912 ULS UNSTABLE RPM

I think the fuel pressure instability is the best clue.

1. What kind of fuel are you using and have you changed fuels recently? If this happens only with the cowl on that could indicate a fuel vaporization problem.  

2. Do you have a fuel return line, and if so, is it working properly?

3. Have you cleaned all fuel filters and screens and checked the fuel gravity flow from the tanks to the fuel pump?

Are you sure that the engine is actually varying RPM?  Perhaps it is a tachometer issue? Perhaps the fluctuating tachometer and fuel pressure readings are a related electrical issue??  Just a thought.

KAIO please can you give some history:

1. Did this problem begin suddenly, or did it begin gradually?

2. You have given a long list of items checked/replaced on the engine.
Were any of those items (or any other items) checked and/or replaced on the engine before this problem began?

3. You say the engine loses 200RPM. Can you explain this further. Is this 200RPM from the wide open throttle RPM?

Thanks.

Hi KAIO

I see you are from Brazil.  Are you using AVGAS?  If not switch to that right away.  As some others have pointed out check your return line.  The line has to have a restrictor in the line, be sure that is not plugged up with anything.  Be sure that the fuel line is wrapped with some insulation also to try and stop heating of the fuel.  Check the temperature at the fuel pump and on the top of the engine where your fuel lines are.  If you are getting more than 36 C you could be having issues with boiling fuel. I would also check your electric boost pump.  Switch to a newer one if you can.  The pump should be in series with the mechanical one on the engine and it should be as near the fuel supply as possible.  I think on your aircraft it may be on the firewall. Check the screen inside your gascolator also and be sure it is not getting contaminated. 

I would also give Bruno and call at CDS as he might be able to help you.  

Cheers

If it were me, I'd want a "second opinion" on rpm loss.  Right now you only have your Dynon to tell you the rpms, and what if there was a bad electronic component in it, or what if there was some electronic interference in play.  The easy 30$ solution that I did in the same situation was to buy a Tiny Tach, put it in the cockpit, run the signal line through the firewall around #1 spark plug and find a ground, then select the right Tach setting and BAM - I immediately had independent verification of what my rpms actually were.

If you find both sources give you the same number, then you know it's NOT the Dynon and can look elsewhere.  But if the numbers are different then you know it's the Dynon.  Lastly, if you are gaining and losing rpms in flight - my humble suggestion is NOT look at the fuel pressure or Dynon - and turn off any noise cancel on your headphones, and simply "close your eyes" (not really, and try and listen and "feel" intently - and try and sense any small amounts of engine surging.  There are amounts we can't detect, but when I had a similar problem I could feel the engine surging a bit.

Bert