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Assistance Request - Interesting Engine Troubleshooting Problem

Canadian_JOY

Well Known Member
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!
 
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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 didn?t mention it.

I?d look at mechanical and electromechanical stuff before electrical components. Things like switch and connector contacts, chaffed wires, etc.
 
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.
 
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.
 
Check for intake leaks as well. Beleive it or not, leaks can cause problems at higher power setting.

Vic
 
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.
 
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.
 
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!
 
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.

That is normal. The combustion burning (heat release) is slightly faster, in effect, advanced with both firing. You can note that anytime with an inflight mag check. This assumes both ignition systems are timed the same.

Also, an errant jumping EGT (elevation) when leaning means that cylinder is beginning to misfire on one plug.
 
My 2 cents. move the ignition controls to toggle switches for testing to isolate the ACS switch (low probability given symptoms) If that is not the issue, I would suspect that the LSE is throwing in too much timing; either the advance curve is too high at higher RPMs, or more likely that it is has lost it's TDC reference or drifted (likely due to a reset or failing VR sensor or CPU. Remember, the two ignition timings are additive, so with just the lightspeed, the engine is soaking up the extra advance from the LSE without problems, but when you add in the 25* from the mag, the total advance is high enough to cause rough running or detonation, which can feel like rough running when it gets to the severe stage.

You can confirm the LSE timing with a simple timing gun attached to the #1 LSE plug and a little TDC marker clamped to the baffling.

Someone recently posted that they were having problems and found a failed LSE brain box that was causing agressive advance. So this seems like something worth validating.

Larry
 
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Thanks again for the great inputs.

Additional troubleshooting conducted today resulted in an engine run with both the magneto and Lightspeed p-leads removed from the ACS ignition switch - they were just dangling in the air. The only way to kill the engine was via mixture idle-cutoff. The engine ran poorly again at high power settings.

The complete train of MAP plumbing was removed, cleaned, inspected and re-attached. One suspect connection at the GRT MAP sensor was re-terminated by shortening the hose a half inch, cutting off the bit of hose that seemed to have become slightly harder than the rest at the point where it connects to the GRT MAP sensor nipple.

Looks like it's time to get the timing light out and see what that Lightspeed ignition is doing.
 
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Which LS pickup do you have?

I had similar symptoms once.
Discovered that the D-sub connector on the back of the Hall Effect unit was slightly loose. Problem didn't manifest itself until at high RPM.
A little blue Loctite on the screws & tightened them down; no probs since.
Just a data point.

Sam
 
SNIP.... Remember, the two ignition timings are additive, so with just the lightspeed, the engine is soaking up the extra advance from the LSE without problems, but when you add in the 25* from the mag, the total advance is high enough to cause rough running or detonation, which can feel like rough running when it gets to the severe stage.

You can confirm the LSE timing with a simple timing gun attached to the #1 LSE plug and a little TDC marker clamped to the baffling.

Someone recently posted that they were having problems and found a failed LSE brain box that was causing agressive advance. So this seems like something worth validating.

Larry

While I agree trouble shooting has mostly eliminated everything other than the Light Speed ignitions, note that the comment on timing being additive is incorrect. For a mag and a EI setup, the timing is set by the EI, the mag is just along for the ride as the flame front started long before the mag plug fires.

Call Klaus and have him send you a loaner Plasma II box to verify it is the issue. Other things:
- If running the Hall Effect, pull it per the Light Speed maintenance section and check the bearing and seal for oil leakage. 300 hours was when mine stared to leak.
- Check each Light Speed plug lead. For whatever reason they don?t last very long.
- If you don?t have a spare coil on your shelf, get one.

Carl
 
Carl - just curious... The points you've made all seem valid, however I still struggle to understand how the engine will run smoothly at full power on the Lightspeed alone, or the mag alone, but runs like an old clunker when both ignitions are selected. There definitely is something here that I'm not understanding. Please help clear my foggy brain! :)
 
Mark - I can?t offer cause and effect for Lightspeed. Perhaps my assumption that the Lightspeed ignition is the issue is wrong, but as I been around this tree I think a swap out with a loaner from Klaus is the appropriate next step. Note - my experience with Lightspeed Plasma II+ is over 10 years ago so perhaps things have changed.

