wavering RPM

G

Glasair

Filing Flight Plan
Joined
Oct 3, 2021
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ermed5
My Dynon units are amazing... so accurate. But, they also let me notice small aberrations. I've had this for some time, and remedies have not helped.

At cruise, or while sitting on the ramp, my RPM's waver a bit. I can idle at 1000, then see it occasionally go up to 1020 or 1030, then maybe later decrease to 990 or 985. Does it on its own, even with the friction lock tight. Sometimes I will notice the change in engine noise... its very slight... but I notice it.

What have we done. ... where do i start! Installed new carb (standard M-S) unit, removed old fuel pump and installed new, all new fuel hoses (with heat wrap), new gascolator, checked all cables,( BTW this is an O-320 B2B 160hp with dual EFII electronic ignition, about 200hrs SMOH), and more. Sensenich fixed pitch prop. I'm sure I forgot a few more items we did.

Its a minor item, but it bugs me..... in previous airplanes, you set your throttle and RPM, and it doesnt change. This one wavers a bit. Wondering if anyone else has experienced this.
 
I doubt it. Like today.... no winds, sitting at idle.
 
Glasair said:
I doubt it. Like today.... no winds, sitting at idle.
Click to expand...
Hmm. With fixed pitch prop, my guess was prop loading from wind. Do you get any fuel-flow change? And difference with the fuel-pump on or off?

On the other hand, what's +/- 20rpm between friends if the engine otherwise runs well. :D
 
I was also thinking that.... maybe looking TOO closely at the Dynon. Have to agree... +/- 20rpm between friends can always be a good thing. No fuel flow changes. It doesnt have a backup fuel pump... all gravity.
 
I wouldn't even notice that. But if it's bugging you, send dynon an email. You'll get response Monday morning.
 
Did those previous airplanes have a Dynon multifunction instrument, or a mechanical tachometer?

What's the published sampling rate and accuracy unit of the Dynon? That displayed 20 or 30 RPM disparity is a .334 to .667 RPM per second variation, or a two or three percent change in idle speed. Your engine speed isn't changing 20 or 30 RPM, it's a reflection of an instantaneous miniscule measurement increment, extrapolated into a rotational speed over one minute calculated and displayed by the Dynon.
 
Are the throttle linkages tight, or is the rod end bolt/nut loose, or the cable housing clamp loose?
 
Your engine always did this, you just did not notice because mechanical tachs dampen it out. Nothing to fix, stop throwing parts at it.
 
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