Mag Troubles Take 2

[rupertjl]

Bruce asked me to start a new thread to re attack this monster...I'll try to summarize what I've done so far:
First off, lets start with the symptoms: 200-300 RPM drop during run-up on the left magneto (right magneto grounded). I suspected I needed the mag overhauled because of oil we found in the mag last year at annual checking the timing. So the mag comes off and goes to Columbia Aircraft in Bloomsburg, PA. Scotty at Columbia tells me there was oil all through the mag and for $392 he'll overhaul it, so that's what I did.

So I get it back and the mechanic and I slap it on the engine. Checked to get the red tooth in the center of the timing window, removed the #1 top plug and found the compression stroke and set the prop flange to 28 deg BTDC referencing the engine case split. Scotty assures me the internal timing of the mag was set correctly at his facility. Hook up the timing box and check timing, adjust slightly and tighten it down, check timing again to ensure it didn't move while securing the mag. Run it up, same exact 200-300 RPM drop.

troubleshooting techinques: Memory is a little vague at this point but I believe this is the sequence of troubleshooting..

1) checked spark plugs and rotated top plugs to bottom plugs (rational-if it was the plugs, it would swap mags since left mag fires bottoms, right mag fires top)
2) New wiring harness. Installed, run-up, produce same 200-300 RPM drop (rational-old harness looked ratty and cracked on the shielding at the ends, thought it wouldn't hurt to get new ones anyway)
3)pulled P-lead from left mag and ran engine, grounded right mag and engine produce proper 50 RPM drop (rational-I thought I now eliminated mag, wiring harness, and spark plugs. Thought it had to be from the p-lead back.)
4)continuity check with the p-lead pulled and the switch in OFF/R/L/BOTH. All checked good.
5)swapped p-leads, run-up-same 200-300 rpm drop (rational-if it was the p-lead and/or noise filter it would swap sides. I only swapped the p-lead and noise filter, i unhooked from the back of the mag and the top of the noise filter and swapped those)
6)New Bendix ignition switch, run up, same 200-300 RPM drop (rational-thought I had an intermittent short when engine was running and the vibes were shaking the switch, grounding the left mag intermittently with the right mag already grounded.)
7) bench checked the spark plugs, all looked good.
8 ) tested the new wiring harness, checked good.
9) checked engine timing, both mags, both are set correctly 28 deg for the left, 26 deg for the right.
10) cursed, kicked the dirt, sat and drank a beverage, watched others flying around while I was on the ground for about 2 months now. (rational-nothing worth noting)

SO my plan of attach now is to recheck everything, go over everything again and see if we missed something. I have 12 new spark plugs i could put in, but that's my last resort, I haven't taken them out of the package yet so they can still be returned...

I might have forgotten something but it's hard to tell at this point...anybody see anything blatant that we forgot/missed?

v/r,
Jud

[GAHorn]

What kind/type of tachometer do you have? (electronic? or mechanical.) Does the rpm drop SOUND like it's dropping that much?

What make/model magnetos? (Slicks perhaps?) (edited to correct a brain belch) Internal timing is also important, but that would not explain why the mag runs fine with correct drop except when hooked up to the switch. This problem points strongly to the P-leads/switch or an electronic tach failure. P-leads that are tightly gripped by grommets/firewall sleeves/tywraps can injure the insulation and cause intermittent grounding with the shielding, and it can also (rarely) cause an "inductance" cross-over between leads due to close proximity.

(BTW, taking this to a new discussion topic was a bad idea in my opinion. It now requires the reading of BOTH threads to make full comparisons back/and/forth, despite the restated progression of the problem.)

[rupertjl]

it's a mechanical Tach George, but yes it sounds that bad, I do my run ups at 1800 RPM and it will drop to 1500 RPM and bounce around 1500-1600. Again, the right mag gives the 25-50 RPM drop and is solid enough that I can consistently tell it's the same drop on the right mag. This problem sounds like it's about ready to kill the engine, it's that rough.
These are Bendix mags, S6LN-21

v/r,
Jud

[GAHorn]

I want you to very CAREFULLY inspect the harness and be CERTAIN they are correctly routed in the correct FIRING ORDER on that mag.

Then before you ship that mag back to the overhauler.... you need to do the nasty thing....and swap mags left/right. (write down their serial numbers before you lose track tho'.)

...This problem points strongly to the P-leads/switch .... P-leads that are tightly gripped by grommets/firewall sleeves/tywraps can injure the insulation and cause intermittent grounding with the shielding, and it can also (rarely) cause an "inductance" cross-over between leads due to close proximity....

Swapping the mags themselves will establish the difficulty is with the p-lead/switch and/or harness with certainty, if the problem remains with the left side.

[N1277D]

When they overhauled the mags, did they check the coil ? Sometimes the coils go bad, it usually shows up after the mag gets hot during a mag check, but checks ok when it is cold. If the mag was full of oil it is possible that the coil has a weak path to ground.

It is easy to misalign the gears when reassembling the Bendix mag. Try rotating the mag the full range of motion to see if that helps the mag drop. If it does their could be an issue with the internal timing of the mag.

