Inflight Misfire

At leaded auto gas peak in about 1970 it contained 2.4 gLead/gal.
Today 100LL contains 2.12 gTEl/gal (1.36 gPb/gal

Depending on whether the 1970 value is TEL (64% Pb by weight) or actual Pb content. 1970 autogas contained 1.3-2.4 gPb/gal.
 
Domenick said:
Depending on whether the 1970 value is TEL (64% Pb by weight) or actual Pb content. 1970 autogas contained 1.3-2.4 gPb/gal.
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Falling to 0.5 g/gal 1975-1979. Same as Avgas 80. It stayed there until leaded Mogas was finally phased out sometime in the '90s.
 
Update - The shop said today that one mag is serviceable for the 500hr SB and the other is beyond serviceable limits and will need an overhaul. They’re giving us credit for a core and starting fresh, so hopefully this will fix the problems experienced.
 
RyanB said:
Update - The shop said today that one mag is serviceable for the 500hr SB and the other is beyond serviceable limits and will need an overhaul. Hopefully this will fix the problems experienced.
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Has SureFly been ruled out for the unserviceable mag?
 
MIA said:
Next thing that happens is the mag quits and ya get to roust out an A/P on a Saturday (or Sunday) to fix it. Suggestion: land at Mexia, TX airport if ya wanna try this. Not that I know anything about it...
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Having driven through Mexia, I’m not confident that your luck would be much better on a weekday.
 
Huckster79 said:
can you share why? As I’m coming up on my 500hr and if one’s unserviceable I’m thinking about it...
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Just owner preference. Not my airplane, otherwise I probably would consider it.
 
-KLB- said:
Having driven through Mexia, I’m not confident that your luck would be much better on a weekday.
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There is (or was) a substantial, as in total rebuild of crashed aircraft, maintenance facility at the airport. Rebuilding the mag that day was just another AOG job for them.
 
"That last one is so fine that water won't go through it if it's wet with fuel."

Actually, not true for everyone. In the A-65, the gascolator and the fuel inlet screens are the same mesh.

I've had misfires in the Bendix mags, not when cold and starting up and preflight checks, but after an hour or two in the summer. The mags heat up and the coil opens up. The next flight the
preflight is just fine. You wouldn't know if you didn't do a post-flight mag check. The other issue was the points had too little a gap and when it heated up the gap when away.

So check the mag with someone who can put it in the oven at 160 degrees and then run it on the tester. Coil, condenser and points. Then check the wires. Plugs are next. Then
you have a pickle to work through what's going on.
 
An update -

My IA installed the new mags, ignition harnesses and all new spark plugs on Thursday. I went out for a test flight and unfortunately the problem still persists. It seemed to be somewhat better at first, but as I got further into the flight, the misfiring/stumbling seemed to become more consistent. I did a couple inflight mag checks that seemed to be fine. Takeoff and low power (2000RPM and below) are fine, but it seems to happen like clockwork when I reduce power after a climb and start pulling the mixture back to lean for cruise. When I am in cruise flight, these very brief misfiring stumbles happen every so often. Sometimes one or two every couple minutes and other times, it’s several per minute. As odd as it may sound, sometimes I notice it more prominently if I roll into a turn (like I’m doing turns around a point), than when I’m flying straight and level. My IA checked for intake and exhaust leaks and did find a few bad gaskets and replaced with new.

Anyway, it’s very frustrating to say the least. Since we can basically rule out the ignition system being the cause of this and no apparent intake or exhaust leaks, my next assumption is that it must be a fuel issue, but I can’t figure out how. I’d hate to have the carburetor rebuilt only to STILL have the problem. If that’s not it, than my next guess would be an intermittently sticking valve or something internal.

Any of our mechanics have any thoughts?
 
Could this maybe be an air intake issue. At higher IAS the problem seems most reproducible. Maybe the higher intake air velocity (or pressure?) Is resulting in a leak or something. Perhaps the turn aggravates it. Can't remember...during cruise when this is happening have you tried pulling carb heat. That woyld alter the airflow. Probably not it.
 
A bud had a similar problem on a PA28-140.

There is a Lycoming Service Instruction ( ?) with a title

similar to “Alternate Ignition Wiring “ dated 70s.

It helped his issue.
 
