VAF Forums

VAF Forums (https://vansairforce.net/community/index.php)
-   Traditional Aircraft Engines (https://vansairforce.net/community/forumdisplay.php?f=21)
-   -   Intermittent power loss - high RPM (https://vansairforce.net/community/showthread.php?t=189408)

RedRaider94 11-27-2020 03:14 PM

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!!

David Paule 11-27-2020 03:20 PM

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

RedRaider94 11-27-2020 03:25 PM

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.

RVDan 11-27-2020 04:26 PM

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.

RedRaider94 11-27-2020 04:54 PM

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).

pat76cj 11-27-2020 04:59 PM

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).

terrye 11-27-2020 05:02 PM

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?

RedRaider94 11-27-2020 05:07 PM

I am running 100LL and full rich the entire time. Electronic fuel boost is on when it happens (happens without it on also - checked that last weekend).

Don’t know about fuel pressure but always run between 26 - 28. I am going to record everything tomorrow so should have good data on egt, flow, fp, etc if I can get it to do it again.

RVDan 11-27-2020 07:14 PM

Quote:

Originally Posted by RedRaider94 (Post 1480790)
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.

PilotjohnS 11-27-2020 07:17 PM

Sounds like.....
 
Sure sounds like the fuel is getting hot. When the engine is warm, like during touch and goes, and the fuel flow drops, like during landing, the fuel will be flowing slower in the fuel lines and get warmer. I would make sure all the fuel lines are firesleeved and away from heat sources. But that is just me


All times are GMT -6. The time now is 05:50 PM.