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RPMs drop >200 during Run Up on Slick MAG while PMAG only drops 100 RPMs

saldarro

Member
Our RV-14A with a Thunderbolt 390 Lycoming delivered by the factory late 2018 has been running well for 400 hours and almost 3 years.
It has a Slick MAG on the Left installed by Lycoming and a PMAG on the Right.
Recently we noticed that the Slick MAG drops more that 200 RPMs when we do the Run Up at 1,800 RPMs.
We have tried the following actions, but have not been successful at eliminating the >200 RPMs drop on the Slick MAG:
- Inspected Plugs: clean, spark, gap all OK
- Checked Timing at specified 20 Degrees OK
- Sent Slick MAG for inspection, no issues noted.
Had the 500 hour service performed.
- Installed the Slick MAG back and Timed it at the specified 20 Degrees
- Ran Up at 1,800 RPMs, >200 RPMs drop on the Slick MAG continue

When we cut the PMAG on cruise at 2400 RPMs
, 24 Inches MP at 4,500 Feet, the RPMs only drop 10 RPMs.

If anybody has solved a similar problem or have suggestions or additional actions to solve, we will appreciate your thoughts.

Thank You!
 
To be clear on how I’m reading this:
- RPM drop when turning off the mag: 200
- RPM drop when turning off the pMag: Something a lot less than 200
- Both mag and pMag timing, plugs and wires verified good.

Do I have this right? If so, I would point to timing difference between the mag and the pMag at 1800 RPM. The mag will be at 20 degrees, the pMag at something less than 20 degrees (as the pMag timing is RPM dependent as well as MP). So turning off the ignition with the most timing advance (the mag) at 1800 RPM will yield a larger RPM drop.

I assume you have a CS prop. Here RPM drop at 2400 RPM does not mean much.

Carl
 
To clarify:

During Run Up at 1,800 RPM

When the switch is on L, meaning the engine is running on the Slick MAG only, the RPMs drop more than 200 RPMs

When the switch is on R, meaning the engine is running on the PMAG only, the RPMs drop around 100 RPMs.

Thank you for your thoughts.
 
And you set the 20 degrees for the pMag timing by placing the crank at 5 degrees after top dead center when you did the pMag “blow in the tube”?

Carl
 
The 20 Degrees is for the Slick MAG.

The PMAG was timed over 2 years ago using the PMAG instructions.

Thank you for your thoughts.
 
The Mag checks have been on the ground at 1,000 feet over sea level, using full rich mix.
We are planning to do Mag checks with some leaning next week to see if it makes a difference, however, for over 2 years we have been doing full rich the Mag checks at this airport without >200 RPMs drop on the Slick Mag.

Thank you for your thoughts.
 
The Mag checks have been on the ground at 1,000 feet over sea level, using full rich mix.
We are planning to do Mag checks with some leaning next week to see if it makes a difference, however, for over 2 years we have been doing full rich the Mag checks at this airport without >200 RPMs drop on the Slick Mag.

Thank you for your thoughts.

I was thinking that a leaned mixture may be causing a larger mag drop than a richer mixture.. sounds like you will find it..
 
Timing, plugs, wiring harness, magneto cap, then dig into the magneto. If you have an engine monitor, look to see it one of the cylinders is falling off more than the others. I've had new harnesses and new plugs arrive broken. So, don't ignore it just because it's new.

Check your timing, magnetos can move. If a cylinder is dropping off, move the plug to another cylinder to see if the problem follows. Lead test your harness. etc.

The ignition is just making spark. Where is makes the spark and whether the spark is hot enough to generate ignition in the fuel/air mixture are the variables.
 
Plugs, wires, and mixture likely fine. Spark is binary. It either successfully lights the AF mixture or it doesn't and when it doesn't, you will notice it on a 4 cyl engine. At 1800 RPM, RPM is directly related to timing advance. More advance = higher RPM. A mag check is designed to indicate advance differences between your ignitions. Clearly you do not have the same advance on the L and R ignitions at 1800 RPM. Many ways to investigate and address, but all that I have read indicates that the Pmag will be way more advanced at 1800 and low MAP compared to a mag. I don't think Pmag folks publish their advance table, so not sure how to figure out the advance for a given RPM/MAP combination.

Assuming your pmag timing is the same as where you set it years ago doesn't seem smart to me. Many posts here with people finding their Pmag timing magically changing. Do some searching.

I highly doubt the Pmag is at 20* advance at 1800 RPM and low MAP, so I would expect a larger RPM drop when running on the mag.

Larry
 
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