Rpm jumping about and reading very high on take off

[Richard]

I have a Skyview SE connected to a Lycoming IO540. Max prop RPM for takeoff is set at 2700. At full throttle RPM can read up to 3700 jumping about in the red. If we delete either mag in Skyview setup it reads max 2700 and is stable. Reinsert both mags and the high readings return. Any ideas?

 

[GKC Aviation]

I had this initially on a C210 install. I had installed the 30Kohm resistors where I picked up the RPM from the P-Leads, but I had to install another two to stop the RPM from jumping around. It was especially bad at full throttle. No more problems after that. The install manual details it.
The resistors must be installed right at the point the P-Leads are picked off. In my case it was at the key.

 

[DBRV10]

100K resistor usually fixes it.

 

[Richard]

Our problem was that the rpm jumped up to about 3500 when it should be reading 2700. Today we tried the 100K resistor, firstly on one mag and secondly of both. In both instances the rpm reading reached about 5,700 !!!!! Any ideas? We are thinking of trying two variable resistors.

 

[preid]

My io540 needed a 150K resistor before it settled correctly. keep going up (both mag pleads) per GKC response until it settles.

 

[vlittle]

My io540 needed a 150K resistor before it settled correctly. keep going up (both mag pleads) per GKC response until it settles.

Check all of your plug gaps and wire connections for integrity. If there is a problem there, you’ll have trouble finding the right resistor value. The IO540 is worse because there are more cylinders and more opportunities for problems.

 

[Richard]

Yesterday, we fitted two variable resistors and fiddled around until the rpm readings were stable at all levels. This confirmed that two 150k Ohm resistors would do the trick. These have now been soldered in and seem to have solved the problem.

Thanks for all the help.

Oh and Merry Christmas.

 

[M20Driver]

My Mooney just started doing this (rpm spiking occasionally) in the last 5 flights after 20 minutes or so. I started playing with setting the other evening after returning to determine which side could have been causing it, and found my engine almost dies on my right mag. I suspect this is what's causing the overspeed warning all of a sudden. I have near 80 hrs with the HDX system now and no problems until now.

 

[preid]

My Mooney just started doing this (rpm spiking occasionally) in the last 5 flights after 20 minutes or so. I started playing with setting the other evening after returning to determine which side could have been causing it, and found my engine almost dies on my right mag. I suspect this is what's causing the overspeed warning all of a sudden. I have near 80 hrs with the HDX system now and no problems until now.

I would be quite concerned about the (L) Right Mag causing the engine to almost die.. or do you mean the RPM on the HDX shows no RPM but engine is running normally? The spike is from lack of resistance between the plead and switch, which would cause spikes, not sure why there would be no RPM indication, that is normally a bad connection or too much resistance.

 

[M20Driver]

I would be quite concerned about the (L) Right Mag causing the engine to almost die.. or do you mean the RPM on the HDX shows no RPM but engine is running normally? The spike is from lack of resistance between the plead and switch, which would cause spikes, not sure why there would be no RPM indication, that is normally a bad connection or too much resistance.

The HDX is showing spikes up to 3500 rpm in flight after some 20 minutes or so. After flight I was going to turn off one of the rpm inputs in an effort to determine which side was causing the spiking, and that when i realized one mag is failing. Never had a mag check issue, but I never thought to do an inflight mag check. My point was it appears a failing mag might have been causing the spike / engine overspeed warning after it becomes heat soaked.