Carl
 
As mentioned by Larrys' insightful comment above, I agree that the next step is connect an automotive timing light and see what is going on. It is a very simple test that will quickly eliminate that possible issue. You can borrow one from Parts Source, or buy one from CT for $60 CAN.

Good luck!!

Phil
 
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While I agree trouble shooting has mostly eliminated everything other than the Light Speed ignitions, note that the comment on timing being additive is incorrect. For a mag and a EI setup, the timing is set by the EI, the mag is just along for the ride as the flame front started long before the mag plug fires.

Carl

Not correct Carl. In a twin plug setup, two flame fronts exist, being lit at the sides and meet at the middle. Twin plug setups will shorten the duration between ignition and peak pressure by approx 10* compared to a single plug setup. Furhter, in a typical single plug setup, the flame starts in the center and concludes at the edges. In a twin plug setup with only one spark, the flame front has to travel the full diameter, not just the radius like a single plug and not to the center as when both plugs fire. This substantially adds to the time between ignition and peak pressure. Optimum ignition timing for one spark vs two will be quite different.

Given that the mag isn't doing anything, turn it off and see what happens and please report back with results. Assuming you are running near optimum timing, drop the mag offline and watch CHTs drop and EGTs rise, proving that the peak pressure point has retarded. If you wait a minute or two, you'll also see your speed drop.

Larry
 
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Carl - just curious... The points you've made all seem valid, however I still struggle to understand how the engine will run smoothly at full power on the Lightspeed alone, or the mag alone, but runs like an old clunker when both ignitions are selected. There definitely is something here that I'm not understanding. Please help clear my foggy brain! :)


Explained above. Our engines run well from around 0-45* of timing. Once you get past this you can detonate and run rough. Let's say the LS is giving you 45* of timing. Engine runs fine on mag alone at 25. Engine runs fine on LS alone at 45. Turn on both and you get 70 degrees of timing and you detonate which causes the rough running. Detonation potential increases with torque produced, which increases with RPM (up to torque peak anyways). This is why you don't see the symptoms at lower RPMs.

In fairness to Carl, it is not truely additive, as in A + B, but I don't have the math to give you the formula for how additive it is, only can assure you that it is. It is also true that the more you advance one EI's timing, the less additive they combination becomes. This is why it is more noticeable to drop a mag with a two mag setup than a mag/EI setup, assuming the EI is running above 25*

Regardless of whether or not you believe this or not, I feel strongly that you need to validate what the LS is doing to resolve this issue. Just too many issues reported with this setup to not check.

Larry
 
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Explained above. Our engines run well from around 0-45* of timing. Once you get past this you can detonate and run rough. Let's say the LS is giving you 45* of timing. Engine runs fine on mag alone at 25. Engine runs fine on LS alone at 45. Turn on both and you get 70 degrees of timing and you detonate which causes the rough running. Detonation potential increases with torque produced, which increases with RPM (up to torque peak anyways). This is why you don't see the symptoms at lower RPMs.

In fairness to Carl, it is not truely additive, as in A + B, but I don't have the math to give you the formula for how additive it is, only can assure you that it is. It is also true that the more you advance one EI's timing, the less additive they combination becomes. This is why it is more noticeable to drop a mag with a two mag setup than a mag/EI setup, assuming the EI is running above 25*

Regardless of whether or not you believe this or not, I feel strongly that you need to validate what the LS is doing to resolve this issue. Just too many issues reported with this setup to not check.

Larry

The math does not make sense if the reference point for both ignition systems is TDC. The advances should not be additive.

I was always told the EI system mag drop was less than a straight magneto because all of the EI systems create a better spark and the 'bang' is more efficient.
 
