[sprucemoose]

I?m in the middle of trouble shooting an ignition problem. I think I?ve got it figured out, but I?d like to get some more opinions. Here?s the relevant facts:

Engine: Superior/ Aerosport O-360, LSE Plasma II on right side, Slick 4371 impulse coupled mag on the left side. About 350 hours since new on everything.

A couple weeks ago, I noticed two problems start at the same time. #1 is that the tach indication on the Grand Rapids EIS4000 was erratic, going up and down by several hundred RPM. This happened intermitantly, at speeds from idle to full power, with a cold or warm engine. At the same time, the engine began having fouled plugs (or what I believed was fouled plugs) which it has never done. A minute at high power leaned out would generally clear up the ?fouled plug.?

The ?fouled plug? problem is now believed to be something other than a fouled plug. The fact that it started happening at the exact same time as the weird tach behavior, leads me to believe that it?s a magneto problem. The EIS4000 takes it?s tach signal from the P-lead of the left mag, run through a 27K ohm resister. I have checked the wiring from mag to switch to EIS. I replaced the resister (it checked out OK, but I replaced it anyway.) I swapped out the EIS for a known good unit, and it did the same thing.

Having tried all the easy stuff, I pulled the mag. Under the supervision of the local A&P greybeard, we pulled it apart. Everything looks good in the mag. The point gap was just slightly over spec so we fixed that. The coils ohm?d out good (I forget the readings, but we looked it up and it was good.) Everything else looked good, so we cleaned it and buttoned it up.

Today was my first chance to fly it since. At startup, the tach was going up and down several hundred RPM, but the engine ran fine on the mag at runup power, so I took it up. In flight, at cruise power, the engine ran very roughly on the left mag alone, as though it was stumbling and missing. It obviously ran just fine on both. I switched the EIS reading to show all 4 channels of EGT, and switched to the left mag. The engine ran very roughly, but all 4 EGTs made a slow, steady climb of 30-40 deg, which is normal behavior. If it were one cylinder not firing and causing the rough running, I would expect the EGT to drop.

I played around with power settings, and found that the engine only ran rough on the left mag at power settings above 18? or so. Below that power, it ran just fine. Obviously it ran just fine on both ignitions, so I am ruling out any fuel or induction problems.

I have an idea in mind as to what the problem is, but I?d like to hear some opinions before I take the next step.

 

[Mike S]

Check your insulators, the ones at the end of the wire, fit down into the plug.

White ceramic=good.

Brown bakelite=not good.

Of course, should be clean, not cracked ETC.

Mike

 

[sprucemoose]

The insulators are ceramic, and are in good condition. If it were a cracked insulator, it would effect only that cylinder, and I would see an EGT drop on that cylinder when the engine was running rough, or such is my logic.

Thanks for the suggestion.

 

[Chickenlips]

I can't solve the problem other than to say that my experience has shown that when one plug misfires or the ignition lead to a plug fails, the egt RISES for that cylinder. If one magneto is failing completely, the egt's will all rise... not necessarily uniformly but up, not down.

What I think happens is that the mixture in the cylinder does not ignite well when only one plug fires and that leaves a still burning mixture going past the exhauxt valve when it opens... this flame surrounds the egt probe if it is in the normal position about 1" below the exhaust flange.

You see some of this when you change the pistons in an engine to higher compression. Low compression engines always will have higher egt's than high compression engines because the mixture burns more 'completely' at higher compression.

 

[videobobk]

I have an idea in mind as to what the problem is, but I?d like to hear some opinions before I take the next step.

Ah, come on, Jeff, tell us your idea so we can all have a good laugh and shoot you down!

No, seriously, I would suspect the capacitor, or perhaps a coil breaking down at higher load levels. I assume the cam looked good. I would try swapping the plugs to make sure there isn't something wierd with them (I would doubt there is.) You might see if someone has a spare mag laying around that you could try, but I would seriously consider a mag rebuild. Intermitent problems are a pain--just spent good money rebuilding a carb to find I had a fuel pump giving higher pressure than normal, but only on occasion. Four A&P's told me it can't happen! Good luck and let us know what the problem turns out to be.

Bob Kelly

 

[sprucemoose]

That makes sense based on what I saw with the EGTs. When running on the LSE and the mag, the fuel burn in the cylinder is more complete. When I turn the LSE off and run on the mag, less combustion occurs in the cylinder and more of it in the pipe, hence the higher EGTs. I have onserved this before when the engine is running well, so i don't think it's a symptom of the ignition problem.

 

[lucky333]

Just a thought..

The LS plasma II has millivolt outputs for advance, RPM and MP that drive their dedicated LCD millivolt meters for monitoring. Shouldn't be any reason why you couldn't bring these signals out to your DVM, set it to millivolt mode and see what the LS PII is thinking about things.

John

 

[sprucemoose]

No, seriously, I would suspect the capacitor, or perhaps a coil breaking down at higher load levels.

That is my theory, especially the coil. My first thought was the capacitor, especially due to the erroneous tach indication, but if the cap goes bad, I suspect it would fry the points pretty quickly, and the points were pristine. Also, in my experience, when capacitors go, they tend to do so in spectacular fashion and leave visible evidence of having done so, and this one was likewise pristine.

Which leaves the coil. It ohm'd out good on the bench, but at higher voltages (higher power settings) it's getting flaky. This makes sense from the coil's perspective, and also explains why the engine only ran rough at higher power settings.

Thoughts?

 

[sprucemoose]

The LS plasma II has millivolt outputs

I looked into this once upon a time, and IIRC, only the Plasma II Plus model had these outputs, so I gave up on it. Are you saying that the II does output this data? If so it would be sweet to wire them into a couple open channels on the EIS4000 and display the advance.

