Interesting P-mag issue...

[RVG8tor]

The P-Mag manual walks you thru a pull thru test. Essentially remove all the plugs and connect them back up to the wires and ground them to the engine, bottom ones are tough, top ones you can lay on metal on top of the engine.

Pull the prop thru and see where how they fire and see if they fire in the same spot, both plugs in a cylinder should fire at the same time.

I had the same issue you did a couple months ago, mine turned out to be the circuit board it has been fine since replacing the board. My mags where a couple years old from when I bought them but only had 65 hours on them since I only started flying back in December.

When I experienced the bang during the mag check, I did the pull thru test and determined the bad mag was firing well late of the other mag. Sent the mags in for a look see and they could find nothing wrong with them. Brad gave me the option of getting a new circuit board that I could install if the mag did the same thing again. I installed the mag timed and went for a flight and all was good till the next flight then loud bang during mag check when trying to run on the bad mag. I put in the new circuit board and smooth running so far, 40 hour on the new circuit board. My thought was some connection on the board was loose and would allow for a good timing and then one flight then during flight something causes the timing to dump you don't notice it because the good mag is running well.

How often do you do the internal power test, I have wondered if that might make something go bad. Mine ran fine for 60+ hours and of course it goes bad with I am on the road. Yours is odd in that even with the new circuit board you are having issues. No heat issues with my set up, non of my sticker turned color.

Good Luck
Mike

[Yen]

You say the EGTs are going up together on one mag. That says to me that the operating mag is retatarded. Late firing and the fire is still burning well when the ex. valve opens.
Try timing them again from the start.

[RV10inOz]

NO........

EGT rise on one mag is EXACTLY NORMAL.

In any mag check the EGTs should all rise together, not necessarily identically but roughly so.

The reason for his is a slower burn completion, later Theta PP, lower ICP and less expansion of the gases.

PV=NRT.

[Brantel]

Not sure about the rough running on one Pmag but it is fairly common to see reports of large drops between two Pmags. Lots of reports of this on here. Not sure if anyone has ever figured out why.

When you say it runs rough, please elaborate.

[RV7ator]

I wouldn't completely ignore mixture. A really lean mixture is hard to ignite, rough running, leading to popping in the exhaust by the wasted spark. Both ignitions firing can overcome this. How does the airplane perform, and what are the climb EGTs at full rich? Do you have around 300 degrees between full rich and peak lean?

John Siebold

[John Collier]

Agree w/Oz...the EGTs should go up.

What curve are you on?

Do you have a orifice / filter in your manifold pressure line that T's into the PMags? I didn't initially...and it caused issues as the cylinder pressure pulses varied the PMags timing at higher power settings. You might try disconnecting the MP line and doing the run-up...this would eliminate the MP / timing curve variable.

Are you leaning out prior to accomplishing the run-up?

I'd start at the plugs (sounds like you've done this) and work backwards toward the PMags. After you test...what are the lights doing when you pull the prop toward TDC? Are they turning green almost in unison?

[cguarino]

Check out Lycoming Service Instruction No. 1437 just to be sure everything is good there. I use the TDC mark on the front of the ring gear and actually retard the timing about one tooth to prevent kickback during starting. Also, make sure your plug wires have some separation between them. I've been running PMags for about 400 hours and I only get about a 50-70 rpm drop between them. Let us know what you figure out.

[steve91t]

Thanks again guys.

Please make sure you read my responses as I've covered most of your questions.


Once again, absolutely NOTHING changed. We pulled the mags off and sent them in to be rebuilt and then reinstalled them. So if the plug wire routing and MP line were ok before, it should be fine now. We used to get only a 20 RPM drop on one mag. There is nothing wrong with wires or the MP line.

Also, we retimed it 3 times. Not only were we using the mark on the flywheel to line up with the case seam, we pulled a plug on #1, pulled it through until we felt compression, and then gently used a screwdriver to feel for the top of the travel. This was to make sure that the ring gear was installed properly.

We aren't dropping a cylinder or two during a mag check. As I've said, all 4 cylinders are firing and all 4 EGTs increase. Now, some of you say that's normal. I've never ran in on one mag for long enough to see that increase.


When I say ran rough, the exhaust note sounded off. The engine sounded labored. It actually got louder and honestly sounded as if the timing was off.


Now, what's his face over at P Mag told us to try setting the timing a couple of degrees retarded to prevent kick back. We did and it ran worse. When we set it at TDC, it ran better, but still getting that off sound and 175 rpm drop on either mag.

Is it possible that the rebuild didn't go well and they screwed something up? One mag, the right, has a brand new circuit board and that one has the largest drop of the two.

Honestly, I feel that we almost need to set the timing at a few degrees advanced to get these mags to fire properly. Alsmost like setting setting TDC causes them to retard while engine is running.


To answer another question, the MAP line comes from a box mounted on the firewall. I didn't take the time to see exactly what it is. Also, both green lights illuminated at the exact same time.

Doing the mag check rich, lean, 1400 RPM, or 1000 RPM produce the same result. It's a timing issue.

[Andy Hill]

Steve...

I've had the P-Mags decide to "re-time" themsevles once.

So firstly retime them (as you have done) - then once you get the "banging" syndrome, confirm they are still timed:
Power to P-Mags On , Ignition (P-Leads) off, rotate Prop until green light - ensure both green lights same time and at #1 TDC marks. This ensures the P-Mags are staying timed.

To confirm mixture / injectors, run engine up to ~1200, full rich, both P-Mags on. Write down 4 x EGT. Pull mixture back a bit, hopefully get RPM rise and write down 4 x EGT again. Repeat until runs roughly. You want to be able to pull mixture lever back quite a long way, and see ~even EGT rise across all 4.

There is no need to use the screwdriver in #1 - it does not matter (unlike a Mag) which rotation it is on.

I would also disconnect the MP lines to both P-Mags and see how you get on for the Mag checks. There will be no advance then, just like Mags. Adds another point of interest for the experts on here re engines.

[selhardt]

Steve;

There is a troubleshooting guide on the EMag web site. I'd suggest (so would Brad) that you go through every step rather than declare it a timing issue. It indeed may be a timing issue, but there is more information that you can gather.

There is data in ohming the wires and doing the pull through test. The data may be that they are not the issue, but for now you don't have that data.