RPM Rollback

[Runtoeat]

We've been working on a problem with a 2010 CTLS.  Originally, engine rpm automatically rolled back to 4500 rpm from 4900 rpm at climbout WOT (throttle full forward for wide open throttle).  After making a quick turn back to the runway with low power from engine and sitting stationary with the throttle at full stop and idling, the rpm rolled back to 800.  Advancing throttle will raise idle rpm.  Checked for full travel at carbs, checked cables, all OK.  Removed float bowls and checked for crud and float position.  Floats looked OK and fuel flow good thru carbs.  Checked vent hose to airbox - OK.  Looked at wire connections at modules - all connections good. Called Lockwood and they suggested we check for carb heat operation at the airbox.  Did this and cable and bell cranks working OK.  When rpm rolls back, engine runs smooth with no roughness so it doesn't seem like ignition or carb problem since this would cause rough running .  We installed a new ignition switch and the WOT (static) rpm was raised to 5000 + with no roll back experienced running static. All seemed fixed with new ignition switch.  Then, when preparing to take off for test flight, the idle rolled back to 800. Back to the hangar.  The idle still will roll back to 800 rpm when throttle is at full stop idle and advancing throttle will still raise rpm to higher levels  WOT now seems to be stable and 5000+ rpm.  All remains smooth when advancing throttle to WOT and when rolled back  at idle.  The next plan is to install new ignition modules. The current modules are retrofitted rotax factory soft start and the new ones are the same. I've looked at the trigger pickup diagram and am wondering if the ignition trigger signal from the stator might be causing this? If anyone has thoughts as to why this rpm roll back is occurring, please let me know.

[Ed Cesnalis]

Maybe the plugs?  I've never heard the term rollback before but I think you are saying the RPM drop from being under load is now greater than normal?  Carbs sound like the right place but not the only place to look.

If it isn't the gas, check the spark, even compression and timing.

[DHeal]

"Out-of-the-box" thought here, but could the Soft-Start function be cutting in and out intermittently thus changing the timing???

[FredG]

I think Roger gives good advice here.  If you have the time, expertise, and regulatory authority to do the maintenance, then disassembling and cleaning the carbs may fix the problem and save the cost of new ignition modules.  

Regarding the recommendation to "Rod out all jets with a piece of wire," I think it is possible to enlarge the hole in a jet if the wrong "wire" is used for cleaning.  I think the recommendation to use solvent and compressed air is safer.  

[Anticept]

Copper and aluminum are both softer than brass. You won't enlarge the hole with these. You can get wire small enough from stranded wire.

[Runtoeat]

Thanks for the replies.  Roger, I got your message to call you back but I hate to bother you.  I figured I'd put a post here on the forum and you'd be providing your thoughts along with others.  When I say "RPM rollback" i am describing an automatic reduction in engine rpm without any change made to the throttle.  Just sitting at idle, the engine rpm  drops 600 to 800 rpms (from 1600 to 800).  Fuel flow is not a factor because the engine now runs at 5000 rpm without loss of rpm.  If there was a carb balance issue, there would be rough running.  Engine operation is smooth.  It is good advice to tear down the carbs and inspect them for blocked jets though.  This will be done. Whatever is causing this, I am thinking that it is uniformly affecting both carbs or both modules based on the "smooth running" observation.  As DHeal suggests, perhaps the modules are being falsely put into a "start" condition which retards the spark?  What might cause this?  Might be the soft start acting up or it might be caused by a damaged or improperly adjusted trigger system in the stator assembly.  Eventually the mechanic will figure this out.  A new set of soft start modules I have on my shelf will be installed on this CT.  If we're lucky, this will fix the problem. If anyone has any other thoughts, please let me know.

