High Mag drop on both mags

[Dave62]

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)

[Toobuilder]

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.

[jjconstant]

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

[apatti]

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.

[AlexPeterson]

Did you check the actual timing with a timing light yet?

[lostpilot28]

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.

[apatti]

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!