Pmag rpm drop

I installed a new Pmag on my injected 320 and notice when I do the mag check at 1700 - 1800 rpm I get nearly 200 rpm drop when running on the Pmag only. I have a Slick on the other side and I get less than 100 rpm drop when running only on that.

Here are the steps I have taken so far:

- Confirmed the wiring
- Reset the timing
- Checked all the plug wires for appropriate resistance.
- Changed the spark plugs (new for new)
- Changed the Pmag unit (I bought two, only installed one)
- Removed the jumper to use the alternate timing curve

The only change I noticed in doing the above was the idle became smoother with the jumper removed, but I still get the ~200 rpm drop.

The Pmag is running the bottom plugs, the Slick is running the top plugs.

What is normal, and should I be concerned?

Check the timing on the Slick. If the timing on it has crept up you will notice a large RPM drop when you turn off the Slick mag.

Example... Both mags 'on' the timing may be 30 degrees BTDC. Turn off the P-Mag and the timing is still 30 BTDC, not much RPM drop. Turn off the the Slick and now the timing is 15 BTDC, you will notice a large RPM drop.

My Rose ignition shows almost no RPM drop when I turn if off because it runs about 15 BTDC at 1800 RPM and the Slick is set to 26 BTDC.

AH HA

Thanks Sid, I never thought of that. I will check it out next week when I get back to the airport.

Hi Mark,

I know it's an old thread but I was wondering if you ever got to the bottom of this issue ? I'm curious because I have a 4 with an IO-320 that I'm thinking of adding a P-mag to .

Marc

Changed the engine

At the same time this was happening I was also having problems with #1 plug fouling regularly (like every flight). I also started going through a quart of oil per hour. Compressions were over 70 on all cylinders but I pulled the jugs thinking I should do a top overhaul. Inspection revealed some corrosion on the cam so I replace the entire engine with a new one from Aerosport Power.

Now I get less than 100 rpm drop on the Pmag, and nearly 200 drop on the Slick (its almost like I reversed the switches!). I am planning to replace the Slick with another Pmag (I bought two to start with) when I have this down for the annual in June.

BTW, new engine is working great.

I'll bet you will be happy with the combo. I had a higher drop on the Slick side before I switched to P-Mag/E-mag combo. Let us know how it turns out.

Bob Kelly

Mine are criss crossed

Just for another data point, my ignitions fire the top on oneside and the bottom on the other. Don't know if it makes a difference.