The math does not make sense if the reference point for both ignition systems is TDC. The advances should not be additive.

I was always told the EI system mag drop was less than a straight magneto because all of the EI systems create a better spark and the 'bang' is more efficient.

And the fire has been burning for some milliseconds before the mag fires. The mag is almost just along for the ride when the EI is pretty far advanced. Turning the mag off doesn't have that much effect because the flame front is almost to the other spark plug by that time. At high MAP the EI retards to about the same timing as the mag and the loss of one or the other would be more pronounced.

Ed Holyoke
 
I was always told the EI system mag drop was less than a straight magneto because all of the EI systems create a better spark and the 'bang' is more efficient.

A spark is a spark. It either lights the mixture or it doesn't. The strength of the spark is immaterial in this regard, though the stronger spark is better able to light sub-optimal mixtures that a weaker spark cannot. This is why you can run leaner with EI than a mag. The mag drop with an EI, as Ed mentions, is less pronounced than with another mag because it has more advance than a mag and the more you advance one spark, the less impact the other spark has on "effective timing" in a twin plug engine.

However, until you get one plug VERY advanced, both ignitions have an impact on the timing of peak pressure. All of this ignition timing is about getting your peak pressure at the desired point (typically 6-10* ATDC). Various other conditions, such as mixture, play a part in how long it takes for the charge to burn and therefore impact the timing of peak pressure separately from igntion timing.

Larry
 
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A spark is a spark. It either lights the mixture or it doesn't. The strength of the spark is immaterial, though the stronger spark is better able to light sub-optimal mixtures that a weaker spark cannot. This is why you can run leaner with EI. The mag drop with an EI, as Ed mentions, is less pronounced than with another mag because it has more advance than a mag and the more you advance one spark, the less impact the other spark has on "effective timing."

However, until you get one plug VERY advanced, both ignitions have an impact on the timing of peak pressure. All of this ignition timing is about getting your peak pressure at the desired point (typically 6-10* ATDC). Various other conditions, such as mixture, play a part in how long it takes for the charge to burn and therefore impact the timing of peak pressure separately from igntion timing.

Larry

OK, but I still don't see how you could add EI and magneto timing advances when the reference is TDC. :confused:

Post #21 above
 
OK, but I still don't see how you could add EI and magneto timing advances when the reference is TDC. :confused:

Post #21 above

When I talk about adding, I am really refering to the timing of peak pressure (which is the ultimate goal in ignition timing). With two plugs, you have two combustion events. If you fire one plug, it must burn the whole chamber of mixture and that take longer than lighting two fires of half the chamber size each.

Peak pressure occurs loosely with the completed combustion process.

Larry
 
When I talk about adding, I am really refering to the timing of peak pressure (which is the ultimate goal in ignition timing). With two plugs, you have two combustion events. If you fire one plug, it must burn the whole chamber of mixture and that take longer than lighting two fires of half the chamber size each.

Peak pressure occurs loosely with the completed combustion process.

Larry

It might take 45* of timing on one plug to reach peak pressure at 8* ADTC. However, it might only take 30* of timing on each of two plugs to reach peak pressure at 8*. OR 35* on one plug and 25* on the other.

With two plugs, the timing of each is interrelated in the goal of managing the timing of peak pressure, due to the converging of two unique combustion events. Inter-related may be better wording than additive.
 
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OK, but I still don't see how you could add EI and magneto timing advances when the reference is TDC. :confused:

Post #21 above

You don't add the timing. If there is only one ignition source, the timing is however far before TDC it fires. If you have two sources at the same timing, the peak pressure would occur just slightly sooner than if you had only the one because the flame front would spread all the way across the combustion chamber from the two sources. That should happen at about 17* ATDC (from the textbooks that I've read). Turn off one and you have mag drop because the effective timing has been retarded. Peak pressure happens later than if both were firing. If one fires at 40* and the other at 25*, the effective timing will fall somewhere between the two, I'm guessing closer to the more advanced firing point, but not higher than the more advanced timing of the two. Two EIs at 40* would have a more advanced effective timing than that, but not by a bunch.

