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Engine "Bogs Down" On Takeoff

VF84Sluggo

Well Known Member
Lately when advancing the throttle for takeoff, passing the 2000-2100 rpm range the engine seems to stumble/"bog down" for a couple of seconds as the throttle is still being pushed forward. Then, the engine bursts on up to full power. Runs smooth from then on. In flight, no hesitation at all if going from idle to full power, even if simulating a go-around starting from approach speed/full flaps.

Also I've noticed the pre-takeoff mag checks seem a little rough, almost like the engine is "missing" a bit. Leaning during the mag checks seems to smooth things out to a normal single-mag sound/feel.

Not sure what the cause is. It's almost like the engine is too rich when at full-rich mixture. I do lean aggressively on the ground after start-up. The engine is a Lycoming IO-360-B1B, Whirlwind constant speed prop. Slick mags, harness, plugs all new as of Aug 2021.
 
What kind of fuel injection system? Seems like mixture adjustments are needed there
Got no idea how to do this on a fuel injected engine. Or what the setting should be.

The servo is Bendix. And I forgot to mention, the engine has GAMI injectors, also installed at the August condition inspection.
 

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It could be some vapor/hot fuel working its way through the system, caused by high cowl temperatures and low fuel flow (just prior to throttle advance).

Do you see any correlation with OAT? Does "lately" perhaps mean higher OAT's? Have you flown the plane through a FL summer yet?

Try running it at perhaps 1700-2000 rpm for 20 seconds on the runway prior to your takeoff roll. Come back to idle, then immediately begin your normal takeoff roll/routine. This will run some cooler fuel through and may help diagnose your roughness if it doesn't stumble in this case.

If I fly around for several minutes at warm OAT's with minimal or idle power, slow flight, I can experience what you described for a couple seconds on throttle up.

What is your full throttle takeoff/climb fuel flow?
 
Yes, I've run the engine in hot weather. It was fine initially after the new mags, harness, plugs, GAMI injectors. That was in August. Still plenty hot in Florida then. This started when the weather cooled off some.

When I do a run-up, no hesitation when I set 1800 rpm. The gremlin seems to only emerge once it's time to take off.
 
The idle circuit is pretty much out of the equation after about 1200 RPM, so that is not the cause. If the bog was immediate upon throttle application from idle, it could be idle mixture. However, if you get a good transition off idle with the bog later, it is not. A bog is somewhat typical of a lean condition, but that is subjective when inexperienced folks describe symptoms. Suggest you have someone watch your takeoff. If they see black or gray smoke from the tail pipes as you apply throttle, that is a good sign of overly rich condition. I definately would not rule out a lean condition, as your description matches a lean condition. generally speaking, Rich is rough, lean is smooth, but low power. Cold weather means a need for richer mixtures. The fact that this started when the wx cooled, further points to a lean condition.

There are adjustments inside these servos that can make them overly rich or lean, including throttle transitions. On my 320, I pull the mixture back 1/2" for each take off, else I lose a bit of power from being overly rich.

It is not common for these units to have throttle application problems; much more typical on carbs that have accelerator pumps. How old is the servo?

An ignition problem is very unlikely as this sounds like a classic mixture transition issue. Ignition problems would show symptoms at other times as well.

Do you have a glass panel? If so, please post a chart of the EGTs /MAP/RPM from a take off that shows the symptoms. That will help.


Larry
 
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The idle circuit is pretty much out of the equation after about 1200 RPM, so that is not the cause. If the bog was immediate upon throttle application from idle, it could be idle mixture. However, if you get a good transition off idle with the bog later, it is not. A bog is somewhat typical of a lean condition, but that is subjective when inexperienced folks describe symptoms. Suggest you have someone watch your takeoff. If they see black or gray smoke from the tail pipes as you apply throttle, that is a good sign of overly rich condition. I definately would not rule out a lean condition, as your description matches a lean condition. generally speaking, Rich is rough, lean is smooth, but low power. Cold weather means a need for richer mixtures. The fact that this started when the wx cooled, further points to a lean condition.