Check the P-lead wiring. If there were problems at the mag end the mechanic should have spotted them. Under the panel at the switch is another spot where stuff comes loose and terminals can short to ground intermittently. Misfiring when "rolling into a turn" implies a couple of possibilities: the controls are fouling wires and moving them, or increased load factor is making it harder for the engine-driven pump to lift the fuel. A weak pump should show up on the climbout, though, unless you're in the habit of running the boost pump for takeoff and climb. What does the fuel pressure look like? Does it flicker when the engine stumbles?

Does it feel like one cylinder doesn't fire for one cycle, or does the whole engine lose RPM or shake for a fraction of a second?
 
Dan Thomas said:
Misfiring when "rolling into a turn" implies a couple of possibilities: the controls are fouling wires and moving them, or increased load factor is making it harder for the engine-driven pump to lift the fuel. A weak pump should show up on the climbout, though, unless you're in the habit of running the boost pump for takeoff and climb.
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Good points. I do run the electric fuel pump during takeoff, but turn it off at about 400-500AGL, so it’s not running for any extended period of time. Once I do turn it off, I don’t notice any difference in engine performance for the remainder of the climb out. There have been times though, where even with the electric fuel pump on, I’ll see the fuel pressure decrease, maybe to around 3psi or so (mid range is 5psi), but no change in performance. Almost as though the gauge lags for a second and then comes back up to mid-range a second or two later. The stumbling mis-fires always seem to happen right after I reduce power when leveling off and start leaning the mixture back for cruise flight.
Dan Thomas said:
What does the fuel pressure look like? Does it flicker when the engine stumbles?
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Actually, I haven’t noticed much of a change on the fuel pressure. During flight it seems to hang right around mid range all the time. After I land and during my taxi, I’ll see fuel pressure fall again, to around 3psi. I’ll have to watch it closely next time!
Dan Thomas said:
Does it feel like one cylinder doesn't fire for one cycle, or does the whole engine lose RPM or shake for a fraction of a second?
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Hard to definitively say. Some of the stumbles are worse than others, but generally, it seems as though it’s just a single cylinder that doesn’t fire for a split second. Seems as though I recall seeing the single probe EGT gauge move just a hair each time it stumbles.

Have you ever had any customers with similar experiences like this? I thought for sure it had to be ignition related. Of course I’m not ruling it out entirely, but now that everything has been redone, the odds seem very slim. I recall the engine behaving this way since I started flying it, but it seemed to go away for awhile and now it’s come back again. Probably worse now, than before.
 
“ Everything has been redone”. NOT.

See post #53.

It did not make sense to me then or now.

However ; I for one can assure you that Lycoming knows more about their engines then I do.

Likely others as well.

There are a lot of possible causes and many have been mentioned.

Don’t overlook the simple solutions.

I’ll search my paper books and see if that could help you get this resolved.
 
RyanB said:
Have you ever had any customers with similar experiences like this? I thought for sure it had to be ignition related. Of course I’m not ruling it out entirely, but now that everything has been redone, the odds seem very slim. I recall the engine behaving this way since I started flying it, but it seemed to go away for awhile and now it’s come back again. Probably worse now, than before.
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Ignition or other electrical systems are responsible for 90% of engine performance problems. It's hard to say how it could still be haywire, with two good mags, new harness and sparkplugs. That's why I had suggested the P-leads.

Reducing the throttle tends to lean the mixture a bit all by itself. The mixture control takes it further. A stumble during either of those implies a weak spark.

What are those mags? Bendix or Slick? Some Bendixes had an extra condenser on the outside of the mag to suppress ignition noise, and it's wired to the P-lead terminal, which puts it in parallel with the condenser inside the mag. That has the effect of increasing the overall capacitance, which can weaken the spark. It absorbs too much of the primary flow after the points open before the primary flow finally stops and the flux collapses. It makes the collapse later and weaker. If it's there you might try going without it. Or maybe someone has put noise filters in the P-lead lines on the firewall.

There are ADs on older Bendix mag switches and all ACS/Gerdes mag switches. The Bendix one is a safety issue--the switch might not ground both mags when it's shut off--and the ACS AD involves burned contacts inside the switch that can cause mag issues. The start contacts of the switch are right up against the mag contacts, and debris can make trouble. You have to crawl under the panel to see which you have. Don't trust the AD signoff your mechanic might have made; I have found many of them signed off as "Bendix mag switch tested and found OK" when the switch was actually an ACS. Many mechanics don't seem to know that there are more than Bendix switches out there. Cessnas are full of ACS switches.

https://rgl.faa.gov/Regulatory_and_...B8ABD56539B4684886256A3E00759DBF?OpenDocument
 
Dan Thomas said:
Ignition or other electrical systems are responsible for 90% of engine performance problems. It's hard to say how it could still be haywire, with two good mags, new harness and sparkplugs. That's why I had suggested the P-leads.

