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  #1  
Old 09-08-2018, 04:46 PM
blaplante blaplante is offline
 
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Location: Southern California
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Question 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!
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  #2  
Old 09-08-2018, 06:00 PM
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Default 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.
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  #3  
Old 09-08-2018, 06:40 PM
blaplante blaplante is offline
 
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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.
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  #4  
Old 09-08-2018, 11:26 PM
lr172 lr172 is offline
 
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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."
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  #5  
Old 09-08-2018, 11:43 PM
Ralph Inkster Ralph Inkster is offline
 
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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?
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Last edited by Ralph Inkster : 09-08-2018 at 11:48 PM.
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  #6  
Old 09-09-2018, 12:41 AM
scsmith scsmith is offline
 
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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.
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  #7  
Old 09-09-2018, 06:51 AM
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Carl Froehlich Carl Froehlich is online now
 
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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
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  #8  
Old 09-09-2018, 07:59 AM
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Larry DeCamp Larry DeCamp is offline
 
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Default 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.
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  #9  
Old 09-09-2018, 08:21 AM
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Alan Carroll Alan Carroll is offline
 
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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?
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  #10  
Old 09-09-2018, 09:25 AM
blaplante blaplante is offline
 
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Default Lead buildup

Quote:
Originally Posted by Ralph Inkster View Post
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?
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.
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Last edited by blaplante : 09-09-2018 at 09:26 AM. Reason: typo
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