EI manufacturers sometimes claim a 10% increase in efficiency with one and an addition 5% with a second ignition. Some of this would be because of the slight increase in effective timing. I don't hear a lot of people complaining of detonation that started up when they added the second electronic ignition, though. If the EI is advancing too far, detonation is possible, but it would be hard to lay that at the feet of a properly operating 25* magneto. Timing light checks of both ignitions would be a good idea. If you can do it safely, do it when it is misfiring for the most accurate diagnosis.

Ed Holyoke
 
It might take 45* of timing on one plug to reach peak pressure at 8* ADTC. However, it might only take 30* of timing on each of two plugs to reach peak pressure at 8*. OR 35* on one plug and 25* on the other.

With two plugs, the timing of each is interrelated in the goal of managing the timing of peak pressure, due to the converging of two unique combustion events. Inter-related may be better wording than additive.

But your example above would have the OP's engine probably runner better at high speeds rather than worse when both ignitions are active.
 
But your example above would have the OP's engine probably runner better at high speeds rather than worse when both ignitions are active.

Unless there is so much ignition advance that it detonates. I am speculating that the LS is throwing 40+ degrees of timing at the engine under high MAP. When you add in the mag it is moving the peak pressure enough that it begins to detonate agressively. Remember ignition only improves performance up to optimum. After that further advancement cause a reduction in performance, even before detonation begins.

Detonation is when the peak pressure occurs a few degrees or more before TDC. The further before TDC, the worse the detonation. Often you won't get noticeable detonation until you are in the upper band of the torque curve even with very aggressive timing. THis is why I believe he only sees detonation (i.e. rough running) at 2000+

For reference, at cruise power, you couldn't tell the difference between 25 and 35 degrees of timing without instrumentation or an ASI/tachometer.

Larry
 
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Explained above. Our engines run well from around 0-45* of timing. Once you get past this you can detonate and run rough. Let's say the LS is giving you 45* of timing. Engine runs fine on mag alone at 25. Engine runs fine on LS alone at 45. Turn on both and you get 70 degrees of timing and you detonate which causes the rough running. Detonation potential increases with torque produced, which increases with RPM (up to torque peak anyways). This is why you don't see the symptoms at lower RPMs.

In fairness to Carl, it is not truely additive, as in A + B, but I don't have the math to give you the formula for how additive it is, only can assure you that it is. It is also true that the more you advance one EI's timing, the less additive they combination becomes. This is why it is more noticeable to drop a mag with a two mag setup than a mag/EI setup, assuming the EI is running above 25*

Regardless of whether or not you believe this or not, I feel strongly that you need to validate what the LS is doing to resolve this issue. Just too many issues reported with this setup to not check.

Larry


It just flat out doesn’t work that way!!!! Where do you come up with this?
 
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Unless there is so much ignition advance that it detonates. I am speculating that the LS is throwing 40+ degrees of timing at the engine under high MAP. When you add in the mag it is moving the peak pressure enough that it begins to detonate agressively. Remember ignition only improves performance up to optimum. After that further advancement cause a reduction in performance, even before detonation begins.

Detonation is when the peak pressure occurs a few degrees or more before TDC. The further before TDC, the worse the detonation. Often you won't get noticeable detonation until you are in the upper band of the torque curve even with very aggressive timing. THis is why I believe he only sees detonation (i.e. rough running) at 2000+

For reference, at cruise power, you couldn't tell the difference between 25 and 35 degrees of timing without instrumentation or an ASI/tachometer.

