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  #1  
Old 11-27-2020, 03:14 PM
RedRaider94 RedRaider94 is offline
 
Join Date: Nov 2014
Location: Celina, Tx
Posts: 54
Default Intermittent power loss - high RPM

Hoping someone might have an idea as this is driving me absolutely crazy.

RV7 - 1547 hours
Superior IO360
Slick mag
Light speed II EI
Silverhawk Bendix EX FI
Constant Speed prop

Problem: when doing touch and goes, the second takeoff and beyond sometimes has a feeling off power loss for a second or two then comes back and then a loss again, and then comes back. This is during climb out at max throttle.

*** it does not happen on the first takeoff of the day ***

The couple second feeling would be light switching off the Lightspeed and running just on the mag.

Fuel flow is in the 18 gph range when the issue is happening (that is normal fuel flow for me at takeoff) and RPMís donít go down while this happens - they stay steady.

The engine is not stopping, itís just a slight loss of power for a second or two, comes back for a second or two, then loss again and so on.

This doesnít happen on the first takeoff of the day and I am just stumped. Any initial ideas or how in the world would you go about troubleshooting something like this?

Thanks in advance for any thoughts!!
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  #2  
Old 11-27-2020, 03:20 PM
David Paule David Paule is offline
 
Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,759
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Just some questions - Does the manifold pressure show a drop? And do any of the cylinder's EGT readings show a drop at that time?

Dave
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  #3  
Old 11-27-2020, 03:25 PM
RedRaider94 RedRaider94 is offline
 
Join Date: Nov 2014
Location: Celina, Tx
Posts: 54
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MP stays in the 27ís when it happen (RPMís stay around 2650) so nothing noticeable with my eyes. I meant to record the takeoff in the GRT today but forgot to hit the record button. Will do that tomorrow to see if anything looks weird. If either are happening I am going to need data to really see it as it isnít perceptible to me eyes.
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  #4  
Old 11-27-2020, 04:26 PM
RVDan RVDan is offline
 
Join Date: Jun 2012
Location: Frederick, MD
Posts: 902
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I might suggest that you are getting some generation of fuel vapor in the line between the servo and the flow divider. I had this happen several years ago on a FI RV-6. It will only happen when the engine is hot. I typically would see this at its worse when doing laps in the pattern. A high power takeoff and climb, followed by a pattern at low speeds which doesnít cool the engine well.
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Frederick, MD
PA28-140
Hph 304CZ
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N199EC RV6A flying
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  #5  
Old 11-27-2020, 04:54 PM
RedRaider94 RedRaider94 is offline
 
Join Date: Nov 2014
Location: Celina, Tx
Posts: 54
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Hadnít thought about that Dan. This started happening about 6-8 months ago best I can remember. Any idea if something specific might be causing it (outside of high heat generated with touch and goes) or anything that i could try to do to fix it)?

If I do a normal climb up to 4-5k ft with CHTís running warm around 400, it doesnít happen if it is the first takeoff of the day (best I can recall it hasnít happened in that situation).
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  #6  
Old 11-27-2020, 07:14 PM
RVDan RVDan is offline
 
Join Date: Jun 2012
Location: Frederick, MD
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Quote:
Originally Posted by RedRaider94 View Post
Hadnít thought about that Dan. This started happening about 6-8 months ago best I can remember. Any idea if something specific might be causing it (outside of high heat generated with touch and goes) or anything that i could try to do to fix it)?

If I do a normal climb up to 4-5k ft with CHTís running warm around 400, it doesnít happen if it is the first takeoff of the day (best I can recall it hasnít happened in that situation).
When the fuel leaves the servo it is under low pressure and when you look at the boiling temp for Avgas, you can see it coild boil creating vapor in the line. The flow divider will normally hold a back pressure on the line from the servo which then raises the boiling temp of the fuel. If the flow divider is not holding the pressure on the line, it can induce vapor issues.
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Frederick, MD
PA28-140
Hph 304CZ
RV6 built and sold
N199EC RV6A flying
Retired Aerospace Engineer and A&P/IA
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  #7  
Old 11-27-2020, 04:59 PM
pat76cj pat76cj is offline
 
Join Date: Jul 2020
Location: Western edge, Iowa
Posts: 31
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When on downwind/base/final, are you still running full rich? Possible fouling of plugs/deposits from running too rich at low power settings. Might consider leaning after leveling out and turning crosswind/downwind. I know the reason we run full rich is if we have to go around/missed, you want full power. per the POH. Just remember to advance to full rich again, after touch down, if going back up again (or if going around).
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  #8  
Old 11-27-2020, 05:02 PM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 931
Default Intermittent power loss - high RPM

Avgas or Mogas? Fuel boost pump running or not? You mention 18 gpm, but what is the fuel pressure at the servo?
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  #9  
Old 11-27-2020, 10:18 PM
rv8ch's Avatar
rv8ch rv8ch is offline
 
Join Date: Feb 2005
Location: LSGY
Posts: 3,737
Default GRT

Quote:
Originally Posted by RedRaider94 View Post
MP stays in the 27’s when it happen (RPM’s stay around 2650) so nothing noticeable with my eyes. I meant to record the takeoff in the GRT today but forgot to hit the record button. Will do that tomorrow to see if anything looks weird. If either are happening I am going to need data to really see it as it isn’t perceptible to me eyes.
Two things - I recommend recording all the time and just leaving a small USB key in there for cases like this. You can put the data on savvyaviation.com and get some interesting information.

You may also want to set the recording speed for this issue at the highest frequency, which I believe is 200 milliseconds. 1000 milliseconds (1 second) is probably not going to catch this issue.
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  #10  
Old 11-28-2020, 08:18 AM
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Carl Froehlich Carl Froehlich is online now
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,940
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If this is a new issue, I would first suspect the Lightspeed ignition. If a coil problem (not unusual) it will show up with two EGTs (1&2 or 3&4) going a little higher.

Do the plug wire and spark plug resistance checks as well (Lightspeed install manually). High resistance in either will end up killing the associated coil.

Carl
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