[rupertjl]

George,
I will do that and as a last resort swap mags before I send it out. The 100 series manual has the wiring harness schematics for a few aircraft, i followed that and it matched the new wiring harness labels, but again I will double check all my previous work...

[rupertjl]

Columbia Aircraft told me they checked the coil, I have no reason to believe otherwise, but the mag swap would tell me that as well I believe.

[N1277D]

The other item to check is the mixture. The mixture may be set too lean.

[Brad Brady]

Jud, I hate to reiterate....but as I said on page one or two of the last thread.....IT'S THE MAG...we had a newly rebuilt mag come from Tulsa, for the Prat 1340 I was flying this summer....hung it and it run like crap...got another mag from a neighboring company, hung it and finished the rest of the season out with the (off the shelf Mag).....I don't know how you might get a AN style mag to be 180 degrees off but that's what I believe they did....they said that it bench checked good when we sent it back... The boss said hang it on an engine and it won't run......The mag will show running correctly (in my belief) but can be 180 out with internal timing. With out a check board...as we had in AP school, the mag will fire at the proper time....but at the proper place? once again just my two cents worth.....

[Bruce Fenstermacher]

OK

First double check the spark plug wiring. Having two plugs wires swapped will cause this but it runs fine with the other mag firing the correct cylinders Not the plug wires go on.

Remove both P leads. Run engine and ground each mag with a fresh ground wire from the mag p lead terminal to the mag case. Good mag drops one each mag indicate a problem in the p leads or switch. A bad mag drop leaves the mag, wire harness or plugs.

Swap plugs as it is probably the easiest. Still a problem swap the harness side to side if you can or swap mags side to side if not. At this point you will have narrowed it down to a single source. Plugs mag or harness.

If after you removed the P leads and grounded the mags with another wire everything is right then you must trouble shoot the p-lead/switch system.

Connect both p-leads to the mag and disconnect them both from the switch. Run the engine and ground each wire to a known ground. If good mag drop on both sides it's got to be the switch.

This is a simple process of elimination. Shouldn't take more than two hours to get to the bottom of it.

BTW I hold Scotty and Columbia Aircraft in high regard though any shop can make a mistake.

As far as the new thread I was half kidding but I think Judd did a nice job starting this one and summarizing what he did. A person could just start reading this thread and skip the whole first one and be up to speed. We simply need to link the first thread at the end to this one so anyone following it will jump to this one. We could also wait til Jud fixes the problem and the thread is dead and then combine them back together. No big deal either way in my mind.

[Brad Brady]

Jud, something I just thought about......see if when you pull the prop through for number one...... are the leads for number one sparking at about the sane time? of course I'm not counting for the lag in the timing but they should spark at the same prop pull not several pulls later......

[Jr.CubBuilder]

A couple thoughts.

If you haven't already tried this I would suggest taking a squirt bottle with water to the field, run the motor for a couple of minutes on the one mag that runs crappy, then shut it down and go around and squirt the exhaust pipes to see if one jug in particular isn't firing while it is running with that two to three hundred RPM drop. This will immediately tell you if the problem is limited to certain jugs, or if it's the mag (if it's the mag all the jugs will probably fire intermittently so all the pipes will be warm).

Second get a note pad and write down your trouble-shooting steps methodically. At this point it sounds like you are starting to loose track of what you tried, when you tried it, and exactly what difference if any was noted.

You've tried several solutions without actually isolating the problem. Stop throwing money at it till you find the problem.

You know without a doubt at this point that it is ignition because you can replicate the problem by turning the ignition key (ie, it isn't intermittent) so forget stuck valves, lean mixture etc.

From your description I think it's dropping one or maybe two jugs.

[N1277D]

All the system checks done to date indicate that the ignition system is within specs:

Sparks plugs, Harness, Ignition Switch and P-Leads all checked ok
Magneto Overhauled, internal timing ok, engine timing set correctly, coil ok, new points, condensor, block, distribtor gear, oil seal, bearings

If the ignition system is within specs the next item to check is fuel. If you lean excessively you can get a much higher mag drop on the 28 degree mag. If the ignition system is fine check to be sure that the mixture is set correctly, look for induction manifold leaks, carb heat fully off, etc.

[Brad Brady]

Jud, I had to leave last night and couldn't elaborate on the prop pull.....Bendix mags have a CW and a CCW timing mark (easy to switch about three or four teath off) although that doesn't count for the same exact mag drop as you had to begin with....I'm still looking at a crack in the distributor housing....(hard to find).....Brad

[Brad Brady]

OK.....now I'm reaching way back into my memory......a long time ago I drove a 150 that every morning, mag tested just fine .....if you checked the mag when the engine was hot ...one mag had a huge drop......I just never did a hot engine mag check and everything was fine we would routinely pull the spark plugs and test them....(cold) everything was fine...still when the engine was warm .....a bad mag check....we finally found one spark plug that had a crack, and when cold was closed up but when the engine was warm would open up to ground that plug.....something else to think about....