Hot starting io-540
- Thread starter drotto
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drjcustis
Pre-takeoff checklist
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Drjcustis
In the case of my engine when I had this similar problem, and is what I think you have, the mag was weak. At cold temps, with priming there was an even and proper fuel air mixture in the cylinder and the weak mag was able to ignite it. The plane ran fine and the normal mag check was fine. At higher temps, the air fuel mixture is uneven and difficult to know if lean or rich. That’s why there’s so many recommendations to flood the engine and then use those procedures, because at least you know the starting point.
Take John Deakins engine course. I learned about mag checks and diagnosing electrical problems (along with a ton of useful info). I do my mag checks significantly different now.
In my case, I took the plane to a different mechanic. The mag failed and refused to start the plane. An hour later with a new mag and the plane starts great.
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flyingcheesehead
Touchdown! Greaser!
I don't have a Lycoming, but I do use @Ted DuPuis' works-for-anyone hot start procedure and it is by far the best of any I've used. And since he used to work for Lycoming I bet it does work on them. 
It's simple: Prime as you would for a normal (cold) start. Then pumps off and engine controls fully forward, and start cranking while pulling the throttle back from full. Works every time unless something's broken, no need to do the three-handed trick, and you don't have the big RPM spike/rev if you don't do the three-handed trick fast enough.
It's simple: Prime as you would for a normal (cold) start. Then pumps off and engine controls fully forward, and start cranking while pulling the throttle back from full. Works every time unless something's broken, no need to do the three-handed trick, and you don't have the big RPM spike/rev if you don't do the three-handed trick fast enough.
drotto
Cleared for Takeoff
Well, plane is in annual and shop says he sees 2 problems with the mags. They have 310 hrs on them.
First the impulse coupler is not catching properly. He showed me on my mag that when you turn the coupler by hand ( very low speed) it should grab and turn the shaft immediately, and not slip. Mine is slipping often, so it is not turning at the right speed and producing the spark at the wrong timing. It is catching enough to start the engine cold, but when hot the metal expansion is causing it not to catch at all. No catch, wrong timing, not enough spark, no hot start.
Second, there seemed to be some excessive wear on the metal fairing/disc ( not sure on exact part name) just above the impulse coupling. He had me feel it, and the edges were burred to touch. He said he could fix the coupler but did not have the machines to rework the shaft.
A new mag is on the way. Hope it fixes everything.
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First the impulse coupler is not catching properly. He showed me on my mag that when you turn the coupler by hand ( very low speed) it should grab and turn the shaft immediately, and not slip. Mine is slipping often, so it is not turning at the right speed and producing the spark at the wrong timing. It is catching enough to start the engine cold, but when hot the metal expansion is causing it not to catch at all. No catch, wrong timing, not enough spark, no hot start.
Second, there seemed to be some excessive wear on the metal fairing/disc ( not sure on exact part name) just above the impulse coupling. He had me feel it, and the edges were burred to touch. He said he could fix the coupler but did not have the machines to rework the shaft.
A new mag is on the way. Hope it fixes everything.
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Including mixture?
flyingcheesehead
Touchdown! Greaser!
Yup. While you're over-rich, the full throttle when you start cranking lets in lots of air, and as long as you don't pull the throttle back too fast, you'll get to the point where the mixture is just right, and it'll start right up. As a bonus, since you've already got your hand on the throttle and you're already pulling it back, you can pull it to idle as soon as it catches and avoid the big over-rev common to many hot starting techniques.