Ok some professionals working on this but I’m impatient enough I’ll open it up to the infinite wisdom and kindness of the internet.

Started noticing engine surging/power oscillation on climb out, particularly at high DA. Plane is a cub with a Lyc O-360, Trailblazer CS prop, ignition is one mag and one electroair. Felt it first in the seat, sure enough manifold pressure oscillating with the surges. Engine data shows MP oscillation with about 10s period, up to 1-1.25” peak-peak. Digging into old data also shows it happens in a variety of scenarios—full power at sea level, economy cruise leaned out, etc. Also plenty of times when it doesn’t occur. Suspect it’s only noticeable in the seat on takeoff because I'm hanging on the prop and max vigilance, and I didn't see it on the gauge because who stares at that for 20 seconds. Have tried different mixtures, switching tanks, prop settings, fuel boost pump on/off, etc.

Prop seems to be behaving. Most promising theory to date was that a leak or sensor issue in the MAP sensor for the electroair was causing the electronic ignition to varying the timing and thus the engine power. Just ruled this out by disconnecting the MAP sensor, issue persists with static electronic ignition timing. Carb issue? Induction leak? (some suspicious EGT behavior). Oh one more stumper.. the fuel pressure (but NOT flow) sometimes seems to track the MP oscillation. Armchair experts.. weigh in!

Two engine data files here.. one cruise, one takeoff.

And no change with carb heat applied?

Have you shut-down either ignition in flight? Results?

What maintenance was performed in last 50h?

Before flying, I’d leak check the MAP system by disconnecting MAP line at engine, connect a hypodermic syringe, then apply a Gentle suction to line until MAP gage indicated 10 inches. Should be less than 1 inch change after 1 minute. If no leaks then I’d check the induction system, to include play in throttle system (rod ends, throttle shaft bushings, throttle body-bowl screws, etc). Be suspicious of any fuel stains.

Curious: Does subject ”cub” have both engine-driven & electric fuel pumps?

BTW, Oil temp & oil pressure legends on charts appear transposed.

iPat wrote:And no change with carb heat applied?

Have you shut-down either ignition in flight? Results?

What maintenance was performed in last 50h?

Before flying, I’d leak check the MAP system by disconnecting MAP line at engine, connect a hypodermic syringe, then apply a Gentle suction to line until MAP gage indicated 10 inches. Should be less than 1 inch change after 1 minute. If no leaks then I’d check the induction system, to include play in throttle system (rod ends, throttle shaft bushings, throttle body-bowl screws, etc). Be suspicious of any fuel stains.

Curious: Does subject ”cub” have both engine-driven & electric fuel pumps?

BTW, Oil temp & oil pressure legends on charts appear transposed.

Thanks for the input. New at this so getting the crash course. Have only done carb heat and ignition checks on static ground runs, both at runup power and full power. Carb heat doesn't seem to have an effect--this is truckee so humidity = 0--or are you thinking heat for mixture? Ignition results are complicated, intermittent, and contradictory. Sometimes it's worse on electroair only. Sometimes worse on both. Seems a little better on mag only? Sometimes just fine. Problem with static runs is I hate running for as long as I need to to get good data.

Annual was in last 20h so I was suspicious until I looked at historical data.. pretty sure it shows this effect from aircraft manufacture--but maybe getting worse? Only 110 on the tach.

Thanks for leak test suggestion good idea, was wondering how I'd do that. Concur on throttle checks I won't be doing those myself but I'll be oversight. There are also some EGT indications that suggest an induction leak affecting 2 of the cylinders? Finally, one puzzler is that if I look closely at the fuel pressure data when it tracks the MP variation, I could swear the pressure data leads. If the pressure that pulls fuel out of the float chamber is varying how could that be?

Yes engine-driven and electric boost in parallel. Thanks for catching the plot labeling I've been staring at them so long I'd never see that..

B

I mentioned carb heat because they often have a valve system that can leak and/or possibly oscillate.

Induction leaks tend to be more apparent when MAP lessens (vs atmospheric), due to the greater ambient air pressure “forcing” outside air in. On a Lycoming typical culprits are
- a degraded paper gasket atop intake tube at cylinder head (usually older engines)
- loose or wrong clamp at intake tube connection to cylinder head (grasp tube by hand and attempt to move up & down)
- loose clamp on rubber coupling at intake tube bottom
- loose swaged induction tube in oil sump
- previously mentioned carb issues
- #4 cyl cruise EGT trace notably differs: if MAP line connects to #4 cyl head then suggest checking sealing & torque on the adapter (usually an AN-fitting)

Interesting: takeoff chart data indicates #1 & #2 cyl EGT trace rises when decreasing MAP, in contrast to #3 & #4 cyl falling EGT. Which cylinders is the primer connected to?

I do not have experience with Electroair’s ignition system. Does it fire upper spark plugs on one cylinder bank & bottom spark plugs on opposite cylinder bank, or ??

If magnetos pass run-up test then I check them separately during flight sufficiently long enough for a comparative assessment.

What if oil pressure fluctuations are causing the prop pitch to vary slightly?

