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By dont overfill
The last few mornings the temperature has been around minus 5 and the Lycoming O-235's have been more difficult than their usual just a bit difficult to start.

They all have the light weight starters and turn over fast but usually are more likely to start with a hand swing.

I vaguely remember something about changing the impulse mag spring if a light weight starter was fitted. Anyone got a reference?
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By T67M
It may not be fast enough to disable the impulse coupling, but could it be fast enough to shift the timing of the impulse so that the cylinder is already well into the power stroke before the spark plug fires? A slightly earlier timing might give more chance for a successful start.
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By Paul_Sengupta
Possible. Just googled a little and came across this thread about cold starting O-235s. It says that the E gap should be spot on otherwise there will be problems.

"I have a Sky Tec starter. I don't think that is the reason for the hard to start in cold wx with the E-GAP out.

Slick recommended marking the mags when you put them on new. After you had "bumped" them 1/4" from the original marks, i.e. reset the timing during annuals, you should reset the E-gap. I did not heed this warning. June 2004 in Ste. St. Marie, Michigan on a Friday morning it was 35 degrees and it took forever to get the engine started. Ran the battery down, jumped it, checked the plugs, hand propped it, recharged the battery and finally got it going. The next fall I had the same problem. I then remembered a Slick seminar I had attended where I was told to reset the E-Gap at 500 hours. The Mags had slightly more than 500 hours on them so I reset the E-Gap and no more cold start problems. Several years later in the fall again no start. I checked my log books, 450 hours since E-gap reset. I reset the E-gap and again the problem was fixed."
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By Paul_Sengupta
Though this


"The objective of the impulse coupling is to delay magneto firing from 20° before top-dead-center until about 5° before Top-Dead-Center by building in a 15° lag angle into the magneto impulse coupling. This happens only in theory.

The degree of impulse coupling retard is directly proportional to crank speed. Take for example the Lycoming O-235-L2C engine. The original starter cranked the engine too fast. Lycoming Service Instruction 1362 changed the lag angle of the impulse coupling from 15° to 5° to compensate for the fast turning starter and greatly improve starting on this engine. (with the 5° lag angle the engine may have a tendency to kick-back during hand propping. Don't hand prop!)

A later starter (Prestolite MMU-4001R) was specifically designed to turn the engine slower. But be careful now, this slower starter works with the original 15° impulse coupling so the magneto impulse coupling lag angle should remain at 15°.

Now lets say you purchase a starter or magnetos for your O-235L2C engine. To get optimum starting performance you need to match the starter with the impulse coupling.

The same hard starting problem may occur if you switch to a faster turning light-weight starter and don't change the impulse coupling on any impulse coupling magneto."
By cockney steve
AIUI, there is NO automatic Advance- Retard an aircraft magnetos.
There is no need, as the engine spends most of it's life in a very narrow speed-band, so a fixed timing is perfectly easy to optimise and simplifies things. Unfortunately, the advance needed for 2500 RPM, is far too early for the slow rise of the piston when using a starter or hand-propping....thus, the ignited mixture can expand and produce power before the piston/crank has cone "over the top"....Piston is stopped and reverses direction before reaching the TDC position (The classic "kick-back" )

The Magneto is really 2 items in one,- a generator and a transformer (ignition-coil) Timing the "break" ensures that the primary coil produces it's optimum output at the right time. and the points, driven by the same
revolving armature, break that current at exactly the right time . this sudden interruption in the current causes the induction of a voltage into the secondary you get a spike if output, which is taken to the spark- plug.

Instead of a direct coupling to the engine, an Impulse- drive is arranged to drive a "clock-spring" and the magneto is furnished with a centrifugal catch which , when it reaches it's stop, stops the magneto revolving.....continued rotation of the engine winds up the spring, which has it's other end fixed to the magneto-side of the coupling....continued turning then brings the "trip" round to release the catch.....the spring unwinds, "****" the magneto round smartly and producing a nice fat spark.....the delay in timing is a product of the number of degrees rotation the spring winds up until it's released . When rotational speed id high enough, the centrifugal catch parks and the spring is sufficiently tensioned that it holds the mechanism firmly against it's stop in the "unwound - run" position.

From this, you will see that too-fast a cranking- speed will disable the impulse- coupling.

Said coupling not only retards the timing when it operates, but because the magneto gets a fast "break" it makes a strong spark.

A simple device that is a very ingenious solution to 2 problems.

!- sufficient rotational speed to generate adequate output.
2- a later spark to ensure the power of the ignited mixture is produced after Top Dead Centre.

Hope this is helpful.
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By Dodo
Slight thread drift, but am I right in thinking that with key turn ignition switches that have Off, Right, Left, Both, start positions that "Start" earths the right mag so that it is only starting on the left mag.

My ancient Pa 28 140, has a separate starter button, so I have Off, Right,Left, Both only and the rule of thumb I use is left for most starts and both for a hot start after refuelling to taxi to park. Seems to work.
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By T67M
Most often correct, @Dodo , but not always. A few aircraft do start on both mags. It gets a whole lot more complex when looking at non-Lycoming/non-Continental/non-Avgas engines.
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By Rob L
matthew_w100 wrote:My Pup POH says left mag only start. My engineer says both. Neither works when it's cold.

Your engineer may be right if you have both mags with impulse (perhaps when your POH was written, only the left mag had so).

As for your cold starting, that's another issue.
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By Lockhaven
I find priming the engine first (6 primes) then pull the prop through half a dozen times before trying to start it with the electric starter works well in all temperatures.
Seems to give the fuel air mixture time to start evaporating before cranking, starts first time every time after a couple of rotations with only the left mag on to start.
By riverrock
Issue (I understood) is that when you pull prop through, you remove the film of oil from the cylinders, increasing considerably engine ware during start? Certainly turns more easily though!