was the misfires bumping the EGT!! The first peak is best torque, the second is due to random retarded firings due to misfires. SAE papers from decades ago showed the realities of combustion initiation from plus firing. The studies resulted in better understanding of the whole process and focus on some of the variables contributing. One is the random nature of the kernels in the combustion chamber from preflames in compression before the spark lights them off. Some SAE and other papers in Combustion & Flame publications. The latter being much more scientific.

For some reason, at cruise I have a wander of FF. Maybe some build up of vapor, or thermal cycling of parts of the fuel delivery, so there is always some wander of A/F that can go in and out of a missing/roughness cycle. One day I'll figure out precisely what the wander cause really is, but not yet.

My recent west coast trip found lots of time to re-experience these phenomenon.

Two-Mags


I would suggest a failure of one plug not firing once is not detectable in feel (maybe in the EGT data), but rather a stumble as you say you have is more likely an instance of a simultaneous misfire of both plugs.

Do you have mags, or EI? If mags, try a mag check at altitude under the subject conditions - it should work just like on the ground, a slight loss of power, but smooth. You may be shocked to find one mag behaving rather badly, and there is your problem. I had the same behavior as you described recently. Ground mag check just fine, at altitude engine ran perfectly smooth except for an occasional hiccup, and when I switched to the left mag, OMG, it misfired so badly the engine almost quit. Mags only 350Hrs. since new need to be overhauled!
 
I would suggest a failure of one plug not firing once is not detectable in feel (maybe in the EGT data), but rather a stumble as you say you have is more likely an instance of a simultaneous misfire of both plugs.

Do you have mags, or EI? If mags, try a mag check at altitude under the subject conditions - it should work just like on the ground, a slight loss of power, but smooth. You may be shocked to find one mag behaving rather badly, and there is your problem. I had the same behavior as you described recently. Ground mag check just fine, at altitude engine ran perfectly smooth except for an occasional hiccup, and when I switched to the left mag, OMG, it misfired so badly the engine almost quit. Mags only 350Hrs. since new need to be overhauled!

Two-Mags and only 200 hrs, but not impressed with the mags. I do a loaded mag check pretty regularly and do not find that as the cause, but a good thought. I would not really call it a stumble like eating a water slug, but just a roughness and loss of an even tune sound. I don't believe it is a two plug misfire.
 
I've had this on occasion also. I believe it is as others have mentioned - ragged edge of occasional plug not igniting mixture. I just turn the mixture knob a quarter turn or so and it doesn't recur.
 
Engine occasionally stumbles when I'm running extreme LOP. I know I've leaned too far when #3 EGT goes dark. I'm running an IO-360 with CS, Bendix FI (overhauled by AFP) and two P-Mags. Most of time I'm in No Hurry Mode; 22", 1800 RPM, 4.2-4.5 GPH. Bumping mixture closer to 4.8 GPH clears things up.
 
Finally found the culprit!!!

Circling back to this older post from August to report that I FINALLY found the solution. It all had to do with my spark plug gaps! Here's my "journey".

Last week I decided to spring for the Tempest Fine Wire plugs as some have reported that their engine was "smoother" after switching to fine wires. They are HORRIBLY expensive, but after getting over the costs, decided to give it a try.

When the fine wires arrived in the box, they have a note that says "Do not attempt to change the gap on these plugs. If they are not correct, send them back to factory." They have the iridium wires break on some.

The gaps from the factory was from .015" (too narrow) to .019" (in the range). Didn't want to risk breaking a wire, so decided to simply install "as is".

Well, my stumble at altitude and LOP got WORSE!! After spending $1,300 for plugs, I was upset to say the least.

After an email to SureFly, and a call to Tempest, I learned that electronic ignitions need a wider gap than mags! I didn't know that. In fact, Tempest tech support suggested a .028" gap, wider than the .022 max they have in their literature. Surefly support suggested .022" (maximum recommendation from Lycoming and Tempest).

Re-gapped the plugs so the SureFly plugs are at .026" and the Slick mag plugs were at .020". Test flew this morning and VERY HAPPY to report that at LOP ops, plane was incredibly smooth and didn't stumble once!!!

