High Mag drop on both mags

[apatti]

All,
I am getting a high mag drop and rough running engine whenever I switch to one mag; it doesn't matter which one. I have a new Lyc O-360A3A built up by Penn Yann Aero with dual P-mags running auto plugs. First start was about a week ago. I just got around to doing a mag check a couple of days ago and have been having the problem every time I run a check. The engine was test run at Penn Yann and the test data record shows normal drops for mag tests at various RPM settings. Since it is new engine I am only able to run it for about 2 minutes before the CHT's get up to 300 deg. I always shut it down at this point.

Here is what I am doing:

Start engine and let idle at 1000 for 10 seconds or so
Increase RPM to 1500 (sometimes 1600)
Ground P-Lead on left mag
Note RPM drop of 300 plus and engine runs somewhat rough
Unground P-Lead on left mag and let engine settle at higher RPM
Ground P-Lead on right mag
Note RPM drop of 350 plus and engine runs somewhat rough

Here is what I have done so far:

Verified timing of P-mags (the first time I did this I found that the left mag was not timed properly (i.e. solid red light at TC) so I re-timed both mags and now get two greens)

Checked resistance of each ingnition wire. All seem to be near spec of 180 ohms per foot.

Removed and checked gap of each spark plug

Re-installed all plug wires on the plugs and P-mags and verified they were seated properly.

So, does anyone have any suggestions as to what to check next? I talked with Brad at Emag Air and he suggested several of the above checks. However, he also thinks that since both mags are behaving the same that it is unlikely an ignition problem. That makes sense to me. I am going to replace all 4 lower spark plugs (BR8ES) with brand new ones and see the problem changes. However, I thought I would go ahead and post this question to see what other ideas might pop up while I am doing that.

Thanks in advance.

[AlexPeterson]

With what you've described, I'd bet on a timing issue. Maybe not base timing, as you've checked that, but something to do with how P-mags choose timing. You can get a old fashioned timing light, and shine it on the back side of the starter ring assy. There should be some marks that read when at the top of the rotation (not indexed to the starter). I don't know if these marks are standard on Lyc parts or not.

In my 500+ hours of running dual EI with auto plugs, rpm drops during "mag" checks are almost not perceptible. However, the system I'm running bumps the timing 5 more degrees advance when the other system is shut off (Lightspeed).

[Bevan]

One thing that is common is ship's power to the P-mags at low engine RPMs. Do the P-mags get their power from the aircraft battery or a aux battery/buss? If so, I would check voltage "in" at the P-mags. I'm thinking that since this is first start, maybe the battery has been sitting around for a while and not well charged.

Also verify that one mag is connected to all the top plugs and the other to all the bottom plugs. Not left and right as one may think.

Let us know what the final solution is.

Bevan

[PCHunt]

Could it be as simple as two bad plugs; one on the left P-mag, and a different one on the right P-mag?

"Check the cheap stuff first."

[N941WR]

Check your PM's

[apatti]

Thanks for the responses so far. I agree with the "check the cheap things first." So, I replaced all of the bottom plugs (which is the left mag) and no joy. If this had fixed the problem I would expect to see smooth operation and little drop when running on the left mag. Also, I keep double-checking the timing after almost every run. No more problems with the left mag being off. Also, I double-checked that the #1 piston was at the top of it's stroke when the timing mark on the starter indicated TC. The Emag manual says it does not matter which stroke it's on.

I don't believe power is an issue. First of all, I have kept the battery charged. It runs both EFIS screens OK and they draw as much or more current as the Pmags. And, the EIS says the alternator is putting out 14 volts. Also, the Pmags are self-powering above 900 RPM or so. And, all of my tests have been at higher RPM's.

Density altitude is running around 2500' here or so here today. So, I did try to lean to peak RPM but didn't see RPM rise as I retarded the mixture.

Brad had suggested looking for a momentary dip in EGT for one of the cylinders when I was running on one mag. He said EGT respond very quickly and I might be able to tell if a cylinder was missing based on that. However, with the digital display and all the EGT's bouncing around I couldn't discern if a particular cylinder's EGT was dropping or not. #2 EGT is a little lower than the others and its CHT is as well. But, the EGT may be a function of probe placement and the CHT is probably lower since #2 is up front. So, I don't think this is telling me anything.

Here are the numbers I got on the last run of the day:

Both 1320 rpm
R mag 1140
Both 1340
L mag 1180
Both 1350

These numbers are not quite as bad as I was seeing before. Perhaps I was using a higher RPM during the check before. These numbers are almost or perhaps even in spec. However, from what I have heard about electronic ignitions, I was expecting to get lower drops and smoother running on just one mag. So, I am still considering that this is an indication of a problem somewhere.

I am not sure what to try next at this point. I may need to find an A&P that is willing to work on an engine with Pmags. At any rate, any more suggestions would be appreciated. Also, when I finally get this resolved, I'll report back here for the benefit of the knowledge base.

Thanks,

[Walt]

Try leaning it out, if the mixture is too rich it will cause an excessive RPM drop on both "mags".

[lostpilot28]

Tony, it seems to me that this is a spark issue. There are far more intelligent guys on here than I am, but at first blush, if you think about how "most" P-mags operate, you should only drop 30 RPM when only running on one. Either one, for that matter.

You've probably already done this, but check your spark plug gap. It should only be .032. Use a spark plug gapper tool. Also, who made your spark plug wires? If you did, make sure they're right. You can ohm them out to check if the resistance is too high. It's not likely, but if they're running next to some other wires, you may also get the spark signal grounded, which gives a weak spark.

Finally, with both P-Mags, are you getting full power? Do you know what your static RPM is with your prop? If you're not getting that, then both P-mags could be affected.

The only other thing I would think it could be is bad fuel, too lean or too rich. Just a quick thought...if you're carbureted, how far can you pull the mixture before the engine dies? I use an MA4-5 carb and have to pull it about 3/4 back to kill the engine while running at 1100 RPMs.

[PCHunt]

Originally Posted by apatti

(SNIP)
Here are the numbers I got on the last run of the day:

Both 1320 rpm
R mag 1140
Both 1340
L mag 1180
Both 1350

(SNIP)

I would think you need to try the mags at considerably higher RPM. You aren't developing much power at 1300 RPM. What happens at 1700 or even 2000?

[G3i Ignition]

Originally Posted by Walt

Try leaning it out, if the mixture is too rich it will cause an excessive RPM drop on both "mags".

I'm with Walt, your problem sounds like your mixture is too rich. What did your plugs look like when you inspected them, black powdery, black glossy, brown, etc.?