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My engine has about 450 hrs. (new in 2018) I use Unleaded 98 fuel. I normally cruise at about 4,150 - 4,300 rpm. I have been getting a  slight vibration/change in engine noise (best described as  bzzzz)  but no change in power mostly in these instances:  

1) when reducing power.

2) running under 4,000 rpm 

I first noticed it at about 200 hrs.  was very occasional then.  Progressively increasing in frequency and duration. I did have a prop strike at low rpm just prior to 200hrs and engine was checked deemed to be ok.

Previously it would occur for a couple of seconds, it will stop if I increase RPM.

My last service at 400 hrs (this year) did a full carby service and balance and checked ignition coils.  The sensation still occurs and has been getting more pronounced and lasting longer now possibly around the 10 -20 seconds at a time or until I change RPM.

I have notice it to be random in duration  a typical example is:  bzzzzzzzzzzzz ...............bzzzz.......bzzzzzzzzzz. ......bzzzzzzzzzzzzzzzz

Initial thoughts were to consider the coils or carby doing something hence the full carby service and balance. .  Now I am thinking gearbox after reviewing some of the videos on this site.

Any suggestions please?

 

  • Re: 912 80hp vibration bzzz under 4,000 rpm

    by » 2 weeks ago


    I had this noise, the rubber protection of the small black expansion tank was broken.

    this bottom of this part vibrated on the engine

    Maybe the same problem?

    Jacques 


    Thank you said by: Allan McMillan

  • Re: 912 80hp vibration bzzz under 4,000 rpm

    by » 2 weeks ago


    Allan - could be so many things.  A buzz related to rpms...  If this were my plane, I would do what you're doing here; but also:

    Take some inflight audio to demonstrate the issue.  My iphone takes horrible in flight audio but my go pro does a solid job.  Then post the audio for us.

    Is the buzzing coming from the firewall forward?  Behind the back wall?  In the cockpit area?

    Next - you make no mention of what happens on ground.  I would tie it down, bricks in front of tires, take the cowl off, and see if you can get it to buzz.  With someone in the cockpit to kill engine in an emergency, I would run the engine and try and pin down if I can get it to repeat on the ground.

    If so, wear thick gloves and randomly push and squeeze items around your engine to see if you can identify source.

     

    My 2 cents!

    Bert


    Thank you said by: Allan McMillan

  • Re: 912 80hp vibration bzzz under 4,000 rpm

    by » 2 weeks ago


    If you download a spectrum (frequency) analyser app for your phone, assuming it can discern the buzz from the ambient noise then you can see the frequency of the buzz relative to the engine rpm. That will tell you whether you have an engine issue or a prop/gearbox issue.

    Alternatively, make a decent audio recording and note the engine rpm. You can then post-process it on a PC using something like Audacity (free). At least that might narrow down where to look.

    You'll have to convert rpm to Hz and don't forget that the prop frequency is the engine rpm divided by the gearbox ratio.


    Thank you said by: Allan McMillan

  • Re: 912 80hp vibration bzzz under 4,000 rpm

    by » 2 weeks ago


    I mean you destroy the engine fast with so few revolutions, my opinion and many others also say that you should fly with 4800 revolutions preferably not below 4700 for the engine to last the longest otherwise 5000 revolutions but then you may be eating of Total hours on the engine but I know I do not fly even between 4800 and 5300 rpm.


    Thank you said by: Allan McMillan

  • Re: 912 80hp vibration bzzz under 4,000 rpm

    by » 2 weeks ago


    I should mention, the engine is on a microlight trike.  The engine is in push config.   I do vary it it from time to time to gain altitude so its not constantly running at cruise rpm.  4200 rpm is trim speed.  With 2 POB it will be 4700rpm for trim.  Take off is 5400 rpm. any RPM above trim will make it climb. 


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