[Runtoeat]

Thanks Roger.  I've always used copper wire to clean out the carburetor jets on my cars and boat motors.  My old spools of various diameters of stranded copper wire will be dusted off and made available to our mechanic.  I'll let him decide.  Could the baro control be dropping the slides somehow?  At idle, with the throttle remaining at it's "stop" and engine running at 1600 rpm the engine speed drops to 800 rpm.  Advancing the throttle off it's "stop" brings rpm's back up to the "normal" idle.  All the while, the engine is running smooth.  I don't recall if the slide and/or needle affects the idle or how idle circuit works.  Need to look at Bing diagram and look back into discussions on this.

[Tom Baker]

With the ability to run 5000 RPM I really doubt it is a fuel issue affecting both carbs at idle. Are the ignition modules original or retrofit? Have you noticed if the amp draw goes up when this happens? Have you tried an ignition check when it happens?

[Runtoeat]

Thanks for those thoughts Roger and Tom.  I didn't ave a bottle to check fuel flow thru the gascolator but when we pulled the bowls and let the fuel run thru the carbs, it looked like good fuel flow.  We ran @ 5000 rpm for 3 minutes statically and no reduction in rpm.  Don't think fuel flow is involved.  The ignition modules are soft start and this is a 2010 CTLS which didn't have soft start from the factory.  I believe the modules are retrofitted. Didn't check amp draw during the drop in rpm but will look into this.  Tom, if you can, please provide a few words why amp draw might be important.  We also looked at EGT before, during and after the epm drop.  As I recall in our debriefing after the last runup, the EGT rises - meaning a richer mixture?  Pretty sure ignition (mag) check was done but will ask the "test pilot" about this.  I'm good with mechanical stuff but I'm not a wire wizzard.  I'm learning a lot about Bing carbs and Rotax ignition. Engines rely on spark and fuel to run but the devil's in the details when it comes to electronic ignitions and altitude compensating carbs.

[Tom Baker]

I ask about the retrofit and amp draw because of where the soft start modules get their power. I am wondering out loud here. The soft start retrofit gets its power from the starter solenoid when the starter is activated. If the ignition is jumping into soft start that would mean that the starter is likely also being energized, because that is where the modules get their 12v to trigger the soft start. If there is a bad spot in the wire that goes to the starter solenoid and it is picking up stray voltage it would also trigger the soft start feature.  I would unhook the wire that triggers the soft start and try a test run.

[Runtoeat]

Thanks Tom.  I wanted to unhook the soft start by disconnecting at the solenoid but didn't get a chance to do this.  We'll be returning Tuesday to try out the new soft start modules.  I'll check on disconnecting the current modules before we install the new ones.  From what I can tell, the modules operate in "start" mode and then go into "run" mode based on the rpm signal they get from the stator.  The soft start modules take this a step further with a change to their programs which delays the transition from "start" to "run" for a few seconds during starting.  Wondering if it might be possible that the CTLS we are working on is getting a intermittent, false, low rpm signal, from the stator which causes the modules to mistakenly go into "start" mode which results in retarded timing?

[DHeal]

OK, here's another "out-of-the-box" thing to check and hopefully dismiss:  Are all of your fuel tank vents open and clear?  Back-pressure due to a clogged or partially clogged fuel vent(s) might temporarily reduce fuel flow to the carbs.

[Runtoeat]

Roger and DHeal, thanks.  We'll do a fuel drain test tomorrow and see what the rate of flow is thru the gascolator.  But, again, the fuel flow is sufficient to sustain 5000 rpm running for 3+ minutes as things now stand.  Flow MUST be OK for idle running. 

Roger, we are thinking the same.  Whatever is limiting rpm's is uniformly acting on the system.  Whether this is fuel, ignition, or??  The fact that we seem to be getting good fuel flow and the inference that this is a uniform problem leads me to ignition.

[Anticept]

My bet is on your soft start modules picking up voltage. I don't know if the delayed timing will still net you 4500 in flight, and I'm not recommending you try, but there's one way to test this on the ground for idle: remove the softstart connection to the starter solenoid and touch it to the negative POSITIVE battery terminal briefly when it's warmed up.

