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High Mag drop on both mags

A

apatti

Well Known Member
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.
 
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).
 
Somethings to rule out.

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
 
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." :rolleyes:
 
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,
 
Try leaning it out, if the mixture is too rich it will cause an excessive RPM drop on both "mags".
 
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.
 
apatti said:
(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)
Click to expand...

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?
 
Walt said:
Try leaning it out, if the mixture is too rich it will cause an excessive RPM drop on both "mags".
Click to expand...
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.?
 
I would also check for a lean mixture. You stated you did not get the RPM rise when you slowly pulled the mixture at shut down....this may be a clue.
A lean mixture will show a large RPM drop on mag check....we used to lean at altatude by turning one mag off and leaning until the engine started to get rough and then going back to both. Old school. An easy check for the mixture being too lean causing the mag drop problem would be to pull on the carb heat. This will richen the mixture. I would check for mag drop with carb heat in the on position. It this helps then investigate further with a possibe intake leak or carb issue.
Hope this helps.:)
Dave62 (Swift Driver)
 
I agree with getting the idle mixture correct first. Based on your earlier post, (no RPM rise before shutdown), that's off. Second, make sure you lean your mixture at runup. At 1800 RPM, I slowly lean until I get a peak RPM rise (on the RV, it's 20 RPM), then just richen it up a hair. With brand new dual P-mags and auto plugs, I get less than 10 RPM drop per side. This is typical on the Hiperbipe as well, so I think it's what we should expect from P-mags.
 
other possibilities

Possibility #1: Have you verified that the spark plug wires are standing off from anything metal, all the way from the pmags to the plugs? They should not be bundled together anywhere, nor secured to anything metal. Look for tiny black dots on the outide of the wire to indicate localized arcing.

Possibility #2: You didn't say if you were using aviation or auto plugs. I am using PMags and aviation plugs. On initial installation I noticed that it was VERY DIFFICULT to screw on the nut from the wire to the plug without really twisting the wire. There is no secondary nut to hold it from twisting like there is on the standard aviation harnesses. It ran fine for a while and then I had one big mag drop and rough running engine on that side and the EGTs told the story of a cold cylinder on that mag. Did everything normal to trouble shoot with no joy. I was in the process of swapping leads top to bottom to see if the problem stayed with the plug or moved with the wire. I wanted to re-lube the wire/nut and when I went to pull off the nut, about an inch of the end of the wire came off with it! It turns out that the insulation was sheared from twisting and was arcing (you could see the tell tale black soot) inside the wire. I cut it shorter, reinstalled with silicone grease (DC4) NOT silicone spray lub and everything is fine now.

I don't believe this would have showed up with an ohm-check. It needed the high voltage to arc to where it didn't belong. If you are using aviation plugs, use DC4 to lubricate. It's still difficult to avoid twisting the wire, but not as bad.

Jeremy Constant
RV7A 110hrs
 
Hi All,
Sorry to take so long getting back to you.

I tried the mag check with the mixture control about 1/3 of the way back from full rich. I still get a mag drop of over 200. I read about setting the idle mixture and it requires that the engine be warmed up. Well, the problem is that I have to keep the CHT's down and they reach their limit long before the oil temp gets above 110. Can idle mixture be set without waiting on the engine to warm up?

There are some other things that are muddying up the waters. One is that my RPM won't stay set very well. When I run it up to 1600 and leave it, it will vary 50 RPM or so on the digital readout. I have double checked that the throttle friction lock is set and it is holding the throttle in the same location. Then, when I cut off one of the mags, the RPM will drop about 250 and then over about 5 seconds or so, it will rise to a drop of only 175 or so. Then, when I go back to both mags, the RPM will be above 1600 for a while then retreat back to something close to 1600. I don't know if the one Pmag is sensing the RPM drop and making some adjustments to try and recover or not. BTW, the plugs I have pulled have a black powdery look to them.

I have checked the gap of all of the plugs (BR8ES auto plugs everywhere) and they are between .032 and .034. I feel that I took care with the routing of the plug wires and used spacers to keep them separate in many places. Also, I used adel clamps and tie wrap standoffs to keep the plug wires from touching grounded areas. However, I do believe there is a place or two where one plug wire my cross over (and perhaps touch) another plug wire. So, I need to double check this.

Bottom line is that I am having trouble getting consistent data with the RPM moving around so much. So, it is difficult for me to provide all of you with good enough information so you can help. I think my next step (after checking the plug wire routings) is going to be to find a local A&P (with emphasis on the "P") and see if he can offer some assistance. I'll let you know what happens.
 
Tony, are you using a fixed pitch prop? If not, it's possible you have a CS governor that's causing your surging issue. Sacramento Skyranch lists the following as causes for surging:
1. Injector nozzles dirty.
2. Faulty governor.
3. Air in turbocharger controller oil lines or wastegate actuator.
4. Engine breather tube plugged.
5. Injector nozzle pressure reference system leaking.
6. Incorrect propeller governor.
7. Defective oil pump. Erratic oil pressure may be traced to the pump sucking air.
8. Propeller blades sticking in hub intermittently.
9. Malfunctioning or sticky wastegate. On engines using mechanical wastegate's check the wastegate control cable.
10. Engine bootstrapping.
11. Cessna 172 with Avcon Conversion to Lycoming O-360. Check carburetor part number against STC. This conversion requires a richer carburetor than what is standard for the O-360 to compensate for poor mixture distribution caused by the air box design. Standard carburetor p/n 10-3878, richer carburetor p/n 10-4164.
12. Continental 520 series engines with early style Woodward propeller governor. Not enough leakage past oil transfer collar. Propeller governor bypass valve of insufficient capacity to handle volume of oil. Not all the oil pressure is bled off. Try installing a McCauley governor or updated part number Woodward.
13. Intake or exhaust system leak.

I would think that the surging is a separate issue than your RPM drop, but maybe they're tied together...or they're both symptoms of a different problem. I'm guessing that if you fix the surging, you'll probably find that your RPM drop issue is fixed.
 
All OK now

Well, I did finally resolve this issue. The main problem seems to have been that the EIS (GRT) was expecting 1.5 pulses per revolution (this must be the default setting since I don't recall changing it). However, the PMag's output 2 pulses per revolution. Thus, I was not getting a correct RPM reading. Once I changed the EIS to expect 2 pulses per rev the top mag only dropped about 50 and the bottom mag dropped a couple of hundred. I noticed that the CHT on the #2 cylinder was low compared with the others and the EGT was high compared to the others. So, I removed the bottom spark plug from the number 2 cylinder and sure enough it was fouled. Actually had a little bridge of carbon (I guess that was it was) across the contacts. After I replaced the #2 plug I got mag drops of around 50 - 70 on each mag. So, life is good again!

For those that asked:

I am using auto plugs (BR8ES).
I am using a Catto 3 blade fixed pitch prop.

Thanks again to all who offered assistance!
 
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