Carbon Cub Rough Running Engine

[Steve Pierce]

Anybody had an issue with light Speed ignition cutting out. I have a carbon Cub here that keeps cutting on and off. Really gets your attention. Does it on left, right, both, emergency, with the key switch disconnected. Have checked all connections, read the book twice. called Klaus but he is in Germany and I think I irritated him. I don't want to start tearing things apart but hen you get up to pattern altitude and the engine starts missing like you are turning the key on and off it gets your blood pumping. Cub Crafters has never heard of this scenario either so i am hoping someone here might have some knowledge. I am out of ideas.

 

[Bill Watson]

I had a very minor miss in flight, leaned the engine and determined which cylinder head temp would not come up then traced plug wire to proper coil and found a small spade connector loose, crimped it and solved the problem. Again it was minor.

 

[Pete D]

Did this problem turn up after recent work? Confirm clearance between plug wires and sensor wires. Also check clearance of pickups to flywheel. Manual has a spec for that.

 

[Steve Pierce]

Starter was changed about 2 flight hours prior. Spade terminals are tight. Have about .040" clearance on both crank sensors, manual calls for .030- .060". Have at least a 1/2" clearance on the between the sensor wires and anything else and quite a bit more than that between them and the plug wires. Appreciate the feedback, I'm still looking and reading.

 

[N86250]

Water in the fuel?

 

[wireweinie]

IF it's an electrical issue, check the common points. Grounds, breaker, switches, etc. Maybe hook an LED to the power terminal of each and run it up. If the LED flickers, you have intermittent power.
Dumb question but as N86250 brought up, are you sure it's an electrical issue?

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[Steve Pierce]

Tanks sumped, gascolator drained and checked along with carburetor bowl with no water found. I have run tanks dry and switched to good tank and this feels like you turned the ignition switch on and off. It isn't like one plug is not firing it is like you are turning the engine on and off.

 

[Farmboy]

Blocked fuel flow? Although it sounds more electrical related. Any chance of corroded terminals supplying grounds to the light speed itself? Whole system is losing connection?


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[tempdoug]

Did the check engine light come on?:evil:

 

[C130jake]

Klouse is German...don’t take it personally. He seemed really irritated with me when I had some issues on my first engine start. CC and the guys in Kamloops were stumped. Klouse listened on the phone and told me it was an induction leak. It was.

I am sure you checked the ignition switch. Any chance of wire chafing thru the firewall?


Jake


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[spinner2]

Assuming this is the O-340 Steve, how many hours on it? It is recommended to change out the leads between the coils and plugs after 500 hours. Otherwise the misses get worse and worse. I went 700 hours on mine before changing and won’t do that again. I thought I had big problems with the Lightspeed boxes but this was a simple fix. Leads are about $20 each.

I also had the scat leading into the air box get a loose flap on it that partially closed off the air flow and caused a cough, like carb ice.

A dead spot in the key switch shuts it down if you do an ariel L/R check. That gets your attention too.

 

[wireweinie]

I'm assuming it is the dual set up? If so, it will be a common point for both sets. That's why I'm thinking breaker/switch/grounds. After looking at the diagrams, I'd open all backshells on the d-sub connectors and check for broken or chaffed wires. Also check closely for any stray strands of wire at one socket, touching an adjacent socket.

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[Steve Pierce]

Checked the trigger wires and moved them a little. Checked the entire set of wires and found no abnormalities. Took the d-sub connectors off the two ignition boxes and then opened the connectors and everything looked fine. Key switch is disconnected from the system and using the circuit breakers to isolate the systems. Ran it with the tail tied to my truck and made a video that will take a while to upload but it should give you a better idea of what it is doing.

 

[Gordon Misch]

A shot in the dark, knowing nothing about these systems - - Is it possible that the two units communicate with each other in any way, such that one could share a fault with the other?

 

[Steve Pierce]

Supposed to be completely independent. I am starting to think I have something else wrong.

 

[Gordon Misch]

Could you possibly hang an electric fence tester on a plug wire to observe if the spark continues when the engine cuts out?

Or maybe remove the lead from one plug, attach it to a spare plug that's in free air but grounded and watch the spark?

Check correct fuel flow to the carb?

 

[flynlow]

Did you pull the finger screen/strainer on the carb?

 

[Steve Pierce]

I did about 10 hours ago when I did the condition inspection. Gascolator screen was clean both then and now. Drained the carburetor again last night before I came home. All cylinders were 76-78/80 except one which was 70/80. Loaned the owner a set of compression gauges and my hose for automotive spark plugs but haven't gotten them back yet but pulling it through I can tell I have a soft cylinder and hear it through the carburetor.

 

[Steve Pierce]

Here is the video I took of my engine run with the airplane tied to my truck. I went through the left, right, both and emergency ignition system with no change in how the engine was running. Sorry it is so shaky, that left window really flexes.

 

[RaisedByWolves]

What’s Jon at cubcrafters have to say?


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[skywagon8a]

Steve,
Judging by the audio and tach indications in your video it doesn't appear to be rpm related which is making me think that it is not ignition. Not being familiar with that model airplane, spinner's suggestion sounds plausible. Can you completely remove any intake tube and retest?

I also had the scat leading into the air box get a loose flap on it that partially closed off the air flow and caused a cough, like carb ice.

 

[Steve Pierce]

What’s Jon at cubcrafters have to say?


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Jon though fuel or induction leak Friday before last. Has the hoses and they are all tight. Gonna run fuel through the carburetor and try again.

 

[Steve Pierce]

Steve,
Judging by the audio and tach indications in your video it doesn't appear to be rpm related which is making me think that it is not ignition. Not being familiar with that model airplane, spinner's suggestion sounds plausible. Can you completely remove any intake tube and retest?

I am not varying the throttle, put it 2200 rpm or so and left it.

 

[Steve Pierce]

Steve,
Judging by the audio and tach indications in your video it doesn't appear to be rpm related which is making me think that it is not ignition. Not being familiar with that model airplane, spinner's suggestion sounds plausible. Can you completely remove any intake tube and retest?

Also, I was switching between the two ignition systems in that video and there was no change what so ever. I think if it had a bad ignition lead it would change depending on which ignition system, coil, plug wire and spark plug I was firing.

The system has two ignition boxes run from two separate buses running 2 separate coils each and an emergency battery that can isolate everything on the right side ignition system. The only common thing was the conventional L-R-Both-Start switch which I removed from the system.

 

[Bill Watson]

I wonder if priming while it is missing would change the RPM? Maybe give a clue to fuel or air problem.

 

[flynlow]

I have had this happen with high time irrigation engines. They build up carbon and it gets hot and fires off mixture before compression stroke. We would water inject them but I wouldn't advise that.
You may have a cracked piston from detonation or broken rings but that would be a result and not likely a cause.

 

[skywagon8a]

It does sound a lot like an intermittent air intake blockage. Check that it is not sucking in some of the induction hose.

 

[flynlow]

Dang, I deleted my thought on sluggish or sticking intake valve. It sounds like a intake backfire to me. Maybe a valve not getting completely closed before ignition on compression stroke.

 

[spinner2]

There is a SB on the induction hoses if it has the glue-on style. Easy to tell if they’ve been changed. The new style looks like the Lycomings with hose clamps on each end.

The old style developed leaks.

 

[Steve Pierce]

I have done the SB on the intake hoses, heard some pretty good horror stories on those. Also installed the insert in the induction and the peppered fuel nozzle.