EDIT: Had one of those moments. Thanks tom for pointing it out.

[Tom Baker]

7 hours ago, Anticept said:

My bet is on your soft start modules picking up voltage. I don't know if the delayed timing will still net you 4500 in flight, and I'm not recommending you try, but there's one way to test this on the ground for idle: remove the softstart connection to the starter solenoid and touch it to the negative battery terminal briefly when it's warmed up.

I would think positive terminal since it needs 12v to activate the soft start, not grounding.

[Tom Baker]

41 minutes ago, Roger Lee said:

I still don't see this as a soft start issue. The symptoms don't seem to be aligned with that type of issue. Soft start has nothing to do with rpm. It is all timing.

Roger, what would happen if the engine is running at idle and the soft start circuit was activated? The retarded timing would cause the RPM to drop off, right?

[Anticept]

3 hours ago, Tom Baker said:

I would think positive terminal since it needs 12v to activate the soft start, not grounding.

Yes, positive. Editing. Had a brain fart.

 

Peak pressure in many properly running engines is shortly after TDC, and the greatest torque generated is commonly around 30 degrees ATDC. Depending on stroke length, the peak leverage on the crankshaft can change as well (there's some trig involved due to engine geometry) but it's going to be close to that peak torque. The problem is, it takes time for combustion to complete, that's why we advance the timing. Retarding the timing (such as what soft start does) will lower the total power available for the engine because the peak pressure will drop considerably, and come too late for the engine to take full advantage of it.

Side note: on old old cars, there was a lever for changing engine timing. The car would be started with highly retarded timing, then advanced slowly until the engine starts pinging, then backed off a little bit. These days, we determine the initial timing settings with a dyno at wide open throttle until it pings or we find max torque, and fine tune from there.

[Anticept]

Which is weird, because if it's fuel, it shouldn't be running smooth like that either, unless we somehow are dealing with a condition that affects both carbs almost equally as well.

[Tom Baker]

Roger, the engine only runs rough on soft start because of the low rpm. If you advance the throttle before the timing changes it smooths out, and when the timing does change the rpm increases almost the exact amount thar the say theirs is dropping.

[Runtoeat]

I'm a fly on the wall and appreciate the thoughts here.  We kicked things around today before flying to the hangar queen CTLS.  We ended up not going.  We figured this is bigger than any of us after we pulled the heavy maintenance and talked to Brett at LEAF.  We thought that there was just one trigger coil running the modules.  Today looking at the diagram and talking to Brett, we found that there are four coils - 2 to each module - and if I recall the conversation correctly, these provide a sign wave which the modules use to determine crankshaft speed.  This problem will require someone with diagnostic equipment and knowledge of the Rotax ignition and fuel system to straighten out.  It is beyond our abilities and we do not want to find that we guessed wrong once someone is in the air.  We did kick around the step where we would pull the soft start off the starter relay but reasoned that this is a "non event" during the engine idle so it would provide no result.  Now, with Tom's idea that there could be a 12v. leak and Corey's idea to touch the soft start to a 12v source while idling, this might provide some data.  If applying 12v. takes the modules back into the soft start mode with retarded spark, and the rpm demonstrates the 1600 to 800 drop we intermittently see, and if we have smooth running, which we shouldn't based on Roger's comments...but....if we do get the similar rpm drop, this would be the failure state we might be experiencing.   Not sure if I mentioned it but when the idle has drifts down to 800, with the throttle lever at full stop, we can advance the throttle to bring the rpm back to 1600.  Then, once at 1600, the throttle can be returned to the stop and 1600 is maintained. Could both idle jets be temporarily picking up dirt simultaneously and slowing the engine down and are we then bypassing the idle circuit by advancing the throttler which brings rpm up?  I believe the next step might be to find expert Rotax repair locally.  I have found Rainbow Aviation's state by state Rotax repair facilities and we will probably start calling to see if there is someone who may have dealt with similar problems on the Rotax and if not, if there is a person or facility with diagnostic equipment to analyze this. The "ah, ha" for me here is to realize that our Rotax engine electronic ignition is fairly complex and trouble shooting and fixing problems involving this, if this turns out to be the case here, requires technical training and equipment.  Guess I'm rambling on here.  Thanks for the comments.  If you think of something, please let me know