There are adjustments inside these servos that can make them overly rich or lean, including throttle transitions. On my 320, I pull the mixture back 1/2" for each take off, else I lose a bit of power from being overly rich.

It is not common for these units to have throttle application problems; much more typical on carbs that have accelerator pumps. How old is the servo?

An ignition problem is very unlikely as this sounds like a classic mixture transition issue. Ignition problems would show symptoms at other times as well.

Do you have a glass panel? If so, please post a chart of the EGTs /MAP/RPM from a take off that shows the symptoms. That will help.


Larry
Hmmm...could be lean. And I agree, I don't think idle mixture is it. The engine spools right up just fine when doing the pre-takeoff run-up. No hesitation or anything.

I posted a reply a few minutes ago that I do lean aggressively for all ground ops. Pulled the plugs earlier, they looked ok. Not fouled or anything, no carbon deposits. Cleaned them anyway, rotated them on reinstall.

As I mentioned, it seems like the mag checks have been a little rougher than a couple of months ago. Leaning a bit on the run-up, 1800 rpm, actually seems to smooth single-mag ops. So that's why I was thinking the engine might be too rich at full rich/boost pump on when getting up to 2100 rpm. But like you said, the colder/denser air, if anything, would lean things out. And it does seem like this annoying bog/hesitation did start with the cooler weather.

Perhaps I'll see if this hang up at 2100 is more pronounced on a cold day.

Again, no hesitation at all when set up in flight at 60 knots, full flaps, idle descent, flight idle rpm, and then add power like I would for takeoff. The engine spins right up to 2700 rpm nice and smooth. I mentioned this "flight test" to my mech buddy, he thinks it's because I have ram air inflight. If this is the reason, then that points to not enough air when starting a takeoff and the engine hesitating at 2100.

So I don't know what the **** is going on. Plugs clean. Air filter clean. 25 hours on mags/harness/plugs and on the GAMI injectors. Good grief.
 
I would look at fuel delivery, and perhaps a variable obstruction or perhaps some water. I get the sense that once the "restriction" clears - all is well? I think that the fuel flow is adequate until that rpm, then it runs too lean to support the power requested. If you retard the throttle a bit when it is "bogging" will it accept that rpm? Is the fuel flow correct at each power setting? Make a habit of checking/knowing the fuel flow at 2000 rpm when things are normal, and what it should be at the "bogged" rpm. Perhaps you will be able to "predict" when it will bog by the lower fuel flow reading prior to the actual "bog".

I would start at the servo fuel strainer, then pull the supply line to the servo and measure the flow rate. (I believe 150% of max take off fuel flow is required) Don't take my word for the procedure is in FARSec. 23.955 — Fuel flow. Check the Fuel filter, selector, tank finger screens... How many years on the airplane? This debris was found in the selector of a C150 with an rpm loss problem.
 

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Randy - do you have an efis, or at least a fuel flow indicator? As others have mentioned, this information would go a long ways towards understanding what might be going on.
 
Hmmm...could be lean. And I agree, I don't think idle mixture is it. The engine spools right up just fine when doing the pre-takeoff run-up. No hesitation or anything.

I posted a reply a few minutes ago that I do lean aggressively for all ground ops. Pulled the plugs earlier, they looked ok. Not fouled or anything, no carbon deposits. Cleaned them anyway, rotated them on reinstall.

As I mentioned, it seems like the mag checks have been a little rougher than a couple of months ago. Leaning a bit on the run-up, 1800 rpm, actually seems to smooth single-mag ops. So that's why I was thinking the engine might be too rich at full rich/boost pump on when getting up to 2100 rpm. But like you said, the colder/denser air, if anything, would lean things out. And it does seem like this annoying bog/hesitation did start with the cooler weather.

Perhaps I'll see if this hang up at 2100 is more pronounced on a cold day.

Again, no hesitation at all when set up in flight at 60 knots, full flaps, idle descent, flight idle rpm, and then add power like I would for takeoff. The engine spins right up to 2700 rpm nice and smooth. I mentioned this "flight test" to my mech buddy, he thinks it's because I have ram air inflight. If this is the reason, then that points to not enough air when starting a takeoff and the engine hesitating at 2100.