Reducing the throttle tends to lean the mixture a bit all by itself. The mixture control takes it further. A stumble during either of those implies a weak spark.

What are those mags? Bendix or Slick? Some Bendixes had an extra condenser on the outside of the mag to suppress ignition noise, and it's wired to the P-lead terminal, which puts it in parallel with the condenser inside the mag. That has the effect of increasing the overall capacitance, which can weaken the spark. It absorbs too much of the primary flow after the points open before the primary flow finally stops and the flux collapses. It makes the collapse later and weaker. If it's there you might try going without it. Or maybe someone has put noise filters in the P-lead lines on the firewall.

There are ADs on older Bendix mag switches and all ACS/Gerdes mag switches. The Bendix one is a safety issue--the switch might not ground both mags when it's shut off--and the ACS AD involves burned contacts inside the switch that can cause mag issues. The start contacts of the switch are right up against the mag contacts, and debris can make trouble. You have to crawl under the panel to see which you have. Don't trust the AD signoff your mechanic might have made; I have found many of them signed off as "Bendix mag switch tested and found OK" when the switch was actually an ACS. Many mechanics don't seem to know that there are more than Bendix switches out there. Cessnas are full of ACS switches.

https://rgl.faa.gov/Regulatory_and_...B8ABD56539B4684886256A3E00759DBF?OpenDocument
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Thanks, Dan! I’m going to have my mechanic take a look at the P-lead as well as the ignition switch as suggested! Will report back soon.
 
RyanB said:
Thanks, Dan! I’m going to have my mechanic take a look at the P-lead as well as the ignition switch as suggested! Will report back soon.
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Before removing any filters or condensers, the mechanic should check the wiring diagrams for that airplane. If the extra condensers are not optional you're sorta stuck with them, but they could be checked to see that they aren't failing internally.

Old airplanes are fun.
 
Dan Thomas said:
Before removing any filters or condensers, the mechanic should check the wiring diagrams for that airplane. If the extra condensers are not optional you're sorta stuck with them, but they could be checked to see that they aren't failing internally.

Old airplanes are fun.
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Fun indeed… :eek:

I assume this would not be something that would’ve been looked at during the 500hr inspection/overhaul?
 
Don

Thanks for finding that.

Some of the “ chrome hair crew” may be interested in knowing that the

change was recommended by the “ Mr. Lycoming “ of the era.

“ Mr. Honest Engine “ Joe Diblin.
 
RyanB said:
Fun indeed… :eek:

I assume this would not be something that would’ve been looked at during the 500hr inspection/overhaul?
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It's not part of the magneto and would have been taken off before they were sent in.
 
Magman said:
Don

Thanks for finding that.

Some of the “ chrome hair crew” may be interested in knowing that the

change was recommended by the “ Mr. Lycoming “ of the era.

“ Mr. Honest Engine “ Joe Diblin.
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I was learning to fly when that was issued. So long ago.
 
Back when a call got you Technical Advice not Legalese BS.
 
RyanShort1 said:
It sounds like a potential manifold leak...
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If so, why would it only show up at higher power settings and not at lower power settings, like an intake leak generally does?
 
RyanB said:
If so, why would it only show up at higher power settings and not at lower power settings, like an intake leak generally does?
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And you said the mechanic had checked it out and replaced some gaskets, right? Unless he missed a badly displaced connector hose, or a loose carb, it's not likely.
 
Dan Thomas said:
And you said the mechanic had checked it out and replaced some gaskets, right? Unless he missed a badly displaced connector hose, or a loose carb, it's not likely.
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Yeah he replaced a handful of intake and exhaust gaskets a couple months ago. Like you say, unless there’s a loose hose or carburetor, it doesn’t seem likely to me either, but that’s another thing worth looking at.
 
I read of a partially collapsing injector fuel line doing similar problems. No issue at low flows, only problem was reducing fuel flow from high to low.
Is such a situation possible here?

