P-Lead Issue? - Need assistance

[JVolkober]

I am at the end of my first annual condition inspections. I have already clean, adjusted and rotated the plugs and check the timing, adjusting the left mag slightly. During my engine test a ran through a mag check. I do not get a mag drop when the switch is set to "L". Also, if I turn the ignition of, the engine continues to fire weakly and erratically. I have disconnected the P-leads and ground wires at the mags and tested for continuity from the P-lead through to the ground connector with the following results:

Switch Position

Off: right mag .03 ohms, left mag .04 ohms
Both: right mag open circuit, left mag open circuit
Right: right mag open circuit, left mag .05ohms
Left: right mag .03 ohms, left mag open circuit

I read this as the switch and circuitry operating correctly; however, I still have the problem. I would appreciate any suggestion as to what the issue is and what I should be looking for.

John

[Vac]

John,

Foremost, make sure the p leads are hooked up correctly. If you have an impulse coupling on both mags, and swap the ground wire with the p lead, you'll get the no mag drop symptom you describe (especially if the only path to ground is to the grounding stud on the mag, vs a jumper from the switch directly to the airframe). If everything was working properly BEFORE the inspection, check whatever may have changed during disassembly first--it's probably the likely culprit.

You can make a set of jumper cables that you can run from your mag p lead studs directly into the cockpit to isolate your switch and all of your wiring (old mechanic's trick). This will help you confirm no mag issues and allow you to concentrate on switch and wiring...use caution around the prop and make sure the airplane is choked or tied down when working/operating with hot mags. If you only have an impulse coupling on the left mag, make sure you start on the left first and then unground the right jumper after start. Perform a normal mag check using the jumpers and shut the engine down by grounding both jumpers simultaneously. If everything is good using the jumpers, then your problem is wiring or switch.

The next thing to check is the connections at the switch switch. Make sure everything is hooked up properly and you have or don't have a jumper (as required) installed. The switch jumper is small and easy to drop and lose if you loosen the screws. Re-check all the connections as well and shoot the wires for continuity--a loose ground can wreak havock!

If all that fails, it may be the switch itself, they do fail even with low time...but before you swap that out, check all your wiring runs first. Assuming all of that is good, then the next link in the chain is your mag itself (if you didn't use the jumper trick first to confirm normal mag operation).

After you get things working a couple of tricks to put in your bag is an occasional in-flight mag check (engine operating at cruise power, properly leaned) and shutting the engine down with the mag switch occasionally as well--just to confirm proper switch and wiring operation...radial engine folks do that all of the time to make sure the mags aren't hot when they pull the prop through to move the oil out of the lower cylinders prior to starting a cold engine.

Best of luck,

Vac

[JVolkober]

The issue turned out to be that I incorrectly set the timing on the impulse coupled mag. I misinterpreted how the timing should be set, setting 25 degrees in advance of dead center at the point the magneto click. After having discussed with a friend and A&P, I now understand that for the impulse coupled mag, I have to let the induction timing device indicate that I have reached the appropriate point and then back up the prop about 30 degrees and then go for the appropriate set point. Also, I needed to confirm that cylinder one was at top dead center, something I did not do originally. Once I did this the roughness cleared and I now get the appropriate response during the mag check.

John

[BobTurner]

John,

Not the timing box, you need to go past top dead center and hear the impulse couplings snap, then go back to the correct timing point. This will insure that the delay ratchet and pawl is not engaged.

Correct timing is important. Too much advance is harmful to the engine, too little will rob you of power. Have your A&P friend show you his tricks.

[JVolkober]

I did go to the point where the impulse coupler snapped before backing off and finally setting the mag as suggested.

John

[BillSchlatterer]

My airplane also fires weakly when I kill it with the switch. I thought you needed to find TDC on the #1 cyl and then back 25 and set with timing indicator. If I get TDC ... what has the position of the implulse coupler got to do with the mag firing when the switch has it grounded.

Not an engine guy what's happening to make it fire weakly ??

Thanks Bill S

[az_gila]

This article is a great help in setting magneto timing -

http://www.avweb.com/news/maint/184370-1.html