[Anticept]

Two things:

Have you tried a new set of plugs? I've had issues with an aircraft slooooowly losing RPM at idle with old plugs, but when throttle is advanced then retarded, RPM would stay up for a bit, then creep down again.

Use another tach or stroboscope on an ignition wire with RPM readout and verify your tach isn't failed. Note that the thick silicone insulation does make it hard for the clamp on stroboscope method to read correctly.

Now that said, your engine won't stay running at 800 RPM if it was really 800 RPM and firing at advanced timing. No way.

[FlyingMonkey]

This is pretty basic, but have you checked the throttle cables end-to-end?  If they were somehow binding up or slipping somewhere, it might cause weird throttle/engine behavior.  It would probably run rough unless both cables were having the same issue, but it's easy to check and eliminate.  I didn't see the throttle cables mentioned in your original post.

[Runtoeat]

Corey, you are right.  The old adage....KISS.  I mentioned the broken insulators to my mechanic early on but he acknowledged this and said the plugs weren't broken and we moved on during our inspection. This should be revisited.  Your comment about the engine not running smoothly at 800 rpm with the spark advanced is a point toconsider. You mentioned elsewhere on the forum that the Rotax uses "pulse width" signal for timing? Would this be the variable voltage versus engine rpm which the trigger coils send to the modules?  Do the modules use this variable voltage signal to incrementally increase spark advance in the +3 degrees ATDC (start) to -30 degrees  BTDC (run) range?  Or, do the modules provide just two levels of spark, i.e., 3 degrees ATDC (for start) and 30 degrees BTDC (for run)?  If the spark is incrementally raised, perhaps there is a small enough change occuring at idle which causes reduced engine rpm but not enough to cause rough running and this might be the problem?  Or, if there are only two levels of spark advance and there is "a jump" from -30 to +3 at idle, then this probably would cause rough running and the suspicion that the ignition is a cause might be ruled out here.

Andy, the cable have been checked and appear to be OK.  Thanks.

[Runtoeat]

Hi Roger.  Mag drop is normal - around 50rpm.  EGT is normal as shown on Dynon.  Sure wish this CT was near to you so you could pull your hair too!   I like the idea of introducing 12v. while @ 1600.  Thinking about Corey's comments on running at advanced spark while at 1600 rpm?  Would you think the engine would be rough?  I'm thinking, if the spark is advanced in the two step method, then after the engine reaches 250 or so rpm during start, the modules will fire the engine with retarded spark but will advance the spark when rpm reaches 800 or so.  So, I'm thinking the engine may run smooth at 800 rpm with advanced spark.  Introducing 12v. would test this theory.

I'm going to work on my old Force / Merc outboard engine with points and condensor today - ahhh, back to simpler times.

[Anticept]

On 8/16/2017 at 8:10 AM, Runtoeat said:

You mentioned elsewhere on the forum that the Rotax uses "pulse width" signal for timing?

Pulse width modulation is a technique used to control the width of the injector solenoid on/off state in the 912iS. The longer it is on, the more fuel that is injected.

As for the ignition system, I'm not sure if I said it uses PWM. If I did, I used the wrong term.

The ignition system is powered by a couple of coils in the magneto generator. There are small tabs on the flywheel that pass by the trigger coils, which are hall effect sensors. This illustrates the idea quite well:

 

If we make the magnets larger, the sensor will stay on longer. It is the falling side of the signal that triggers the spark in normal operation, but it is the rising side that triggers below 600 RPM and during soft start.