Engine rpm too low with full throttle

[Arcusdriver Andreas]

Recently I have trouble to achieve the max rpm with full throttle. The engine rpm does not go higher that 3900 rpm. Various settings have been checked and in summary it does this with either ignition 1 or 2, with air filters removed, with the redundancy system active. All spark plugs have been replaced by new ones and I have checked the ignition system and sensors okay with trijekt diagnosis tool. Both butterfly valve of the injection system have been checked for correct physical operation. I am now at a point where I suspect the fuel supply and/or insufficient ventilation. Does anyone has experienced similar? And more particular experienced any trouble with the check valves downstream of the fuel pumps ?

Regards

Andreas

[Bif Huss]

Are both cylinders firing? I had this problem with my V3 and the culprit was a bad connection to one of the cylinders.

Bif

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[Mark Zivley]

I would tend to agree with Bif.  Sounds like only 1 cylinder.

The check valves in the fuel system are probably not the issue.  I believe (someone can correct me if I'm wrong) that in Normal mode, one fuel pump runs and I suspect it turns on/off as needed to maintain fuel pressure.  In redundancy mode, the other fuel pump runs and it may stay on no matter what the fuel pressure is.  So unless both check valves have failed mostly closed, I suspect that both are working ok.

I base the above on the sounds when the Engine master circuit breaker is pushed in.  You can hear the primary pump whine for a second or 2 until the desired pressure is reached.   If you have the switch in redundancy mode when you push in the Engine master circuit breaker, you hear the backup fuel pump whine and it won't stop.   This operation makes sense as a "backup".

Might be worth pulling the spark plug wires off of one cylinder and seeing what happens then switching to the other cylinder.

Mark

[Mark Zivley]

Any chance you have the plug wires wired incorrectly?

[Andreas]

Thanks Mark, I will check this.

Mit freundlichen Grüßen
Andreas Balk

[Mark B]

A leak in the exhaust manifold caused a reduction in max rpm, but nothing like as much as your reduction.

[Chester Fitchett]

If I was diagnosing this problem on a small engine, I would run the engine without air filters, so extra fuel could be added to the air flow.  If the engine RPM goes up with extra fuel, your problem is fuel starvation, perhaps a blockage between the fuel tank and the pumps.

What to spray into the intakes to act as fuel?  I did this once with throttle body cleaner, but I wonder what that would to do crankcase lubrication.  Likely the safest would be a squirt bottle with pre-mix gas.

[Jorgen Thomsen]

Hi Andreas

In the inlet stud of the fuelpump a fine mesh low capacity filter is mounted.

It could restrict the fuel flow at higher rpm.

I have removed these filters on my pumps, they are not nescesary, as the fine micron filter sits after the pumps.

If you have a pressure meter 0 - 4 bar, you can connect it to the fuel manifold by the redundancy injecters.

The pressure should read 3,2 bar from my memory, also at full rpm.

Cheers Jorgen #061

[Mark Zivley]

Jorgen does have a good point.  If this drop in RPM occurred very suddenly, then it's less likely it's a filter issue, but if it's gotten worse with a little time, it could be the case.

You'll have to pull out the fuel pumps to get to the inlet side and then inside the inlet of the pump is a small screen filter.  Picture below.  The cylinder coming toward the camera is a fiber optic light so I could get a good picture, but you can see the filter.  It's lightly pressed into place.

There have been reports of fibers building up within this filter, but at 6 years, you can see mine is spotless.

Mark

[Andreas]

Thanks Jorgen. These veey inlet filters are already removed. Fuel pressure measurement is on my "to do"list.
Best regards
Andreas

[Henrik Holm Kristensen]

Hi Andreas

We had a similar problem with our Arcus some years ago. 

But not on the redundancy system. 

Only normal fuel pump could not deliver full flow. 

We found a little filter in the inlet nozzels on both fuel pumps. 

The filter in the normal pump inlet nozzle was fould origination from the filter element in front of the pumps. 

In a Dimona aircraft similar pumps is installed, and on the Super Dimona is is required to remove the small filter in the inlet nozzle of the pump

Sendt fra min iPhone

Henrik Holm Kristensen

Tlf +45 23695110

Den 23. mar. 2023 kl. 11.40 skrev Arcusdriver Andreas <quax...@gmail.com>:

Recently I have trouble to achieve the max rpm with full throttle. The engine rpm does not go higher that 3900 rpm. Various settings have been checked and in summary it does this with either ignition 1 or 2, with air filters removed, with the redundancy system active. All spark plugs have been replaced by new ones and I have checked the ignition system and sensors okay with trijekt diagnosis tool. Both butterfly valve of the injection system have been checked for correct physical operation. I am now at a point where I suspect the fuel supply and/or insufficient ventilation. Does anyone has experienced similar? And more particular experienced any trouble with the check valves downstream of the fuel pumps ?

Regards

Andreas

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[Arcusdriver Andreas]

The story has come to an end. The troublemaker was a piece of a piston which did its destructive work in the engine housing. The debris produced a little whole in the housing and such spoiled the fuel/air mixture. Other bits of the debris were found in the piston rod bearing but did not block it. How the piece could have broken off needs to be established. No signs of excessive heat or insufficient lubrification.

Arcus is back in the air with new engine....

Thanks to all of your for your valuable hints.

Best regards

Andreas

[Mark Zivley]

I'm not a metallurgist.  Anyone on this group?   I'm curious if it might have been a defective casting.