Larry

That's not a real good definition of detonation. Detonation occurs when small pockets of unburned fuel air mixture spontaneously ignite after the ignition event but before the flame front reaches them. It will happen when the CHT is too high, when the induction air temp is too high, when the pressure in the cylinder is too high, or some combination of these. It doesn't have to initiate at any particular position before TDC. Mild detonation can occur well after TDC. High compression ratio and high manifold pressure can bring the cylinder closer to detonating by increasing both the heat and pressure in the cylinder. Uneven mixture of the fuel and air can provide pockets that are ripe to detonate. Advanced timing can contribute to detonation by making the peak pressure occur earlier thus raising CHT and pressure to the point of spontaneous ignition. Once detonation begins, other pockets of unburned fuel react to the increase in temperature and pressure and it can rapidly get a lot worse. The shock waves from the multiple sudden combustion events break up the insulating boundary layer protecting the aluminum piston and cylinder head and erosion damage is the result. If it gets bad enough to heat up something red hot, pre-ignition will be next up and that happens way before TDC, before the timed ignition event in fact.

If the OP's problems are being caused by the LS ignition firing early enough to cause detonation in a ground run, there is something wrong with the ignition module. It should advance the timing with RPM and low MAP, but retard it back to the same as the data plate timing with high MAP. LS Engineering recommends pulling the manifold pressure line at low RPM to check that it is retarding when it suddenly sees ambient (higher) pressure. The RPM should noticeably decline if it is working right. That might be the first test to do with the timing light on it. I can't remember if you said that it was crank triggered or hall effect.

Ignition wires, coils, and spark plugs that seem nominal may not work as well at high power settings. Worth checking the coils and plugs with an Ohm meter. A new plug could have been dropped before you bought it. Magnetos have odd failure modes sometimes. Arcing within the harness cap can actually fire the wrong plug and that will make the engine run poorly indeed.

If it used to run OK, what is different this year? Any wiring changes? I might be wrong, but I don't think you could induce weirdness into the EI with the P lead of the mag next to the power wire, but is it near the pickup wire? That might have an effect on timing. If you've ruled out fuel delivery, what about air? is the air filter clean and oiled? If it isn't fuel or air, it pretty much has to be ignition assuming that nothing has changed with valve timing and no induction leaks. Ruled out a collapsed lifter? This may be off the wall, but any valve springs broken?

Ed Holyoke
 
So....... coming back to the original troubleshooting issue at hand...

Following the sage advice offered in this forum, tonight we timed the Lightspeed ignition. I was very impressed with the smooth changes in ignition advance provided by the Lightspeed. It would start near 0*BTDC then come up to about 30*BTDC at 1000RPM. As the power was increased timing rolled back smoothly until it was very close to 25* at full throttle. There was no detectable erratic change in timing and no significant advance beyond something in the neighborhood of 30*. I know it might have been 34* according to the Lightspeed manual, however this was a rough rather than exact test, shooting the timing light through the windshield and subject to the vagaries of the position of my eyeballs relative to the parting line of the engine case as the fixed timing reference. We certainly did not see wild deviations to 40* or anything that appeared at all untoward.

With all the above having been said, the engine was still misfiring at high power settings. Both ignitions have their P-leads removed from the key switch and dangling in the air so we now know the ACS ignition switch has been completely removed from the equation.

Tonight I was able to find a "sweet spot" in the travel of the mixture control where the engine ran very well at full RPM and with both ignitions active. This finding is leading us in the direction of there perhaps having been two problems - poor ignition via worn spark plugs and poor carburation as a result of perhaps a stuck float valve. Tonight I found evidence of blue fuel dye atop the carb air box - not a good sign in terms of carb health. Next step is to repeat tonight's tests with the original (rather than the loaner) freshly overhauled carb installed.

We're making progress is small bites, with no small thanks going to the very helpful suggestions gleaned in this forum.
 
