Pmag rpm drop

[SMO]

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?

[Sid Lambert]

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.

[SMO]

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

[777drvr]

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

[SMO]

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.

[videobobk]

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

[cguarino]

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.