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912 loses 500 rpm during climb

  • Eric Greenwell
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2 months 1 day ago #17212 by Eric Greenwell
Eric Greenwell created the topic: 912 loses 500 rpm during climb
The 912S is in a Phoenix, an LSA glider (it's a motoglider). The engine has 440 hours on it with no problems. I run it mostly on mogas, and it spends maybe third of it's flight hours with the engine idling while I'm soaring it. Here's the summary of what happened on two flights - Oct 7 & 8...

+ It started and ran normally on the ground, including the 3000 rpm ignition check
+ Full throttle while holding it with the brake yielded about 4800 static rpm (normal), then I began the takeoff.
+ Takeoff and climb were normal for the first 20 seconds or about 300+ feet AGL
+ I felt the engine running rough, and the RPM dropped to 4600 rpm
+ I reduced the throttle to 4400 rpm, where the rough running became smooth, and I could then power up to full throttle with no problems.
+ At 3000' AGL or so, I went to idle to begin soaring, and the engine performed flawlessly, as it always has.
+ There were other full throttle climbs during both flights, but at higher airspeeds, so the pitch angle was only 7 degrees instead of 12.
+ The Dynon Skyview data log showed the EGTs stayed with 20 deg F during the roughness, and with 30 deg F the whole flight

I thought that the nose up attitude during the climb after takeoff might have caused contaminants in the fuel to get picked up, but when I drained samples from each tank after the last flight it was pure mogas.

A friend wondered if it could the plugs, because I do a lot of soaring with the engine idling. A possibility? I've been doing the idling while soaring for the 3 years I've owned it, without any problems.

Any ideas about the cause? And what tests might help find it? At this point I don't mind doing another flight to carry out some tests to pin down the problem. With the 30:1 glide, I only need 200 feet AGL to turn back to the airport safely, so the risk period is about 10 seconds at most.

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2 months 1 day ago - 2 months 1 day ago #17213 by Roger Lee
Roger Lee replied the topic: 912 loses 500 rpm during climb
I would start with popping the carb bowls off and taking a look for any debris. Check floats for weight. Is this just the mechanical pump or is there also an electric pump? What maint. was performed last and how long before this issue? What is the plug gap? Wondering if it is too wide and lots of low rpm time they may produce a poor spark, poor burn and some plug fouling? When's the last time the carbs were synced so they are running efficient, but during this you can see if there are any irregularities.
Out of curiosity what is the rpm when at idle and soaring?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last Edit: 2 months 1 day ago by Roger Lee.

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2 months 18 hours ago #17217 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
It has a low wing and an electric fuel pump that I use during takeoff and landing. The pump was not used during the (mostly) full throttle climbs later in the flights.

The last maintenance was the annual inspection in April (50 hours ago); no work was done on the engine except for an oil change and plug inspection; the plugs have 150 hours on them, and the gap appears to be 0.028". A picture of the top #1 plug is attached (new plug on right), and the data log with a red arrow showing the rough-running, reduced rpm period.

The static idle is 1800 rpm; cruising at 60 knots, the idle is about 2300 rpm; thermalling at 50 knots it's about 2200 rpm.

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2 months 17 hours ago - 2 months 17 hours ago #17218 by Roger Lee
Roger Lee replied the topic: 912 loses 500 rpm during climb
Hi Eric,
The plug gap should be between .023 - .027. I would pick .025 it's in the middle and won't be out of spec by the next annual. Next I would throw those old plugs away. I never go past 100 hrs. and 98% of my customers and I toss them at 75 hours. (I know what the manual says, but I don't know of anyone that goes by 200 hr. plugs) They are only a couple of bucks so they're cheap.It kind of looks like you have anti seize on the plug. If so this shouldn't be used and use the correct thermal white color paste on them. Next I would check the carb bowls for debris and then sync the carbs. Use gauges to sync the carbs as they will help you diagnose an issue. Electronic sync devices can't do that. After these things are done then go for another flight. Leave the electric pump on during your climbs at take off.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last Edit: 2 months 17 hours ago by Roger Lee.

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2 months 15 hours ago #17223 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
Thanks, Roger. I've located someone locally to loan me a tube of the proper paste, so I'll gap the plugs, change them, and do a test flight this afternoon. Jim Scott at Aircore changed them last time, so I'm sure he used the proper paste. I have used plugs as long as 170 hours with no issues.

If I still have the problem after changing the plugs, I'll meet with Jim on Saturday to check the carb and floats. He suggested it's not the carbs, since the EGTs run so close together during the loss of rpm (and the entire flight), but it might be the fuel supply, which could affect both carbs equally.

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2 months 14 hours ago #17224 by Roger Lee
Roger Lee replied the topic: 912 loses 500 rpm during climb
Hi Eric,
It can be a several of things, but just do 1-2 common easy and inexpensive things at a time.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 month 4 weeks ago #17241 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
I replaced all 8 plugs yesterday, put the cowlings back on today, and went flying with a half page of potential tests in hand. Well, it worked perfectly on the takeoff and 5000' climb at 5000 rpm, so I stuffed the list of tests in the side pocket and went soaring. 3.5 hours of idling later, I did a 5000 rpm, 60 knot climb for 3 minutes - still working great. Tomorrow, I'll do a couple takeoffs and climbs to confirm proper operation, but right now, it appears the symptoms were caused by the plugs. I have no idea how, but I'll stick to a 100 hours between changes in the future, probably at every other oil change.

