A

apatti

Well Known Member
All,
Recently I have noticed that when I do a run-up I have trouble with the RPM varying. I throttle up to 1700 rpm and note the RPM will vary from approximately 1660 to 1730. I see this variation before ever testing a mag. And, I see while trying to test the mags. It is hard to get an exact mag check, but they appear to be dropping normally (around 60 on the left and 80 on the right). I have dual P-Mags. I am quite sure the throttle is not moving as I keep my hand on it during the run-up. Also, the friction lock is set the same as it has been for a while. I can hear an audible change in the engine. So, I don't think it is a measurement error. It is an actual change in engine RPM. While the RPM's have varied in the past during run-ups, it was only on the order of +/-20 or so.

I did not have any trouble with this in flight. The plane developed normal RPM on takeoff and climbout. The RPM did not vary in level flight.

BTW, I have a fixed pitch Catto 3-blade prop and an O-360. My instrumentation is GRT EIS and EFIS'.

Any thoughts would be appreciated.
 
apatti said:
All,
Recently I have noticed that when I do a run-up I have trouble with the RPM varying. I throttle up to 1700 rpm and note the RPM will vary from approximately 1660 to 1730. I see this variation before ever testing a mag. And, I see while trying to test the mags. It is hard to get an exact mag check, but they appear to be dropping normally (around 60 on the left and 80 on the right). I have dual P-Mags. I am quite sure the throttle is not moving as I keep my hand on it during the run-up. Also, the friction lock is set the same as it has been for a while. I can hear an audible change in the engine. So, I don't think it is a measurement error. It is an actual change in engine RPM. While the RPM's have varied in the past during run-ups, it was only on the order of +/-20 or so.

I did not have any trouble with this in flight. The plane developed normal RPM on takeoff and climbout. The RPM did not vary in level flight.

BTW, I have a fixed pitch Catto 3-blade prop and an O-360. My instrumentation is GRT EIS and EFIS'.

Any thoughts would be appreciated.
Click to expand...
Is there any chance this was due to gusty surface winds? The load on the engine varies as the wind speed varies, so the rpm would change.
 
Kevin,
Thanks for the reply. This isn't one I had thought of. However, I don't think this could be it. On Friday when I last flew, the winds were pretty calm (less than 6 or 7 knots if I recall correctly) and not gusty. Now, on Saturday when I was ground running the engine to heat up the oil for an oil & filter change, the winds were a bit gusty. And, this is when I was paying more attention to the RPM variation. Still, the gust factor was less than 10 knots. I wouldn't think that would be enough to cause that much difference.

However, now that I think of it, the airplane was pointed down a row of T-hangars and the winds at that point on the field could have been quite different than what AWOS was reporting. The next time I fly, I will keep this in mind and run some experiments.

Thanks,
 
Tony: I had a similiar rpm flux on my last flight when throttle was max or near max. Rpm was steady below 2000 rpm but fluxuated 500-600 rpm, as I said at max throttle or near. I have dual P-Mags, O-360/carb/fixed pitch prop and 825 hours, working perfectly until last flight. I talked to EMagair folks and they gave me several suggestions to work. The first fix was to pull the 6-wire plugs and check tightness of wire screws and reinsert and check. It worked, none of the screws were really loose, a couple I tightened an 1/8 of a turn and the plugs appearded to be seated and the hold down screws tight. Anyway it worked on the test flight of 30 minutes. A cheap and easy fix. The P-Mags probably lonely and just wanted some TLC. Dan
 
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