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  #1  
Old 08-20-2018, 10:59 PM
Canadian_JOY Canadian_JOY is offline
 
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Default Assistance Request - Interesting Engine Troubleshooting Problem

I've been scratching my head, trying to figure out how to help a fellow on our airport resolve his rough running engine. I'm down to only a very few remaining ideas so I thought I'd come to the brain trust for some additional guidance.

Background:
- engine is an O-360-A2A with Marvel-Schebler carb, one impulse-coupled Slick mag and one LSE Plasma II ignition
- engine ran fine late last year and currently has approximately 300hrs TTSN, as do most of the components on the engine (save for the carb - see below for additional details on that) and a starter that was upgraded a couple of years ago
- a wing tank fuel leak necessitated a lengthy repair process, with repair timelines exacerbated by a long, cold winter that prevented the use of sealant until suitable curing temperatures could be achieved this spring
- fuel system has been vacuumed clean, flushed with fuel running out at every fitting in the system - literally every fitting has been disconnected to ensure it was not harboring contaminants from the fuel tank work - something on the order of 50 gallons of fuel have been flowed through the system to flush it out
- the fuel tank finger screens have been inspected visually in situ via borescope and found contaminant free
- all fuel line fittings have been visually inspected for leaks and found to be leak-free
- multiple fuel flow tests (fuel line disconnected at carb) have verified well in excess of 150% of max engine fuel burn is being delivered to the carb
- carb has been overhauled by an approved overhaul shop as part of investigation process... then the overhaul shop provided a second, freshly-overhauled "loaner" carb to help with troubleshooting - symptoms do not change with either carb installed
- new spark plugs have been installed this past weekend (sorry, can't remember the plug PN for the Plasma II ignition but they are the exact part specified by Klaus Savier) and the plugs fired by the Slick impulse-coupled mag are brand new Tempest UREM37BY - replaced after I discovered their predecessors were found to have center electrodes badly worn to an oval shape
- Slick mag has had its timing checked this evening - it was at about 24*BTDC and now is as close as humanly possible to 25*BTDC

SYMPTOMS:
- engine cranks and starts easily and idles well including an expected slight increase in roughness of idle down at about 650-700 RPM
- engine responds well to smooth throttle inputs
- engine runs well on EITHER the LSE Plasma II OR the Slick mag right up to and including sustained runs at full throttle
- when the ACS key switch is set to "BOTH" the engine seems to misfire and surge at high power settings
- prop is a Sensenich fixed pitch so max static RPM is about 2200 - engine misfiring occurs above about 2000 RPM
- engine runs normally on BOTH ignitions at typical 1700 or 1800 RPM mag check power settings
- no difference in operation is noted with engine cowl on or off
- movement of mixture control makes no difference in the rough running characteristics until obvious RPM drop-off from excessively lean mixture
- engine has been run for a good amount of time with ACS ignition key switch in "L", "R" and "BOTH" positions - engine power is fine for L and R, but the bucking and popping starts as soon as "BOTH" is selected at above about 2000 RPM. This symptomology appears very repeatable.

OTHER DATA:
- engine instrumentation is via a GRT EIS4000 with secondary display of engine data on a GRT Sport EFIS
- no unexpected engine indications are present other than the one known failed CHT sensor
- as expected when the EIS uses a magneto P-lead as its RPM source, TACH RPM is zero when the Slick magneto P-lead is grounded via the key switch
- operation on each single ignition system shows EGTs rising, and declining again when BOTH ignitions are selected ON
- when the engine is misfiring on "BOTH" ignitions, EGT drops slightly
- TACH signal input is from the left mag P-lead via a series dropping resistor - I haven't cut open the heat shrink on that resistor yet but since it has worked for about 300 hours I'm suspecting the resistor itself is not the source of the trouble
- other recent work done on the aircraft includes replacement of alternator field wire and its associated firewall feedthrough when engine run-up in the spring time resulted in a popped circuit breaker caused by the field wire having chafed through to ground at the firewall penetration

At this point I'm nearly out of troubleshooting ideas. Since the engine runs very well on each individual ignition but runs poorly when both ignitions are firing, I've been looking for a common tie point between the two ignition systems as a potential cause. The only thing that comes to mind is the GRT EIS tach signal pickoff - its wire is an unshielded 22ga Tefzel wire which is bundled alongside the other wires going to the ignition switch. The resistor connection is made literally right at the ignition switch terminal. I'm wondering if that unshielded tach sensor wire could be coupling p-lead voltage spikes into the LSE Plasma ignition, causing the Plasma ignition to misbehave. Has anybody seen this before? The magnitude of P-lead voltage spikes would be expected to increase with engine speed so this might explain why trouble doesn't occur until higher engine power settings.

