Max rpm becoming too low !

[Daniel GUEDON]

We have the following issue : the max rpm gradually went over time from 6200 @ 95kmh when engine was new down to 5800 now. TTE is 37 hours, ARCUS 1 snr 202 

Spark plugs, spark plug caps and ignition harness has been replaced.

Throttle position sensor replaced and adjusted at 89° instead of 89.6° at full throttle due to free play causing jitter. This did not cause problem in the past.

Now I am wondering where to go from here, if anyone experienced something similar, findings might help.

Thanks 

Daniel

  

[Andrew Georgeson]

Have you tried cleaning the airfilters?

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[Daniel GUEDON]

Hi Andrew,

Air filter and fuel filters have been replaced, no change.

Wondering if the cylinders and/or pistons had been damaged, we inspected with a camera the cylinders, and found very minor scratches.

Then we measured the engine compression pressure, and found 8.2 bars on both cylinder, but we do not know the correct value.

Anyone has the info ?

Tahnks

Daniel

[Chester Fitchett]

Gas engines are fuel, air and spark.  I assume the butterfly valves are wide open at full throttle.  Spark/ignition could be compromised by mistiming, hard to measure  What about fuel?  This would be a terrifying test, but could you spray extra fuel at the air filters to see if you get extra power from more fuel?  Pressure could be dropping in the fuel rail - I'm not sure how fuel pressure is regulated in the Arcus, but in cars there is a fuel pump and a shunt valve that opens at a specific pressure.  If the spring gets tired, fuel pressure will drop.

[Mark Zivley]

If there is a crankcase leak then the engine will see less fuel/air into the combustion chambers

[Andreas]

I made this painful experience. A piece of the piston desintegrated and led to a little hole in the crankcase. Hardly to discover during insitu inspection as it was  located below exhaust pipe connection of the forward cylinder.
You may identify the affected cylinder by measuring the exhaust temperature during an ground engine run ( using a heat gun). A lower temp indicates insufficient,i.e. loss of fuel/air mixture.
Do the check on normal and backup system to exclude a fuel injection nozzle issue.
In addition to exclude air filters trouble do the engine run with Filters removed in an proper environment.
Best regards

[Daniel GUEDON]

Still working on this issue, I am going to check the fuel pumps and their hiden filter

Which pump is the normal mode one, front or rear ?

Thanks

Daniel

[Andreas]

If I recall right the front one. You can check by switching ignition on and put your hand on either pump.

[Pavel Foldyna]

Hi, we had the same problem once on Arcus. The revs dropped to 5800 on the new engine and when we searched for the cause we found nothing, just some minor stuff with the exhaust gasket, but we fixed that. As it is a 2t engine, on the advice of a mechanic I adjusted the oil to petrol ratio. The engine had about 36 hours on it. I put the ratio at 1:40 and the problem was solved. The revs were now 6300 all the time. We flew with this ratio for 5 years.
Let me know then.

Dne středa 26. března 2025 v 10:39:51 UTC+1 uživatel Arcusdriver Andreas napsal:

[Edoardo Jacucci]

Thank you Pavel - so you say a bit richer/more oil in the mix? 

2.5% instead of 2% as per flight manual?

Thanks!

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__________________________
Edoardo Jacucci

[Ugo Raffaelli]

Hello everyone. I have always prepared the mixture with aviation gasoline and 2.5% of excellent quality oil and until now, after 12 years of use, I have never had serious problems with the engine of my Arcus M.

ugo raffaelli

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