Skyview and jumpy RMPs tach - Check Mag Timing

In case anyone out there has issues with their RMP gauge not remaining steady and occasionally jumping around on the Dynon Skyvew, or perhaps other glass panel system that gets it's reading electrically vs. a mechanical tach, I have found that the issue can be caused by mag timing between the left and right mag being off by a few degrees.

There is some info out there about if you have a 4 cylinder that you can increase the in line resistors from the standard 30K to around 60K and that will take out the jumpy RPMs...but before doing that, check your mag timing...not sure exactly how many degrees off between the two mags the jumpy RPMS will show up...but it will at some point - at least in my case it .

It seems that one added benefit to the glass panel system/RPM tach is that you have a built in indicator of mags getting out of sync between annuals.



Paul
RV7A with O360/Skyview panel

Seems weird.

The Dynon system reads either mag, but not both at once.

It is usually a resistance value matter.

So the points open every revolution of the mag, and close again.....so there is one very tiny slim chance that I can think of that might cause this o be possible. But I am clutching at strafers here.

So was your timing advanced or retarded and by how much? Which was it L or R?

Right Mag was retarded...didn't look closely to try and see exactly how much..maybe about 4 or 5 degrees I would estimate.

For what it is worth, I have 56K resistors in as when initially running the engine with the 30K resistors it the RMP tach would jump up periodically (150 + hrs ago). Haven't had the issue since putting in the higher resistors, till the mags where out of sync

Triumph1974 said:

Right Mag was retarded...didn't look closely to try and see exactly how much..maybe about 4 or 5 degrees I would estimate.

For what it is worth, I have 56K resistors in as when initially running the engine with the 30K resistors it the RMP tach would jump up periodically (150 + hrs ago). Haven't had the issue since putting in the higher resistors, till the mags where out of sync

Click to expand...

I agree with David. Seems highly improbable that a timing mismatch would cause a jumpy tach reading if the mag and dynon are working properly. It is more likely that you have too much resistance and that is the cause, just as it was when had too low of a resistance. You need to find the sweet spot between the two. It is an iterative process. Timing has nothing to do with RPM. The dynon is just observing the collapsing current/voltage on the coils primary winding when the points open. The resistance must be right for it to do it properly. The resistance required will vary as the e gap drifts from point wear. This is likely the reason for thesudden appearance in your case. As the e gap drifts, your primary coil power diminishes and at a certain point, the 56k ressitors are degrading that too much for the dynon to routinely read the signal. This issue will likely reappear and you should drop to 45 or 50K resistos when it does.

FYI, it is the point wear that caused the timing drift which helps substantiate the conclusion drawn above, as the point wear causes both timing drift and e gap change (which reduces the power on the primary winding). Whenever timing on a mag drifts due to point wear, the e gap must be resset in order to restore optimal spark power from the mag.

Larry