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Runtoeat

RPM Rollback

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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.

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This doesn't sound electrical because you said it is smooth.

How many hours on the carbs? maybe time for a rebuild?  Bottom line; iI it were me I would strip them down and clean them just like a rebuild. Rod out all jets with a piece of wire. Flush every orifice with carb cleaner and then follow it with high pressure air. Use a MM ruler and check float height and put the floats on a scale. You can sit a play with this for weeks or spend 2 hours to take them off, clean them and then sync them.

If the carb cleaning fails then It might be worth while to pull apart and inspect the filter in the instrument panel.

 

Leave no stone unturned or foe sure that one stone will be the bane of your existence.

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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.

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DHeal   

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

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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.  

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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.

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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.

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When roding out jets I take a 4" piece of 16 ga. wire and strip back the insulation. Then I unwind one strand from that. This is how small the idle jet orifice  is. This copper wire will not enlarge the orifice holes, but will help scrape out any deposits. 

Check the red wire on each ignition module. Pull on it and take it out to see if it is cut by the connector inside.

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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.

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It's the smooth part that has me wondering. If it dropped that much at low rpms and it was dropping a mag then it would run rough. Fuel delivery shortage to both carbs can cause what you describe. Check the fuel filter in the panel. I haven't seen one that clogged, but it's possible.

Also if there has been a fuel hose change make sure one of the fire sleeve clamps isn't too tight.

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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?

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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.

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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.

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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?

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I doubt it is anything to do with soft start. He says it runs smooth. Soft Start timing is at 3 ATDC and normal run timing is 26 BTDC. If it was soft start is would run rough just like it does at start up for the first few seconds. The starter itself can't be involved because the sprag clutch release at approximately 850 rpm and if they ran the engine up to their higher rpms it would trash it.

 

It's the smooth running that makes me think fuel reduction. It doesn't sound like one carb because it would be rough. A central fuel reduction to the system would account for the loss of rpm, but still smooth.

 

???????? This is a tough one for sure. 

You have to start somewhere and just start eliminating possibilities.

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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.

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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.

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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.

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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.

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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.

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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?

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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.

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6 hours ago, Tom Baker said:

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?

And it would run rough just like at start up. not smooth like Dick stated. It would run rough because timing would not be at its 26 BTDC it would be 3 ATDC. and it would run rougher at 5000 rpm than idle. It's the smooth part that is confusing and leads me to believe it may be fuel.

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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.

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