migfighter42(at)gmail.com
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 Posted: Fri May 29, 2015 7:49 am Post subject: Yak-List: Re: Engine roughness, as in stops. starts. stops. |
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Ditto.
Keep in mind that for a lot of us, the mags went in or were replaced at the same time. Mags have a lifespan and when they fail, this is how they fail. if they are installed at the same time, they will fail in similar time spans. Also keep in mind that a mag is not just off and on from a load perspective. The harder the engine is being run, the harder the coil has to work to provide adequate spark across the gap. This work generates heat which affects the windings in the coil. If they cycle hot enough over a long enough time, the coil will internally short. when it cools off, the short can go away. Once it starts down this road, it will completely fail soon.
In our case, one coil had an intermittent failure and the other coil was unable to take the full load by itself. Mark’s approach for flying with just one mag on and then the other showed which mag was going bad and which one was getting ready to go bad. Test it over your field 😊
We replaced both, and the problem Immediately went away. It is not terribly difficult to replace them, call Jill at M14.
Bill
(not an A&P mechanic, but I am an electrical engineer and I did stay at a Holiday Inn last nite!)
PS, Ernie - L-29’s don't have this problem, you should not have sold it!
Sent from Windows Mail
From: Bitterlich, Mark G CIV NAVAIR, WD
Sent: ‎Friday‎, ‎May‎ ‎29‎, ‎2015 ‎9‎:‎58‎ ‎AM
To: yak-list(at)matronics.com
--> Yak-List message posted by: "Bitterlich, Mark G CIV NAVAIR, WD" <mark.bitterlich(at)navy.mil>
Might as well add my 2 cents. First of all, I totally concur with Richard Goode, not only because he has been around these aircraft longer than most anyone else on this list, but also because I have experienced the exact same symptoms troubleshooting a YAK-52 just a few months ago.
Very likely NOT the carb. And this is not meant as sarcasm, but after some "mechs" mess around with it for a while it might very well end up being that as well. Very few people know how to adjust these pressure carbs properly and there is much more to it than turning the "screw" on the outside. Of course there is nothing like "doing" to help in learning, as long as it is on your aircraft and not mine please. What I am saying here is that messing around with the carb should be LAST on anyone's list and never first. Also it is the nature of pressure carbs that if you adjust one thing, it impacts another. Just had a Bendix PS-7BD rebuilt and flowed ($3000 by the way) and while not Russian, the theory is the same and worth reading about.
There is a very simple test to run here. Go up and fly it one more time and just simply fly it until it starts doing this again, and then switch off one mag and then the other to see if it then STOPS doing it or gets much worse on one. It is very unusual for both mags to have something go bad with them at the same time (thankfully). STRONGLY recommend this before applying fingers or tools to the carb.
The problem with the coils is legendary. If you own one of these aircraft it is not a matter of "if" you will have a coil go bad, but when. Other things can cause the coil to over-heat faster and these range from wide gaps on spark plugs, to misadjusted points "or dwell" as mentioned. Also, I have seen the point assembly actually crack leading to erratic ignition (you mentioned that one yourself). Points will not usually come out of adjustment through normal use enough to cause what you are experiencing, but if they are cracked and flexing they can cause these exact symptoms too.
I have had two personal experiences with mag issues on my own aircraft. In one case, the internal gearing in the mag (M9F) let go, and the engine just STOPPED. Ernie (in another message) mentioned that this should not happen should it? Well, that is what we are all taught, but as it turns out it is not necessarily accurate. I did not time how many seconds it just STOPPED, but it was enough time for me to think: "Oh crap, I am going to have to land this thing on the freaking OUTER BANKS! Should I go in gear up or down?". And then it came back running rougher than blazes. I tried the mag switch, and it ran on one mag perfectly and on the other it just stopped. So why did that happen? It should not have. But it did. Lots of "might be maybe" theory, but honestly I do not know.
Second time was a mag coil. Pretty much exactly what you are talking about here. Again, simply switching between mags in flight isolated the problem very quickly. Why did it impact engine performance on both? Well, all I can think of is a spark plug firing way in advance. In my mind, only three things can happen with ignition. It doesn't work at all, it fires early, or it fires late. Not working at all would not make the engine cough and gag, UNLESS you had a problem with the OTHER ignition set that you didn't know about (bad plug, etc.) If it fired late, it would not matter at all. But if it fired early... way early, as the piston was coming up on the compression stroke ..... some weird stuff would happen, bad detonation and engine coughing and puking being one. So that is my "theory".
If this test is performed in flight and it does it again on EITHER MAG, then the next step is a compression test. But really, you need to fly it one more time.
Mark
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