So I don't know what the **** is going on. Plugs clean. Air filter clean. 25 hours on mags/harness/plugs and on the GAMI injectors. Good grief.

If this persists, I would consider sending the servo to Don at AFP for an overhaul. Your issue is one of throttle transition and if the issue appears ONLY in throttle transition, you can expect the problem to be inside the fuel metering device, or the servo in your case. Most any other issue (fuel delivery, air flow, ignition, etc.) would not be limited to JUST throttle transition. An exception to this can be bad coils which sometimes only fail under heavy load. But in your case, the initial movement to WOT would show it and would not wait for 2100 RPM.

Throttle transitions require a metered shot of extra fuel and the servo does this internally and if parts wear or drift in settings, this can affect performance.

Engine needs more air at 2700 than 2100, so it makes no sense that air blockage is your cause.

Again, engine data log would help move us from guessing to making more informed decisions.

Larry
 
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Yep, what I need to do is take a kneeboard card and note gph when the "bog" happens, and gph leading up to it, and after it.

I'll try pulling the throttle back, too, at the point this hesitation occurs, see what effect that has.

But yes, once the engine bursts on through this hesitation point, it runs fine. No "bogging" at all in flight. Now, after my last flight (about an hour), I did taxi back after landing and do another takeoff. Only a slight hesitation at this 2000-2100 rpm point on that takeoff. And as always, right on up to 2700 afterwards and smooth from then on.
 
If this persists, I would consider sending the servo to Don at AFP for an overhaul. Your issue is one of throttle transition and if the issue appears ONLY in throttle transition, you can expect the problem to be inside the fuel metering device, or the servo in your case. Most any other issue (fuel delivery, air flow, ignition, etc.) would not be limited to JUST throttle transition. An exception to this can be bad coils which sometimes only fail under heavy load. But in your case, the initial movement to WOT would show it and would not wait for 2100 RPM.

Throttle transitions require a metered shot of extra fuel and the servo does this internally and if parts wear or drift in settings, this can affect performance.

Engine needs more air at 2700 than 2100, so it makes no sense that air blockage is your cause.

Again, engine data log would help move us from guessing to making more informed decisions.

Larry
My mech seems to think, as you suggest, the servo is the likely culprit.

I have a JPI EDM-700 in the plane, with data dump capability.
 
If this persists, I would consider sending the servo to Don at AFP for an overhaul. Your issue is one of throttle transition and if the issue appears ONLY in throttle transition, you can expect the problem to be inside the fuel metering device, or the servo in your case.
Larry
Larry, thanks for this info. Talking with my mech bud again this morning, what you state here pretty much matches his thoughts.

I've just reached out to Don at AFP. This fuel servo unit could very well be in need of overhaul.

RC
 
data dump

Pull your JPI data and upload it to Savvy Analysis. Then you can get all kinds of nice charts and zoom in on a section looking at fuel flow, and all your CHT and EGT readings.

You can even post a picture of the burp (likely an RPM dip or flattening) area so we have some data to look at.

You may need to go into the JPI setup and change the data storage interval, as I think the default is every 6 seconds. Given that, it will likely miss the event. I think it can go down to 1 second intervals.

I seem to recall that Savvy Analysis is free once you setup a user and password on the website. If you haven't read any of the founder's articles ... Mike Busch, I highly recommend doing so. Once you have an account you get a monthly email with a 'puzzler' too.
 
Gents,

Thanks for all the good replies here. I do have Savvy QA, been bouncing this off of them.

It does seem that the root cause here is that I'm not getting enough fuel flow at takeoff, that yes, the engine is a little too lean, and that's likely the cause for the stumble/hesitation there at 2200 rpm, and why its more prounounced on a cold morning. EGT's are just a little higher, fuel flow a little lower, than it should be for a takeoff at 121' MSL airport.

So, for now that's what I'll try and adjust, and go from there.

Thanks again!

RC
 
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