DeltaRomeo wrote:What if oil pressure fluctuations are causing the prop pitch to vary slightly?

That's where I started in all this, I guess sometimes if the low pitch stop is misadjusted you can get oscillation in the governor.. but this would show up in the RPM? I sent some data to Hartzell and they agreed. But keep the ideas coming, thanks.

It sure looks like fuel pressure tracks the manifold pressure.

Tell us more about the system:

Does it have a standard cub fuel system? Any deviations?

It's pretty obviously gravity fed, but really odd that the fuel pressure changes 25%. I'd fix that first and see if your other issue follows.

So, here is my guess:

Poor fuel tank venting.

I've seen cases in other engines where a poor vent will cause the engine to slightly lean out before the system suddenly pulls in some air and it's good for another few seconds.

Carb float issue.

Kurt

iPat wrote:I mentioned carb heat because they often have a valve system that can leak and/or possibly oscillate.

Induction leaks tend to be more apparent when MAP lessens (vs atmospheric), due to the greater ambient air pressure “forcing” outside air in. On a Lycoming typical culprits are
- a degraded paper gasket atop intake tube at cylinder head (usually older engines)
- loose or wrong clamp at intake tube connection to cylinder head (grasp tube by hand and attempt to move up & down)
- loose clamp on rubber coupling at intake tube bottom
- loose swaged induction tube in oil sump
- previously mentioned carb issues
- #4 cyl cruise EGT trace notably differs: if MAP line connects to #4 cyl head then suggest checking sealing & torque on the adapter (usually an AN-fitting)

Interesting: takeoff chart data indicates #1 & #2 cyl EGT trace rises when decreasing MAP, in contrast to #3 & #4 cyl falling EGT. Which cylinders is the primer connected to?

I do not have experience with Electroair’s ignition system. Does it fire upper spark plugs on one cylinder bank & bottom spark plugs on opposite cylinder bank, or ??

If magnetos pass run-up test then I check them separately during flight sufficiently long enough for a comparative assessment.

Good stuff, I’ll play with the carb heat under more controlled circumstances to see if I can get any differences.

-I did a pretty thorough manifold inspection with my A&P and wiggled all the hose connections. Other than ANC discussed below nothing obvious. I have seen a soapy water manifold leak check on the internet, haven’t tried. I did do a running manifold leak check (after watching 20 hrs of Mike Busch webinars ), comparing EGT drop for each cylinder running wide open vs 10” less than wide open. I guess the theory is at -10” air is going to flow through the leak and affected cylinders will run leaner so drops will differ if there is a leak. sure enough, 2 cylinders drop 60F and two drop 130F. Not sure I have the expertise to interpret.

-MAP sensor does go into the #4, and when all this started the ANC fitting was flopping around. But now it’s got another turn and new sealant and didn’t seem to help.

-There is no prime on this installation, throttle prime.
-Electroair replaces the right magneto, think with the same plug layout. I’ll check.

Some new data to stir the pot

1st page is runup prior to MAP sensor disconnect. Pretty clear oscillation on electro air only, with fuel pressure right with it. Seems damning, but..

2nd page is same runup after MAP disconnect. Electroair doesn’t oscillate but both does, although clearly damping out.

3rd page is running leak test showing clear difference in EGT drops.

Thanks again, got a good list going.

B

akschu wrote:It sure looks like fuel pressure tracks the manifold pressure.

Tell us more about the system:

Does it have a standard cub fuel system? Any deviations?

It's pretty obviously gravity fed, but really odd that the fuel pressure changes 25%. I'd fix that first and see if your other issue follows.

So, here is my guess:

Poor fuel tank venting.

I've seen cases in other engines where a poor vent will cause the engine to slightly lean out before the system suddenly pulls in some air and it's good for another few seconds.

Thanks. Venting would certainly fit pressure leading the MP changes, and the time scale. Wonder if I’d see the variation in the EGT’s or if they aren’t fast enough?

Not too familiar with standard cub, this is Part 23 not CAS so maybe some differences? Gravity fed, left and right selector (no both), vented under the wing (not in the caps). There’s a strainer after the selector, then parallel engine driven and electric boost pump, followed by flow then pressure transducer and carb.

G44 wrote:Carb float issue.

Kurt

Thanks I’ll add to the list. Just curious what’s the mechanism in that case? Sticky float? Something clogged? Any clever tests to differentiate I can run on the ground?

stretch wrote:
Thanks. Venting would certainly fit pressure leading the MP changes, and the time scale. Wonder if I’d see the variation in the EGT’s or if they aren’t fast enough?

Not too familiar with standard cub, this is Part 23 not CAS so maybe some differences? Gravity fed, left and right selector (no both), vented under the wing (not in the caps). There’s a strainer after the selector, then parallel engine driven and electric boost pump, followed by flow then pressure transducer and carb.

Does it matter which tank it's on?

Does the fuel pressure fluctuate on the ground during an extended runup?

Does the fuel pressure fluctuate when the boot pump is on or off or both?

stretch wrote:

G44 wrote:Carb float issue.