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Carbs can be strange animals

I had a weird problem with the carb on a C-150. Over the mountains the RPM suddenly dropped from 2500 to windmilling at about 1000 and then, every 45 seconds or so, I would get a one second shot of full power. Glided to an airport and, once in the pattern, full power came back. Carb ice? Didn't seem right for that. Checked everything. Sent the carb out for repair and it came back with a mark on the old needle and seat. A chunk of something stuck there, running over rich, and curing itself near sea level? Maybe. Cured? Seemed OK until, over the mountains again, the RPM began smoothly cycling between 2500 and 1000 over and over. Made it to an airport and the power came back on full. I checked everything. Pulled gun cleaning patches through the fuel and vent lines. Had the mags and harnesses checked. Plugs, compression, looked at the valve springs, you name it. Bought an overhauled carb, test flew it, and sold the airplane. That, by the way, was the week after we had bought a 6A. Nobody wanted to fly a 150 anymore that kept trying to kill us, to boot.

Ed Holyoke
 
Our engines run well from around 0-45* of timing. Once you get past this you can detonate and run rough. Let's say the LS is giving you 45* of timing. Engine runs fine on mag alone at 25. Engine runs fine on LS alone at 45. Turn on both and you get 70 degrees of timing and you detonate which causes the rough running.

This is horribly wrong advice. It's all I can do to not delete the post so it doesn't send someone down a wrong path in the future. Engine timing is not additive between the 2 ignition systems. Please make sure of your facts before providing advice, or help us understand what reference material you in which you found this.

Thanks.

Vic
 
Our engines run well from around 0-45* of timing. Once you get past this you can detonate and run rough. Let's say the LS is giving you 45* of timing. Engine runs fine on mag alone at 25. Engine runs fine on LS alone at 45. Turn on both and you get 70 degrees of timing and you detonate which causes the rough running.

This is horribly wrong advice. It's all I can do to not delete the post so it doesn't send someone down a wrong path in the future. Engine timing is not additive between the 2 ignition systems. Please make sure of your facts before providing advice, or help us understand what reference material you in which you found this.

Thanks.

Vic

First, this was not advise. It was an attempt to help someone understand the relationship of timing on twin plug engines. If you read further down the thread I elaborate on my admitedly over simplified post that you reference. I have checked my facts and have explained how ignition timing is inter-related on twin plug systems and how this relates to the timing of peak pressure, which is further related to achiving optimal torque output on a gas reciprocating engine. I have yet to hear factual debate arguing this point, other than ED correctly pointing out the oversimplification on the scale of affect.

I have read several threads on this site with folks who have tested with EI/Mag and then 2 EIs. In each of those cases, they found optimal timing of the EI, when installed with a mag (25*) to be several degrees higher than with two EI's. This factually substantiates my argument. Let me know if I need to post references to those.

I'll put the same challenge out to you. If you don't believe that the timing of the two igntion systems are additive in nature, turn one of them off in flight and watch the results. Further, the premise of one component of a Mag drop test is based upon this very concept. The RPM drop will vary based upon how much "effective timing" is lost when one mag drops. This helps pilot to identify a mag that is not properly timed.

Larry
 
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No, Bob, this is an airplane with about 300 hours on it, 200 by its current owner. The airplane suffered some downtime as a result of a fuel leak that had to wait for warmer spring weather to be properly repaired. Upon emerging from nearly 6 months of dormancy the rough-running engine conditions became manifest.
 
No, Bob, this is an airplane with about 300 hours on it, 200 by its current owner. The airplane suffered some downtime as a result of a fuel leak that had to wait for warmer spring weather to be properly repaired. Upon emerging from nearly 6 months of dormancy the rough-running engine conditions became manifest.

Ok. What is the fuel flow when the stumble occurs?
 
The Lightspeed ?runaway? MAP sensor is a known issue thanks to this forum and was my initial thought ? but that?s been put to bed during troubleshooting. Induction leaks can be insidious but should be easy to verify by running the engine at the misfire condition and spraying carb cleaner or WD-40 at every tube connection. If the engine and/or EGT changes, then that?s a leak. Might as well order Ross? induction tube kit because even if they aren?t leaking today, they will eventually. Running the engine at night in the dark with the cowl off will also often show errant spark paths. Either at the boots or along the run of wires somewhere. A (daylight) visual inspection of any metal near the plug wire path can often show evidence of arcing too.

