Constant Speed Not Constant
 

In the last couple of flights in calm air, level with AP on, with constant speed prop set at 2300, I noticed the rpm jumping down as low as 2270 and high as 2310. Frequency is at least once every minute. It jumps quickly and most times its almost unnoticeable unless Im looking at the EFIS. Engine or oil temp doesn't matter. Nothing out of parameters on the CHT or EGT.

On a short flight to get fuel yesterday I ran at 2400 and it stayed dead on, but started fluctuation again when set at 2300. Just did an annual and thought I would find something up with the prop governor or plugs.. but nothing so far. I have a GRT EIS and Dual EFIS set up with 205 hours on a O-540.

Update EFIS software? Check mag timing? More test flying tonight at various RPMs.

what governor?

I would have thought that a 40 rpm shift would be quite noticable without the instrument but you say your primary feedback is the instrument and not "feel"? Especially if it is cyclic?

If that is true, I might look very hard at the rpm measuring system in addition to the gov or prop.

The GRT EIS picks up rpm from both mag P leads, but usually only sends "rpm1" to the efis. Set the EIS display to show both rpm1 and rpm2. Next time this happens look at the EIS. If the readings are 'split' then it's an EIS issue (you may need a larger or smaller resistor in the sensor line to rpm1)

MT P-860-3 Governor
3 Blade Aero Composite

Didnt think of that.. Ill set it up and see what kind of readings I get.

rzbob - It generally wants to stay at 2300 but quickly jumps down 20 or 30, then back to 2300 then up 10 and back down. Almost seems to happen quickly enough that its hard to feel unless its in pretty calm air. In other words in two of the flights the passengers didnt even notice a change... or to scared to say so! :)

Call MT in Deland. We had this problem and they had to change to a different strength spring internally for the 540.

Thanks for the insight. I'll give them a call.

Flew this evening for about 1/2 hour, running at different rpms to see what I could figure out. leaving the MP at 23" I can run at 2250 or 2350 and it doesn't change 1 rpm, but at 2300 it fluctuates from 2270 to 2310 almost every 15 to 20 seconds. I even took off my headsets but cant really feel or hear noticeable change it the engine sound. Ill try some of the other ideas posted here next week. Thanks for everyone's input.

Thats exactly the problem we saw. MT's number is 386 736 7762.

What they said. A relief spring change or thin washers under yours will be the answer.