Partial power loss and running rough

[fatsportpilot]

I was flying in my CFI's plane (Evektor SportStar with Rotax 912ULS) a few days ago and while practicing emergency landings in fields by getting 2 feet above an agricultural field and powering up before really landing, I started climbing and at maybe somewhere around 1000 ft the engine started to run rough. Not very rough but rough and there was a definite change in pitch and smoothness from the engine and throttle response felt a little "weaker". I don't know what the drop in RPM was but there was definitely some. It stopped after several minutes of very gradual climbing but I have no idea what could have caused it.

This happened before too but the other times it stopped shortly. It only happened when we were climbing at full power but the previous times the CHT was around 210 but this time the temps were fine. I hadn't flown in a month and this was the first time it happened since then so my CFI wonders if it's something I was doing, but I'm very gentle to the airplane and I don't throttle up or down too fast and I keep all the gauges in healthy ranges.

His first thought months ago when it first happened was that the smoke from wildfires (we're in California) was messing up the air filters so he cleaned them, but it happened again later. So then he thought hot air was getting into the carbs because the carb heat system is so fragile and might have been leaking, so he disconnected them. But then it happened again and worse this time.

Does anyone know what it could be? I'm not the one maintaining the engine but because it hasn't done it for a month and only started again now that I am here makes me think. The engine is over TBO (I'm not sure by how much), but what sort of things could have caused this? Or is this not enough information?

[3Dreaming]

I don't know the year model of the airplane, but there are some of the older 2006-7 engines that have ignition module problems. Normally it effects starting. but I have heard of partial power loss and rough running before. The other issue is the carb vents. Having a carb vent off or broken can cause a out of balance condition with the carbs that will show with the same issue. Sticky slides in the carbs will also do the same.

[fatsportpilot]

I think the engine is 2008 or newer, but it's new enough to have the soft start module built in. I don't think there was a balance problem with the carbs and it didn't feel like that kind of rough running. Starting and stopping the engine was totally fine and this was a 100% transient problem. Engine ran perfectly otherwise.

[3Dreaming]

If it has the soft start modules, then I would definitely check the two things I mentioned. The vent line would be the easiest thing to check. Both of these would be more likely to show up during high power settings during a climb.

[RV12Heal]

I experienced a similar partial power loss issue during climb-out in my RV-12 / 912 ULS. My Skyview EMS data dump suggested the right-side carb. It turned out to be a small amount of contamination in the mogas. I disassembled and cleaned the carb as well as cleaned a small amount of jelly-like crud out of the gascolator screen. I also drained out all of the fuel, added fresh fuel from another supplier, and flushed the entire fuel system. All is well now.

[drseti]

Since you were practicing approach to emergency landings, I assume the engine was idling (perhaps for a prolonged time). If relative humidity was high, you could have picked up some carb ice. Suggest you pull on carb heat before attempting prolonged glides.

[fatsportpilot]

Since you were practicing approach to emergency landings, I assume the engine was idling (perhaps for a prolonged time). If relative humidity was high, you could have picked up some carb ice. Suggest you pull on carb heat before attempting prolonged glides.

It wasn't idling any longer than when I do pattern work. There was no humidity here (California central valley) and the carb heat tubes have been disconnected anyway. My CFI even thought that they were leaking hot air in even when carb heat was closed because the onset was very sudden, not at all gradual and it only occurred at full power after a few minutes of climb.

[JimParker256]

Since you were practicing approach to emergency landings, I assume the engine was idling (perhaps for a prolonged time). If relative humidity was high, you could have picked up some carb ice. Suggest you pull on carb heat before attempting prolonged glides.

It wasn't idling any longer than when I do pattern work. There was no humidity here (California central valley) and the carb heat tubes have been disconnected anyway. My CFI even thought that they were leaking hot air in even when carb heat was closed because the onset was very sudden, not at all gradual and it only occurred at full power after a few minutes of climb.

I'm curious about whether your CFI obtained an LOA from Evektor to have a mechanic alter the design of the fuel system in that manner? I'm assuming the airplane is still in the S-LSA category, so you might want to check on that, since any modification from the factory design would require that LOA for the plane to be in an airworthy state. Remember that you, as the pilot/operator, are the responsible party (under FAA policy) to ensure the airplane is airworthy before flight.

I'm also curious as to whether that airplane has a "return line" to the fuel tank that is Rotax's current recommendation for avoiding fuel vapor bubbles forming, which could account for the rough running after a period of low fuel flow in a hot engine compartment. Per Rotax, this is far more common than carb icing with the 912 series engines and Bing carbs.

I'd be willing to bet that it would be easier to obtain an LOA to make the plane comply with Rotax installation guidelines (fuel return line) than it would to obtain the LOA to disconnect the carb heat tubes as you described.

[drseti]

I'm also curious as to whether that airplane has a "return line" to the fuel tank

All the SLSA Evektors produced since about 2005 come from the factory with a fuel return line (with flow resteictor) routed back to the left wing tank.

[fatsportpilot]

I'm curious about whether your CFI obtained an LOA from Evektor to have a mechanic alter the design of the fuel system in that manner? I'm assuming the airplane is still in the S-LSA category, so you might want to check on that, since any modification from the factory design would require that LOA for the plane to be in an airworthy state. Remember that you, as the pilot/operator, are the responsible party (under FAA policy) to ensure the airplane is airworthy before flight.

I'm not sure and maybe he only disconnected it to test it.

I'll tell my CFI about 3Dreaming's advice because it sounds like it could be the easiest to test for.

And yes it has a return line to the left tank.

The engine wasn't very hot this time. The last times we did think it could have been too much heat but this time CHT was under 210 so I don't think it would be vapor lock. And the OAT was also under 80 that day (despite it did fine even with an OAT of 110 for many days on end).

[fatsportpilot]

He's pretty sure he found out the culprit and fixed it. It was a small aluminum T connector for the electronic auxiliary fuel pump and is unique to the Evektor SportStar. The illustrated parts catalogue calls it "Distributor MON 3501" (it is labeled number 9 in the graphic on page 122 at https://avsport.org/acft/SportStar/sportstar_ipc.pdf). I'm not sure why but it had apparently began sucking in air bubbles and sometimes when the engine was at full power and needed the most fuel, the carb float bowls got so empty because of air that the engine would begin running rough. It hasn't happened since he fixed that so I think the problem is solved.

[3Dreaming]

Even if it is spliced in after the engine driven fuel pump, I think it would be difficult to suck air when the fuel is under pressure.

[fatsportpilot]

Even if it is spliced in after the engine driven fuel pump, I think it would be difficult to suck air when the fuel is under pressure.

Somehow air was getting in it.

[Dave C]

The T marked number 9, the one on the left side of the diagram, is under suction at all times when the engine or electric fuel pump is running. If it leaked it could suck air.

My 5 year rubber kit, that I got a few months ago from Evektor, has replacement Ts that are now brass instead of aluminum and look more robust.

[3Dreaming]

Even if it is spliced in after the engine driven fuel pump, I think it would be difficult to suck air when the fuel is under pressure.

Somehow air was getting in it.

Did you lose fuel pressure when this was happening?