snap pop and bang

I have a Lightspeed II installed on an o-320-h2ad, and the second ignition is a conventional Bendix mag. Engine & mag have 240 hours. Yesterday in flight I had the engine start making snap / pop / bang noises. At first a few random ones and then quickly frequent. This was at 7500', quick check in the air didn't seem to isolate to one mag or the other. Reduced power, fuel pump on, it continued. Anyhow, long story short, I diverted, landed, and with help from A&P diagnosed this as fouled plugs... on the Bendix Mag. One plug had a couple large lead deposits in it. All of the Bendix plugs needed cleaning. Cleaned plugs, reinstalled, and problem solved. Flew 2 hours and was fine.

So I'm puzzled... #1 I'd expect the LSE to fire the cylinder successfully no matter how bad the plug on the magneto is fouled up. #2 On a misfire I don't expect to be hearing pop / snap / and bang. I'd expect quiet for a moment.

The only thoughts I have are that this could be somehow firing in the exhaust pipe (Vetterman crossover) perhaps aggravated by the way wasted spark works? Or maybe the big lead deposit was causing a hot spot?

Any insight is much appreciated as it will help me diagnose any sort of odd noises in the future!

Conventional Bendix mag is what?

When you say conventional Bendix mag, do you mean a D3000 just using one side or a custom built single? If custom, does it include a impulse coupler? Going to be curious what you find.

It has a custom Bendix 1200 setup for the D3000 hole. No impulse coupler.

One more thought I have, maybe the EGT probes get hot enough to light an unburned charge? But that doesn't explain how the Lightspeed failed to ignite it in the cylinder.

I just can't seem to make the noise line up with a root cause. Cleaning the conventional plugs fixed the issue.

I can't answer why the LS didn't fire the charge. However, with intermittent misfires, you can get afterfires. When the plug doesn't light the charge, the raw air/fuel mixture is dumped into the exhaust. If the mixture lights on the next cycle, the exhaust at the valve is hot enough to ignite the fuel/air in the exhaust pipe and that is your "pop/bang."

What you describe sounds like pre-ignition caused by the hot lead ball buildups. Question is what is causing the lead fouled plugs? Maybe modify leaning procedures for taxi & flight operations?

If your mag plugs are fouled badly, you get a partial misfire even though the LSE plug is firing and lighting the mixture. This is related to an extensive thread on ignition advance and multi-plug combustion processes that was going on a week or two ago. Under normal cruise conditions, the flame front from the EI plug gets somewhat farther than half way across the cylinder (owing to more spark advance) when it is met by the flame front from the mag plug coming from the other side of the chamber.

If the mag plug doesn't fire, the flame front from the EI plug can not burn everything before the expansion from the power stoke and then the exhaust stroke. So you get a partial mis-fire.

If you want to see how important that mag plug is, go out and set up your usual LOP condition at about 8k ft, and then turn the mag off (turn ignition to EI only). You will be amazed how rough the engine gets. Even though the mag flame front is "late" it still helps burn everything.

Bryan,

I agree your assumption that the LightSpeed should have fired the cylinders even with the mag plugs fouled is correct. Standard in flight mag (or EI) checks should result in a few RPM drop (FP prop). For dual EI setups turning off one side should be hardly noticeable.

I suspect you have not yet found the root problem. I would start going through the LightSpeed system.

Carl

Steve and Carl comments re : 2 plug flame front.

I am not nearly as knowledgeable as previous posts, but I can add an experience. While leaning my carbed 0320 at low altitude " like a cessna student pilot" it still felt a little rough after some richening. So I did a mag check ( 2 pmags) and it was rougher than normal. As mentioned above, with two EI, it is almost imperceptible to ground one . So I pushed in the mixture one notch at a time and repeated the single EI test.Normally one notch on my mixture cable is .1 GPH. It progressively got noticeably smoother until the mag check change was insignificant.
So this is now my mixture fine tuning protocol and I think it reinforces Steve and Carl observations regarding two plug flame front characteristics.

I've got dual Lightspeeds and if I turn one off in flight there are no popping noises. I agree with Carl's suggestion, you should check out the Lightspeed more carefully. Could be the the mag has been masking a problem with the EI, which only became evident when the mag plugs became fouled?

Lead buildup

Ralph Inkster said:

What you describe sounds like pre-ignition caused by the hot lead ball buildups. Question is what is causing the lead fouled plugs? Maybe modify leaning procedures for taxi & flight operations?

Click to expand...

Per the two A&P's I spoke with while resolving the issue, they recommended cleaning the plugs at every oil change (and one mentioned that the h2ad seems to deposit more lead on bottom plugs than non-h2ad o-320 engines).

Plugs have not been cleaned at every oil change. So I'm thinking that the leaning procedures are not particularly bad - more an issue of needed more frequent plug cleaning than I realized and I will clean them every 50 going forward.

Alan Carroll said:

I've got dual Lightspeeds and if I turn one off in flight there are no popping noises. I agree with Carl's suggestion, you should check out the Lightspeed more carefully. Could be the the mag has been masking a problem with the EI, which only became evident when the mag plugs became fouled?