I'm convinced that if I were to put my massive plugs back in with a wider gap on the EI (Surefly) side, my problem wouldn 't have cost $1,300 to solve. Hopefully my ignorance about EI's needing a wider gap will help someone else down the road.
 
Thanks for the update Randy!! Good to know the gaps are sensitive, more than I would have thought.
 
Follow up 2 years later!

Circling back to this older post from August to report that I FINALLY found the solution. It all had to do with my spark plug gaps! Here's my "journey".

Re-gapped the plugs so the SureFly plugs are at .026" and the Slick mag plugs were at .020". Test flew this morning and VERY HAPPY to report that at LOP ops, plane was incredibly smooth and didn't stumble once!!!

Well, as luck would have it, the spark plug gaps did help, but the problem (small stumble when running LOP) has continued to be there, just with less frequency. Until recently.....

I was sent post on Facebook RV owners group regarding this same issue from another RV-10 builder. He battled this same issue for years. His cure was to installed turbo injectors into his IO-540 VS the standard non-turbo injectors.

I then called Don at Airflow Performance about ordering these turbo injectors for my plane. After a few questions about my setup, he said he did not think this "fix" would resolve my problem. He did however feel that the stock restrictors for the Precision Airmotive Silverhawk system were not the optimal ones for the flow divider on this system. Stock restrictors were all at .028.

After rattling off some "engineering speak" that I'm not smart enough to understand, he suggested that I try different, smaller diameter restrictors in my system. He said the additional backpressure this created would help the flow divider do its job better.

Don suggested 3 different sizes. .0250 for cylinders 1&2. .0245 for cylinders 3&4. .0255 for cylinders 5&6.

After installation, I test flew for the GAMI spread with these new restrictors. Immediately I could tell a difference in LOP ops. Mainly, where I used to have about a .2 GPH fuel flow tolerance between smooth and some stumble in the engine, I know had about 1.5 GPH tolerance!!! That was HUGE!!!

This weekend I took my first 1,200 mile cross country trip with the new set up. Never a stumble at LOP AND the tolerance between smooth and not-perfect was again, about 1.5 GPH.

But... I also think I gained about 1-2 knots in TAS! I didn't expect any performance changes except for the smoothness, but it did seem to make a small difference. I cruised LOP at 8k-9k at 12 GPH and was getting 171 KTAS all day long. (I could cruise at 10.5 GPH smoothly, just not as fast.)

Here's my take.... If you have an IO-540, call Don at Airflow Performance. His knowledge is impressive and think you may benefit if your running the stock non-turbo injector restrictors that come with the "Bendix" system.

Thank you Don for your help! I think you're a genius and I appreciated your help!!!
 
Well, as luck would have it, the spark plug gaps did help, but the problem (small stumble when running LOP) has continued to be there, just with less frequency. Until recently.....

I was sent post on Facebook RV owners group regarding this same issue from another RV-10 builder. He battled this same issue for years. His cure was to installed turbo injectors into his IO-540 VS the standard non-turbo injectors.

I then called Don at Airflow Performance about ordering these turbo injectors for my plane. After a few questions about my setup, he said he did not think this "fix" would resolve my problem. He did however feel that the stock restrictors for the Precision Airmotive Silverhawk system were not the optimal ones for the flow divider on this system. Stock restrictors were all at .028.

After rattling off some "engineering speak" that I'm not smart enough to understand, he suggested that I try different, smaller diameter restrictors in my system. He said the additional backpressure this created would help the flow divider do its job better.

Don suggested 3 different sizes. .0250 for cylinders 1&2. .0245 for cylinders 3&4. .0255 for cylinders 5&6.

After installation, I test flew for the GAMI spread with these new restrictors. Immediately I could tell a difference in LOP ops. Mainly, where I used to have about a .2 GPH fuel flow tolerance between smooth and some stumble in the engine, I know had about 1.5 GPH tolerance!!! That was HUGE!!!

This weekend I took my first 1,200 mile cross country trip with the new set up. Never a stumble at LOP AND the tolerance between smooth and not-perfect was again, about 1.5 GPH.

But... I also think I gained about 1-2 knots in TAS! I didn't expect any performance changes except for the smoothness, but it did seem to make a small difference. I cruised LOP at 8k-9k at 12 GPH and was getting 171 KTAS all day long. (I could cruise at 10.5 GPH smoothly, just not as fast.)