Any well-considered suggestions from this esteemed group would be very much appreciated. Thank you!

Last edited by Canadian_JOY : 08-20-2018 at 11:12 PM. Reason: attempted to provide more detailed info and better clarity of description
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  #2  
Old 08-20-2018, 11:08 PM
rocketman1988 rocketman1988 is offline
 
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Default Switch?

We had some weird issues and it turned out to be the ACS switch...
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  #3  
Old 08-20-2018, 11:37 PM
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fl-mike fl-mike is offline
 
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What is the condition of the Lightspeed manifold pressure hose? Mine crumbled after seven years. Not sure why it would matter, but you didnít mention it.

Iíd look at mechanical and electromechanical stuff before electrical components. Things like switch and connector contacts, chaffed wires, etc.
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  #4  
Old 08-21-2018, 12:03 AM
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az_gila az_gila is online now
 
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Question Grounds

Check the grounding - both the magneto P-lead and for the EI system.

Higher rpm would mean a higher current draw for the EI system and possiblly more susceptibility if there is a higher than usual resistance to ground.
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  #5  
Old 08-21-2018, 12:30 AM
Cannon Cannon is offline
 
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Run it (carefully) with the p-lead and plasma switch connection disconnected (so both are hot all the time). That will eliminate (or confirm) the switch as the problem.
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  #6  
Old 08-21-2018, 06:17 AM
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snopercod snopercod is offline
 
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There's a Service Bulletin on the pre-1989 ACS switches related to arcing on the starter contact. It doesn't seem likely that this is the problem, though. http://acsproducts.co/wp-content/upl...10/SB92-01.pdf
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  #7  
Old 08-21-2018, 07:04 AM
vic syracuse vic syracuse is offline
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Check for intake leaks as well. Beleive it or not, leaks can cause problems at higher power setting.

Vic
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  #8  
Old 08-21-2018, 08:31 AM
Canadian_JOY Canadian_JOY is offline
 
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Thanks for all the great responses.

Bob, John, Chris - the ACS switch and associated wiring is high on my list of culprits... just can't seem to nail down a failure mode of the switch, other than internal arcing from mag to EI contacts, that might cause similar symptoms

Gil - that's the one thing I haven't done yet is absolutely verified the power ground / chassis ground of the Lightspeed ignition. Engine bonding to airframe has been verified, as has battery bonding (and excellent engine cranking would seem to add a second corroboration to this latter check). Will dig into the Lightspeed harness and see if I can spot a weak ground connection there.

Mike - after the fun chasing fuel flow/carburation as a culprit, manifold pressure sensing and leaks were next on the list. MAP hose to Lightspeed and GRT MAP sensors seem good.

Vic - agree that inadvertent mixture leaning via intake leaks would create the same kind of stumbling at higher power settings, but for the life of me I can't figure out how an intake leak would allow the engine to run just fine on either ignition system but not both. Am I missing something here?

Thanks again for all the great responses.
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  #9  
Old 08-21-2018, 09:44 AM
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Quote:
Originally Posted by Canadian_JOY View Post
I can't figure out how an intake leak would allow the engine to run just fine on either ignition system but not both. Am I missing something here?

Thanks again for all the great responses.
You noted that EGT drops a bit with both ignitions enabled. With both sets of plugs firing, more fuel is being burned inside the cylinders and could be pushing you into the "too lean" zone on one or more cylinders.
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  #10  
Old 08-21-2018, 09:48 AM
Canadian_JOY Canadian_JOY is offline
 
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Hmmm interesting point, Ray. I had mentally attributed the slight drop in EGT to the large drop in actual ignition events. Good point to ponder- thank you!
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