 

[gmcjetpilot]

Use the Plasma II tach output or use a hall effect tach sender

I read all above, not sure this addresses the issue or repeats, but I would suggest you use the Plasma II tach output, not the mag.

My second suggestion, I elected to go around both Left and Right ignition and use a hall effect tach generator from UMA (its small). I also have a Plasma II and EIS4000. The on van sells is bulky. This gets around the issue of the tach dropping off line with a mag check. Hope that helps.

 

[Sam Buchanan]

Today was my first chance to fly it since. At startup, the tach was going up and down several hundred RPM, but the engine ran fine on the mag at runup power, so I took it up. In flight, at cruise power, the engine ran very roughly on the left mag alone, as though it was stumbling and missing. It obviously ran just fine on both.

Jeff, I have seen exactly the same issues twice within the past three years with my RV-6 (O-320, two Slick mags, carb). Initially, plug(s) would "foul" during runup afterwhich I could clear them on the ground with the mixture control, but there would later be a slight misfire on one mag at cruise power. The tach reading on the RMI uMonitor was also erratic on the bad mag.

The first time this happened, I swapped plugs and harnesses and the problem was only solved after replacing the left mag. The most recent occurrence, which I assumed was the right mag which I replaced, turned out to be a year-old plug that was firing through the insulator. That plug cost me $500!

Before you start throwing much money at the problem, run the plugs on a tester to make sure they aren't the problem......wish I had.....

 

[lucky333]

Sorry, don't know about the II (not plus)

I looked into this once upon a time, and IIRC, only the Plasma II Plus model had these outputs, so I gave up on it. Are you saying that the II does output this data? If so it would be sweet to wire them into a couple open channels on the EIS4000 and display the advance.

Have to check your manual on that.. Good idea on connecting to the EIS if it does have the outputs.

 

[DanH]

Jeff,
The erratic tach, if actually related to the ignition problem, might be a clue worth more consideration. Ya'll check me here. A P-lead waveform should be an overall voltage level rising with RPM due to increased magnet sweep speed. At point opening it would spike to a much higher level due to magnetic field collapse, then rapidly drop to zero. The tach probably counts the incidence of zero voltage (or zero voltage crossings) over time; a quick call to GRT would confirm or deny. If confirmed, then for the tach to be erratic something must add or subtract from the number of times the tach sees zero voltage.

A grounded P-lead also shows zero voltage. I know you said you checked your P-lead wiring, but perhaps another quick check is in order before you remove the mag again. This test is to check for an intermittent P-lead to ground short due to vibration. Can you reach the P-lead terminal nut on your mag through your oil door? If so, disconnect it at the mag and go do a test flight.

Obvious caution: with no P-lead ground you must ensure that nobody turns the prop on the ramp.

Dan

 

[sprucemoose]

Thanks to all for the replies and ideas. Let me address a few of them here:

George- as for the tach signal, I think having a tach signal from the mag is more important than from the LSE. On runup, I can see the mag drop 70 RPM or so and know that all is well. Of course the tach drops to 0 on the LSE, but the RPM drop is almost non-existent (per the Mk1 ear.) In this case having the tach on the mag is useful because it is helping diagnose the problem.

Sam- on your second mag replacement, was the rough engine (caused by the $500 plug) accompanied by the same erratic tach as the first one? I am assuming that the RMI unit uses the P-lead for a tach sensor just like the EIS. Also, I tend not to suspect one plug being bad, because that would show itself as a EGT drop on the affected cylinder, which mine did not.

Dan- your idea of disconnecting the P-lead is a good one. I tried this during the head-scratching phase of this problem. The engine ran fine (at runup power) with the P-lead disconnected, and I thought, AHA! But, then I hooked it back up and tried it and it ran fine with the P-lead attached. @#$%% intermitant problems! When I flew yesterday, it ran fine on the ground at runup power (and in the air at the same power settings) but stumbled at higher power, so I know think that the fact that it ran up OK with the P-lead disconnected is not all that conclusive.

A new coil, condenser and points have been ordered (little over $300) so we?ll know next week. I was planning to pull the mag and overhaul it at 500 hours, guess I?ll just do it a little early.

 

[Sam Buchanan]

Sam- on your second mag replacement, was the rough engine (caused by the $500 plug) accompanied by the same erratic tach as the first one? I am assuming that the RMI unit uses the P-lead for a tach sensor just like the EIS. Also, I tend not to suspect one plug being bad, because that would show itself as a EGT drop on the affected cylinder, which mine did not.

Jeff, the uMonitor pulls the tach signal off both mags and uses the leading mag (they are never timed exactly the same) to display rpm. Yes, I saw similar tach issues the second time but not as extreme since the misfire was less significant. Erratic uMonitor tach readings have been accurate indications of ignition problems twice with my O-320.

I eventually isolated the bad plug over a course of a couple of test flights. First flight the plugs were swapped top-to-bottom on the left side of the engine, but the problem remained unchanged, on the same mag. Next flight the plugs were swapped top-to-bottom on the right side and the problem switched over to the other mag. I then knew which pair of plugs were involved. It was then that I had both of those plugs put on a tester and one of them failed under load. One more flight and I could have isolated the individual plug without the tester by replacing one of the pair with a known good plug.

This was perplexing because the plane ran good on both mags during run-up, but at cruise would misfire when running on only the right mag. And it wasn't the ragged, cross-firing normally felt with a bad mag, just an obvious and consistent miss. Since I saw similar symptoms when I replaced the left mag a couple years ago (it really was the mag that time) I assumed (usually a bad thing....) I had another bad mag since this one was older than the one replaced two years ago.

Fortunately the new Slick mag came with two "free" plugs so the bad plug only cost me ~$450, but a good mag was returned as the core and probably crushed.