Here's my take.... If you have an IO-540, call Don at Airflow Performance. His knowledge is impressive and think you may benefit if your running the stock non-turbo injector restrictors that come with the "Bendix" system.

Thank you Don for your help! I think you're a genius and I appreciated your help!!!

I have the 025's in my 10 and 024's in my 6 (320). Based upon conversations with Don, I would NEVER use the 028's. The smaller orifice up's the pressure a bit behind the restrictor and this results in better atomization of the fuel which results in a more even burn/combustion, so no surprise that you saw a slight uptick in performance.
 
I have the 025's in my 10 and 024's in my 6 (320). Based upon conversations with Don, I would NEVER use the 028's. The smaller orifice up's the pressure a bit behind the restrictor and this results in better atomization of the fuel which results in a more even burn/combustion, so no surprise that you saw a slight uptick in performance.

That's why this posting was updated with new info. How many 10's are out there flying with the stock restrictors (.028)? Who would have known without a conversation with Don that this was not optimal? Hopefully someone else can learn from my lack of knowledge.
 
I have a similar stumble LOP as you have described.
I changed to 026 all around (+\-) for GAMI spread.
Didn’t really change it for me.
I’m going to try the turbo nozzles. They’re in the mail.
Given I have evidence of fuel staining at the injector bodies on 5/6 cylinders I figure it can’t hurt.
Showplanes cowl here which also seems to be a common variable for this issue.
I’ll report back when I’ve tried this.
Edit, I have the SDS Oring intake tubes and have done extensive intake leak testing. None found
 
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I have the 025's in my 10 and 024's in my 6 (320). Based upon conversations with Don, I would NEVER use the 028's. The smaller orifice up's the pressure a bit behind the restrictor and this results in better atomization of the fuel which results in a more even burn/combustion, so no surprise that you saw a slight uptick in performance.

Do you still meet Lycoming's spec for minimum fuel flow at sea level full rich, full throttle power? If no, do you have reasons (data) to suggest that their number is overly conservative?
 
data point

Flying my Comanche 260B, IO-540-D4A5, stock Bendix RSA-5 system with stock injectors, routinely run 75-100 deg. LOP 11.4 gph. Never stumbles. 50 LOP 2300/25" will get me 11.7 gph at lower altitudes. Bendix mags with 37BY plugs.
 
Good timing. I just put the same injectors in my IO540 yesterday and going to test today. Randy, are you going to have to move around any injectors or are you good with the combination that Don gave us?
 
Do you still meet Lycoming's spec for minimum fuel flow at sea level full rich, full throttle power? If no, do you have reasons (data) to suggest that their number is overly conservative?

My home field is 800 msl and wot/2700 gives me around 24-25 gph and about 250 ROP. I have been down to about 400 msl with no negative side effects. Don has been around the rsa type equipment for decades and does all sorts of performance work and has flow benches and sophisticated measuring equipment. There is absolutely no one I trust more in this area. If he recommends the 025’s, i have all confidence they are appropriate.

Lyc is very conservative so not surprised that they use much larger injectors to deal with any number of issues that could arise. No different than specifying 10 quarts of oil in an engine that only needs 5, unless you are foolish enough to be flying around with an engine that burns a qt an hour. I am not that guy and therefore don’t need those absurdly conservative guidelines.

Now, i will say that I can’t get any higher than 250 rop like the stock setup can. However IMHO there is no reason I would want to. I would never throw tons of fuel at a cooling deficiency.

Recommendations like this tend to be given to those people like me that are well tuned into what their engine is doing at any given minute. If you have no instruments or never look at them, you are wise to stick to Lyc recommendations, for the built in safety margins.

Larry
 
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Good timing. I just put the same injectors in my IO540 yesterday and going to test today. Randy, are you going to have to move around any injectors or are you good with the combination that Don gave us?

I had to move 2 restrictors around. GAMI spread was .8 when initially installed as instructed. Swapped 2 restrictors to get it down to .4 GPH spread.
 
Don Rivera is absolutely the guy to go to for this stuff, he'll treat you right. Once you get that system dialed in with Don's help, the only way to improve past that point is by installing a computerized system like SDS or EFII with a higher pressure fuel rail.
 
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