Though no longer relevant to this situation, I have to side with Larry on ?most? of his discussion on the composite nature of a dissimilar timed, dual plug ignition system. I say ?most? only because he went too far in his simplification of the ?additive timing?. He?s corrected it now, but otherwise he?s dead on. To be closer to reality (but still oversimplified) the difference in timing can be thought of as an average between the two. i.e. A mag at 20 and an EI at 30 has an effective timing of 25. Flame speed based on mixture and air density and combustion chamber swirl complicate this in reality, but the point remains that in every case I have actually conducted flight test with a split system, the magneto still brings something to the party ? it?s never ?along for the ride?. Even at high altitude and lean enough to squeak ? where the EI is FAR outpacing the fixed magneto timing ? turning off the mag shows an EGT rise and speed dropping off. - Note that this condition is AFTER the optimum composite timing has been established in flight test.
 
It's all I can do to not delete the post so it doesn't send someone down a wrong path in the future.

Vic


I strongly encourage continued restraint from deleting posts with which you disagree. Discussions such as this, as unpleasant as it may seem, is how we collectively debate and dispell the large body of inaccurate knowledge that is based upon decades of hearsay. Imagine if all of Deakin's material was deleted because it was "known" to be wrong.

My apologies to Canadian Joy for the thread drift.

Larry
 
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Fuel flow is passing north of 15GPH - haven't closely watched where it peaked as I was spending more time looking at EGT/CHT.

When it stumbles lean the mixture. If it quits with very little mixture pulling then you're not getting enough fuel. Check that and report back.

Other than than not getting enough fuel then it is possible you're getting preignition at high cylinder pressures. I've seen cylinder deposits do this.

Eliminating the above I would start looking for a broken valve spring.

Eliminating that I would check dry tappet clearances to see if you have a rounded cam lobe.

Sounds like both of your ignition systems are working fine but you have a full-power issue.
 
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Last night I found that if I pulled the mixture out about 2" the engine would run perfectly with both ignitions firing. That's when I once again started to think this a fuel/air problem rather than an ignition problem. Carb is being changed again today...
 
Last night I found that if I pulled the mixture out about 2" the engine would run perfectly with both ignitions firing. That's when I once again started to think this a fuel/air problem rather than an ignition problem. Carb is being changed again today...

You can take a look at the needle/seat on the carb. That is the most likely suspect given these symptoms, as well as the report of blue staining on top of carb, IMO. Are you able to observe black smoke coming out the exhaust while running rough? That would help confirm an overly rich condition. However, I am struggling to correlate that with the ability to run well on one ignition, but not two.

A fuel delivery problem can introduce trash into the carb and that is most likely to produce problems with the needle/seat. Debris caught in the rubber part of the needle will allow the bowl to overfill and cause an overly rich condition. There is an overflow vent on the carb top where excess gas goes when bowl level can't be controlled by the needle/seat.

Larry
 
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Last night I found that if I pulled the mixture out about 2" the engine would run perfectly with both ignitions firing. That's when I once again started to think this a fuel/air problem rather than an ignition problem. Carb is being changed again today...

Bingo. Too much fuel. Carb has wrong jet. What's the part number of the carb?
 
The carb in question is the "loaner" carb from the overhaul shop. I have no idea what jets are in it since it is indeed a "loaner".

Aircraft owner changed out the carb today - removed the "loaner" and re-installed his original, freshly overhauled carb and performed a ground run. Results were encouraging, thus a flight test is to take place in an hour or so. Will report back with results.
 
I strongly encourage continued restraint from deleting posts with which you disagree. Discussions such as this, as unpleasant as it may seem, is how we collectively debate and dispell the large body of inaccurate knowledge that is based upon decades of hearsay. Imagine if all of Deakin's material was deleted because it was "known" to be wrong.

My apologies to Canadian Joy for the thread drift.

Larry

I did not delete it, and won't. But it is flat out wrong that timing is additive to the 70 degrees that was mentioned in that post.

Vic
 
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