Click to expand...

+1

As I mentioned above, the popping is an indication of intermittent misfires (the key here is intermittent) and a misfire typically requires a lack of ignition when a proper fuel/air mixture exists in the chamber (proper meaning combustible - too lean or too rich it becomes un-combustible). Either plug can effectively ignite the charge and it will FULLY burn. The issue about effective timing with two plugs is only relevant to when the combustion event's peak pressure occurs, relative to the desired point in crank rotation.

The fact that you are getting intermittent misfires indicates that in those occassions of popping, at least one cylinder (possibly all) had no ignition from either plug, assuming there were no issues with mixture and those are unlikely with your intermittent condition. That said, intermittent misfires are likely to occur right on the brink of too lean to combust. But this doesn't match your symptoms as I read them.

Note that a spark VERY early or VERY late will not ignite the mixture, so you are also looking at CPU issues on the LS and not just a lack of spark.

Given the fouled plugs on the mag, it seems most logical that you were running on just the LS and it is intermittently not sparking a cylinder or intermittently drifting the timing in a radical fashion. Note that running on an EI with advanced timing will make it very hard to detect if the mag is not firing. It does impact performance, but not that much and typically not enough to notice it without really paying attention.

Given the intermittency, a bad coil on the LS is a prime suspect. They often fail intermittently.

Larry

+1 on what Larry said.

I again suggest you take a close look at the Lightspeed. With all due respect to your two A&Ps, they may not have experience with Lightspeed failures.

Carl

As you have the only hole in the accessory case filled with your Bendix, you must have a crank triggered LSE. If you have the old RG58 (black) coax cables from the LSE box to the coil primaries, get rid of them and use RG400 (what Klaus currently supplies)

Even if that's not the problem, it will be - sometime, someday... I have over 5,000 hours aggregate experience with LSE 2 in 3 EAB planes; PM me if you need help checking the system out. If you decide to call Klaus, suggest you first read the manual again

Electrode short/dirty plugs?

I run an O320H2AD in my -4, with the standard D3000 magneto, and a few months ago I had similar misfire pop/bang during run-up, which I thought was just fouling plug because I hadn't leaned during taxi as I usually do. Cleared it up with the normal power/lean run and took off. During cruise it started again...quick RTF and look under the hood. I had a plug with a cracked insulator and a piece of it jammed in the electrode causing an intermittent short out. Replaced the plug, and good to go. Your problem may have nothing to do with the LS, and simply the dirty plugs.

Carl Froehlich said:

Bryan,

I agree your assumption that the LightSpeed should have fired the cylinders even with the mag plugs fouled is correct. Standard in flight mag (or EI) checks should result in a few RPM drop (FP prop). For dual EI setups turning off one side should be hardly noticeable.

I suspect you have not yet found the root problem. I would start going through the LightSpeed system.

Carl

Click to expand...

No doubt this is true ROP. If you are very LOP, not so true at all.

Alan Carroll said:

I've got dual Lightspeeds and if I turn one off in flight there are no popping noises. I agree with Carl's suggestion, you should check out the Lightspeed more carefully. Could be the the mag has been masking a problem with the EI, which only became evident when the mag plugs became fouled?

Click to expand...

Sure, if you are ROP.

Try it 60? LOP and see what happens

scsmith said:

Sure, if you are ROP.

Try it 60? LOP and see what happens

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I can?t think of a reason why anyone would get afterfires on one ignition instead of two. It shouldn?t matter how far lop or rop you are.

Larry

lr172 said:

I can’t think of a reason why anyone would get afterfires on one ignition instead of two. It shouldn’t matter how far lop or rop you are.

Larry

Click to expand...

Well, that's true, I don't get afterfires. What I get is a very rough-running engine. But you are right, I don't get pops and bangs. As you pointed out, with one ignition, it will still burn, it's just way late in the cycle and acts like a partial misfire. So if the afterfires (pops and bangs) are the only real symptom that the OP is experiencing, then my point is out of line, and it must be an intermittent ignition problem.

Some more findings

Thanks for the many ideas. I've found some more root cause that I think explains the trouble. Since the electronic mag was mag checking OK, those plugs were left alone until after my return. On pulling those... I found one of the recommended iridium plugs had no fine wire left. Surprisingly the LSE was still firing this plug (well most of the time I guess). So I think what happened was the fine wire vanished, making the gap too big, which caused the electronic ignition to periodically not successfully fire, but the leaded up Mag plug wasn't in good shape either... hence afterfire. Notably the mag check before takeoff was fine.

per Lightspeed those Iridium plugs should be good for at least 300 hours. I was under 250 when this happened. I've now replaced all the plugs, and will be replacing the iridium plugs more often in the future.

Also, on the recommendations here, I checked the primary Lightspeed coax (it's rg400) and the connections to the spade lugs on the coils... I did find one that was a bit loose & replaced that.

Lightspeed EI's are notorious for bad coils. They are used on German snow mobiles. Klaus imports them for his ignitions.