Kurt

Thanks I’ll add to the list. Just curious what’s the mechanism in that case? Sticky float? Something clogged? Any clever tests to differentiate I can run on the ground?

Sticky float, “water logged” float depending on the material, sinking float, misadjusted float and so on.

Also, the main jet may be the problems too. A lot of Husky’s have left Aviat (high altitude airport) that run great in Afton but have surging issues and lean at full throttle issues. The main jet in these carbs is the leaner of the available jets. Carb shop here changed my buddy’s jet on his Husky, much better! Are getting at least 18GPH fuel flow at sea level on full throttle full RPM take off. Basic rule of thumb is 1 gph per 10 HP.


Kurt

According to Lycoming's O-360 Operator's Manual, permissible carburetor fuel inlet pressure is either [8.0/0.5 psi, with 3.0 psi desired] or [18/9.0 psi, with 13 psi desired], depending on exact model. So they are somewhat tolerant of variable fuel inlet pressure.

For oil pressure, Operator's Manual says 115 psi is max start/warm-up/taxi/takeoff, with 95 psi max cruise, for all O-360's. Cub's last oil filter check was okay, yes? And when checking oil dipstick, examine both quantity & quality (always brown & Never sparkly).

"New data" chart legend says oil temp (F), but I'd bet it is in (C).

Troubleshooting can get expensive in both time and money, so I suggest starting with simplest/easiest and keep good notes - attention to detail is vital. And learning is fun.

Since data SEEMS to indicate a possible ignition issue (thank you for annotating), some thots:

If Cub's exhaust risers have bead clamps ensure the clamp flanges are not "pointed" at nearby ignition lead; adjust clamp and/or ignition lead position as required to avoid exhaust leakage onto lead (most bead clamps leak, in my experience, & this is easy to fix).

Spark plugs are relatively cheap to check or replace, but check even new ones for proper gap & resistance & overall condition before installation. Resistance ref: http://www.tempestplus.com/Portals/0/PDFs/AT5K%20Instruction%20Sheet.pdf Photograph all spark plug electrodes together while in a tray for future reference, and resistance test old ones before trashing (keep good notes).

Please keep us informed.

G44 wrote:

stretch wrote:

G44 wrote:Carb float issue.

Kurt

Thanks I’ll add to the list. Just curious what’s the mechanism in that case? Sticky float? Something clogged? Any clever tests to differentiate I can run on the ground?

Sticky float, “water logged” float depending on the material, sinking float, misadjusted float and so on.

Also, the main jet may be the problems too. A lot of Husky’s have left Aviat (high altitude airport) that run great in Afton but have surging issues and lean at full throttle issues. The main jet in these carbs is the leaner of the available jets. Carb shop here changed my buddy’s jet on his Husky, much better! Are getting at least 18GPH fuel flow at sea level on full throttle full RPM take off. Basic rule of thumb is 1 gph per 10 HP.


Kurt

Interesting. I get 18" at sea level but it takes 30 seconds to get there after throttle hits the stops.

Thanks for all the input I've got my homework I'll post when I sort more out.

b

30 seconds to get to 18GPH after throttle hits the stop is not acceptable, this is where I would start. There appears to be something wrong with your “fuel delivery system” aka your carburetor.

Kurt

I had a similar surging issue a few years back, I spent a ton of time chasing my tail. changed valve springs, gaskets it was driving me nuts. It was very noticeable at full power and less noticeable but still apparent at lower power settings. On takeoff I was getting power oscillations of 100+RPM. It turned out to be a worn throttle shaft, give it a good wiggle for in/out as well as axial play at various throttle positions, the play in the throttle shaft should be barely perceivable at a maximum.

While reconizing it is the MAP oscillation that is complained about, if propeller governor is McCauley then ensure their ASB273C is complied with, if applicable. See https://mccauley.txtav.com/en/service-bulletins

Not even close to being qualified to offer this up, but,...... when all cylinders are significantly rich of peak, power output of each cylinder is roughly the same. But as fuel is restricted and the mixture distribution puts some cylinders in the lean zone and others in the rich zone, the power output of each cylinder is different.

In a fixed pitch carbureted engine, uneven power generated by individual cylinders from lean mixtures tend to make the engine run rough because the propeller cannot compensate for the changing power output.

In CS carbureted engines, lean mixtures can lead to the sensation of engine surging as the propeller governor attempts to keep the RPM constant under ever changing power from the individual cylinders.

Interestingly, in injected engines with GAMI injectors that flow match the cylinders, there is very little surging as the mixture is reduced in flight and the engine can quit and begin windmilling without too much objection.

You mentioned that the engine surges in takeoff as well as lean economy. This is what CS engines seem to do when power output is unbalanced between cylinders.

I suspect that something is causing a power imbalance between your cylinders and the propeller governors is trying to do its job, but the result is the surging that you experience. The carburetor is a likely culprit as Kurt mentioned. If not the carb, then something else may be creating a significant mixture imbalance between your cylinders. It could be a fuel OR and air input problem leading to a mixture imbalance of one or more cylinders.

What do your CHTs and EGTs suggest when the engine is surging?