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Intermittent Rough Engine

mattwood

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Hi, All:

I've searched the forums and haven't been able to find a solution to my problem - hoping for some insight.

I purchased an RV-7A about a year ago. O360 with dual Lightspeed ignition. Normally it starts up and runs beautifully, and if it does it runs like a top for the duration of the flight.

Occasionally it starts rough, and CHT on #1 doesn't come up, so appears to be that cylinder. If I do a hot start procedure to clear the cylinders and let it sit for a minute, it will usually start up w/o problems. If it starts rough, it never switches to clean. If it's running rough, isolating the 'mags' (it's Lightspeed electronic ignition) has zero effect on how it runs, so I don't think it can be a bad coil.

During the hot months here in Texas (500' MSL), if I go full power with full mixture on the initial takeoff roll, then it can switch from clean to rough but never on touch and go's. Last weekend I forgot to enrich the mixture before doing a runup, and it switched to rough. Again, if it's rough, I have to do a shutdown and wait a bit to restart for it to start clean.

I've replaced the spark plug wires, the plugs, and intake hoses. The builder completed the plane in about 2010, and it has 400 hours since then. He said it did do this occasionally before I purchased it (I knew this was an issue when I bought it).

It's only sometimes a problem, and so far it's never switched to rough running when I'm in the air, but that of course is my worry.

Best regards,
 
Always isolated to the #1 cylinder? You may have a sticking exhaust valve.

+1

I agree. Symptoms say sticking exhaust valve. Stuck intake will cause much more commotion. You have covered most other possibilities I believe.
Further, if an exhaust valve sticks and stays off the seat it may heat up more and stay stuck - consistent with what you are reporting.

I would borescope the cylinder and go from there. If the valve looks uniform at the seat edge it may be possible to deal with it without pulling the cyl. Maybe RocketBob or another has the current best answers posted. Compressed air/rope trick then some mouse milk, electric drill I am thinking.. And measure stem wobble.

Ron
 
I wrote this post a couple of years ago. It does apply to your engine. It sounds like you have a sticking valve. This normally occurs around the 400 hour time frame. I would not run the engine again until it is checked out and the SB is complied with. If it gets worse, the exhaust valve could stick and contact the piston and ruin your day! Here is the link


https://vansairforce.com/community/showthread.php?t=128848
 
Thanks for the comments. I'll have my mechanic look at it and see what he finds. I'm relatively new pilot (130 hours) so mostly doing pattern work, which is probably exacerbating the problem by not having long periods of high temps to clear things.

Thank you again - great community.

Matt
 
Thanks for the comments. I'll have my mechanic look at it and see what he finds. I'm relatively new pilot (130 hours) so mostly doing pattern work, which is probably exacerbating the problem by not having long periods of high temps to clear things. ...

Matt, I recommend looking at this impressive thread which demonstrates how to fix a valve seat.

https://vansairforce.net/community/showthread.php?t=188764

There is some overlap with what you need to do, and there are threads that talk specifically about the valve guide, like this one.

https://vansairforce.net/community/showthread.php?t=128673

Definitely recommend getting a borescope. I know I learned a lot from having one and these VAF threads.
 
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Is this problem limited to cold starts only or have you experienced it when the engine is warm?
 
Rough engine

If you find it’s not a sticky Valve, look to see what series light speed ignition you have.
If you have the Hall sensors that replace the mags one or both could have lost their timing. Depending on settings with ing on the little green lights on the back of the sensors should light at either TDC or 20deg TDC.
 
While I definitely recommend the wobble test to rule in/out a sticking valve, I can't help but think that the symptoms don't match. It seems odd that a shut down and restart will un-stick the valve. Also odd that it would start and run fine up until T/O, then stick. I also would suggest more research on ignition anomalies that could cause this.

Larry
 
Thanks again. I was also thinking the symptoms are not exactly sticking valve, since it *never* smooths out if it starts rough. Also, as another example, once I forgot to push in the breaker for the electronic ignition before turning it over. Sure enough, it had a rough start once I reset. But if I do things in proper order, it basically starts great every time. It's really only on restarts that I have a problem - which is why I was thinking maybe it was electronic. But even if running rough isolating one 'mag' then the other has zero effect.

I'll look into the Hall effect sensors - we hadn't thought about that.
 
Also, as another example, once I forgot to push in the breaker for the electronic ignition before turning it over. Sure enough, it had a rough start once I reset. But if I do things in proper order, it basically starts great every time. It's really only on restarts that I have a problem - .

certainly sounds like a s/w issue in the EI. I would call the mfr looking for common s/w faults that match your symptoms.

Larry
 
ROCKER ARM

Take a look at the rocker arms, sometimes the intake rocker gets installed on the exhaust valve, valve gets to hot gets carbon on it, sticks, Exhaust rocker has small hole in it squirts oil on the exhaust valve keeps it cool,
I have found this on quite a few engines, Lycoming has a service bulletin on this
on later engines both rocker arms have the hole. mike
 
Following up, it turns out that I had 3 - three! - broken valve springs (fortunately on 3 different cylinders). The cylinder shop (Sal's in SE Texas) that my A&P sent it to replaced all the springs, 4 new exhaust valves and guides, worked on intake valves, etc., and now it runs like new. Starts first time every time.

Moral of the story: when the engine is talking to you, listen.
 
causes?

Reading around in the car forums - possible causes can be rust (makes sense, a rust spot will cause a stress riser on the spring)

Or a batch of bad spring steel.

If you know how old the springs are, were they Lycoming or Superior or ? Hours etc. It might be good to file a service difficulty report https://av-info.faa.gov/sdrx/

If you have or can get back the broken springs, hang on to them, the FAA might want to examine them. Before blowing off my suggestion consider someone flying their RV around the world, across the Atlantic, etc. Your outcome was not a big deal but it could be different for someone else. Just my 0.02.
 
Reading around in the car forums - possible causes can be rust (makes sense, a rust spot will cause a stress riser on the spring)

Or a batch of bad spring steel.

If you know how old the springs are, were they Lycoming or Superior or ? Hours etc. It might be good to file a service difficulty report https://av-info.faa.gov/sdrx/

If you have or can get back the broken springs, hang on to them, the FAA might want to examine them. Before blowing off my suggestion consider someone flying their RV around the world, across the Atlantic, etc. Your outcome was not a big deal but it could be different for someone else. Just my 0.02.
I agree with Bryan - we can all learn a lot from these kinds of failures, and if you don't read a forum like this daily, you'll struggle to find them.
 
I've been away from the forums. A&P tossed the springs in the trash in front of me - I didn't see a reason to hang on to them, although if he'd handed me the bag instead I probably would have.

I'd done borescope inspections and there was a *lot* of buildup on the exhaust valves, so I'm sure that was part of it. All 4 valves and guides replaced.

A friend of mine who is a 16,000 hour pilot multi-engine commercial jet rated and former associate dean of Florida Tech Aviation school suggests the timing might be advanced too much, so I'll get that checked out. If anybody has recommendation for A&P to do the timing in the DFW area, please let me know.

As far as I know the engine hasn't been over revved - I suspect not.
 
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