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1 month 4 weeks ago #17245 by Genival Pacheco
Genival Pacheco replied the topic: 912 loses 500 rpm during climb
Hi Eric
If I understand, most of the time your engine is running at idle (low oil temps, rich mixture). Making an analogy with car engines it’s like a heavy-duty situation and in the case of car the makers recommend that you should reduce by 50% the time or km between oil changes.
In my opinion you should consider the above and reduce the time between oil changes.
Take in to consideration that oil and the plugs are one of the cheapest items of the engine maintenance so you should consider changing the oil and filter every 50 hours if using mogas or oil every 25 hours if using avgas and the filter you should go to 50.
Your engine will thank you and more than that you will be on the safe side.

Regards

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1 month 4 weeks ago - 1 month 4 weeks ago #17247 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
Bad news - I did a takeoff this afternoon, and the engine ran rough very soon after liftoff, dropping the rpm to about 4400. I heard popping noises and there was a strong smell of gasoline. Reducing the throttle didn't help any, so I continued my turn back to the runway, and landed uneventfully. Two full throttle run-ups on the ramp went to 4800 rpm (normal for the engine), with the engine running smoothly. I could see nothing wrong in the engine compartment.

I've looked at the data log, and it shows a spread in the EGT readings, suggesting carb/float problems to me. The image is attached.

I'm totally perplexed, so I'm waiting for my mechanic to call back to see what he thinks is the next step. Still open to suggestions, of course.

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Last Edit: 1 month 4 weeks ago by Eric Greenwell. Reason: added more info about the engine symptoms

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1 month 4 weeks ago #17248 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb

Genival Pacheco wrote: Hi Eric
If I understand, most of the time your engine is running at idle (low oil temps, rich mixture). Making an analogy with car engines it’s like a heavy-duty situation and in the case of car the makers recommend that you should reduce by 50% the time or km between oil changes.
In my opinion you should consider the above and reduce the time between oil changes.
Take in to consideration that oil and the plugs are one of the cheapest items of the engine maintenance so you should consider changing the oil and filter every 50 hours if using mogas or oil every 25 hours if using avgas and the filter you should go to 50.


Hi Genival - Thanks for the suggestion, and you might be surprised to learn I already use 50 hour oil changes for Mogas operation, as it is recommended by the Rotax engine manual :^)

I inquired here and elsewhere about the issues of running the engine in flight at idle, while keeping the oil temperature near 190 deg F. The consensus was the rpm was plenty high enough to avoid the problems low rpm idling can cause, and with normal oil temperatures, it was unlikely to suffer from the idling. I don't recall anyone suggesting the oil be changed more often, nor did my mechanic think more frequent oil changes would be useful. If you know of any studies or reports on prolonged idling, I'd like to know about them. It's an unusual "mission profile", and knowledge is thin on the subject.

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1 month 4 weeks ago #17249 by Roger Lee
Roger Lee replied the topic: 912 loses 500 rpm during climb
If your rpm dropped that much from full or high rpms you should check the carb float bowls for debris. Check the vent tube coming off the side of the carb and make sure both are connected and they're getting the same pressure input. Both tubes should be coming off on the standard Rotax airbox or you may have them vent tubes in another setup.
was your electric aux pump on?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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1 month 4 weeks ago #17250 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
Hi Roger - the electric pump was running.

The vent tubes are vented to the atmosphere - are you saying put a pressure gauge on them to measure the pressure in the float bowl? I'll see if I can find someone locally that's competent to check the floats; unfortunately, my usual mechanic is on the other side of the Cascade Mountains near Seattle, so it might be while before he can check them.

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1 month 4 weeks ago #17251 by Dennis Richardson
Dennis Richardson replied the topic: 912 loses 500 rpm during climb
If you are having a fuel supply problem your egt's should increase. If you are having an ignition breakdown your egt's should decrease. You indicated that your egt's remained stable. I would be looking at the exhaust, perhaps a loose baffle that is moving around.
Dennis

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1 month 4 weeks ago #17252 by Dennis Richardson
Dennis Richardson replied the topic: 912 loses 500 rpm during climb
I hate it when I make a mistake. I missed your post with the egt readout. I will go back to reading the posts and predicting the solution.
Dennis

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1 month 3 weeks ago #17273 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
I can create the problem on the ground by running the engine at WOT for longer than two minutes. The data logs show the EGTs are nearly identical until the rough-running starts, and then they are about 30 degF apart; the fuel flow is erratic during the rough period, and varies from 5 to 7 gph (4.9 gph is the flow when it's running properly); and an ignition check at 4400 rpm (and running roughly) has the same drop (about 150 rpm) for Left and Right. The electric fuel pump was not running during this ground test.

I've weighed the floats, and all four weigh 3.1 grams.

My mechanic is perplexed, but still thinks it's fuel problem, and likely a carburetor problem, but he'll have to visit the plane next week to pursue a solution.

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1 month 3 weeks ago #17281 by David Glosser
David Glosser replied the topic: 912 loses 500 rpm during climb
Eric, I had similar problem with my Phoenix upon original delivery and training. We went to Lockwood and had fuel pressure gauge installed for diagnostic purposes. No help. Subsequently one of the carbs was replaced; with instant and durable cure. No subsequent issues.

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1 month 3 weeks ago #17282 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
Thanks, David, that tends to confirm my mechanic's suspicion that it is carb related.

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1 month 1 week ago #17410 by Eric Greenwell
Eric Greenwell replied the topic: 912 loses 500 rpm during climb
I and another pilot spent several hours a week ago, testing things and doing WOT run-ups, and the only thing we found was a speck of what looks like red rubber (like the red rubber on the hose insulation) in the left float bowl.

The run-up after that was perfect - no rough running, no RPM sagging. The next two day, I did another full power runup, a takeoff, and two 5 minute, full power climbs; the following day, I flew a two leg, two hour total trip to another airport. During the runup, climbs, and the out and return flight, the engine performed smoothly just like it did for the first 450 hours. So, I'm persuaded it's fixed, but I'll be extra cautious for the next few flights, keeping an airport in easy reach while flying.

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