Sent from my HD1907 using Tapatalk
 
tspear said:
I read of a partially collapsing injector fuel line doing similar problems. No issue at low flows, only problem was reducing fuel flow from high to low.
Is such a situation possible here?

Sent from my HD1907 using Tapatalk
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It's carbureted. No injector lines.
 
These kinds of issues are where the multi probe EGT/CHT engine monitors earn there keep. If you had one you might have recognized and identified the issue by now, or even before you noticed the rough running.
Recently, had an engine (fuel injected) start showing abnormally High EGT on the #2 cylinder. Mag check showed no significant change so ruled out an ignition issue. Actually without the engine monitor we would not of known there was even an issue unless the single probe guage happened to be on the cylinder having an issue. Determined we could lean the #2 cylinder run lean of peak and run the rest of the cylinders significantly rich of peak to continue the flight with reasonable CHT’s to a location where we could get the injector cleaned or replaced.

Brian
 
Dan Thomas said:
It's carbureted. No injector lines.
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yup, but is the concept of a partially collapsing fuel line:
1. even possible with carbs (maybe upstream of the carb)
2. Would it exhibit the same behavior?

Tim
 
tspear said:
yup, but is the concept of a partially collapsing fuel line:
1. even possible with carbs (maybe upstream of the carb)
2. Would it exhibit the same behavior?

Tim
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If a fuel line to the carb was collapsing it would show up at full power, where the flow is the greatest, not during power reduction or leaning the mixture. The engine would stumble in the takeoff roll and climb and possibly cause an accident.

The occasional stumble or misfire is almost always ignition. If there's a weak spark in one cylinder it will cause an almost-constant misfire. If there's a mixture problem to one cylinder, leaning it will do the same. If there's an induction leak to one cylinder, letting air in, it will show up instantly at idle, where manifold pressure is lowest, causing the cylinder to suck lots of air through the leak.

This sort of intermittent misfire can be very difficult to trace. In my experience it's usually ignition. A sticking valve can do it, too, but a sticking valve will usually show up when the engine is cold, after start. The valve guides are bronze, which has a coefficient of linear thermal expansion twice that of the steel valve, and if the valve's clearance is disappearing due to carbon buildup on the stem, that guide will shrink around the stem as it cools and seize it enough to hold it open. It can seize it so hard that the pushrod gets bent during startup, trying to open a closed valve. That often shows up as a bent pushrod tube.

It's no fun, believe me.
 
Also recently dealt with a Magneto Condenser that would only Fail only when very hot. Never actually got it to show a problem during initial run up or even inflight mag checks. But if we did a mag check on 90degree+ day after taxiing back in or a quick fuel stop (when we discovered it), it would drop 1000rpm during the Runup. Wait an hour and it would run up fine.

I read your post earlier about the mags being rebuilt IIRC you said one checked and one rebuilt. Might be the one checked still has a problem. Were the Condensers replaced in both Mags? Have also heard the insulation in the coils can cause a similar issue. Also have heard of Sparkplugs that only show issues when running hot and with lean mixtures. Maybe try doing the mag check with a lean mixture, or a mag check in flight when you are detecting the roughness(an engine monitor would help in when doing this, but I understand you don’t have one)

Brian
 
Dan Thomas said:
It's not part of the magneto and would have been taken off before they were sent in.
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So I asked one of the A&P’s today about the possibility of a bad condenser, and he said that would have been replaced when these mags were sent in for OH. Evidently they didn’t do the 500hr SB, but rather did an OH. Does that sound right to you?

Nonetheless, the IA is going to re-check the intake tubes, carburetor, P-lead and ignition switch. Will report back on it soon.

BTW - can you be my mechanic? :p Wish you were closer!
 
RyanB said:
So I asked one of the A&P’s today about the possibility of a bad condenser, and he said that would have been replaced when these mags were sent in for OH. Evidently they didn’t do the 500hr SB, but rather did an OH. Does that sound right to you?

Nonetheless, the IA is going to re-check the intake tubes, carburetor, P-lead and ignition switch. Will report back on it soon.

BTW - can you be my mechanic? :p Wish you were closer!
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Sounds right. The 500-hour thing is an internal inspection. An overhaul replaces the bearings and distributor and some other stuff. For a Slick it's almost as costly as a new mag.

I'm retired. Occasional work might be ok. But an intermittent thing like is can be really hard to find if it won't just die and stay dead so one can figure it out. I'm happy to leave that frustration to your mechanic:)
 
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