More F-20 Questions?

erskine2017 · Jul 11, 2017

Thanks to everyone who responded to my F-20 Idle questions. I have some more timing questions. I presume that with the advance lever retarded that you want the impulse to snap when the TDC mark appears in the window. Question #1) What should the timing be with the tractor running and the advance lever still retarded? Question #2) What should the timing be with the engine running and the lever fully advanced?

Oldiron29 · Jul 11, 2017

Rough timing Full advance TDC Than retard to start!

SW EM · Jul 11, 2017

There is 35 degrees of advance by moving the spark control rod. If I understand correctly from all the F-4 magneto articles I've read on this forum, the impulse doesn't affect timing. The points will close at the same regardless. The impulse just aids in retaining energy in its spring so when it releases the rotor turns quick enough to generate a spark. (The rotational speed of the rotor when the impulse releases it is approximately the same speed as if the engine were at idle.)

James Williams · Jul 11, 2017

Your mag in the off position should snap at the TDC mark centered in the sight hole,Your delay or retard allows your mag to fire a little late to keep the motor from kicking backwards

teddy52food · Jul 11, 2017

Way late. Engine will sound like it is lugging with no power. tdc for starting & advance from there.

n9lhm · Jul 11, 2017

An impulse coupling HAS to retard the spark. That's how it works. The output shaft is stationary while the input shaft continues to move, winding up the spring. The difference between when it starts winding up the spring and when it releases is the lag angle, or the amount of retard.

The determining factor of when the plug fires at cranking speed is when the impulse trips. That MUST be set at or slightly after TDC to avoid injury when starting the engine. The position of the spark rod only influences the timing after the impulse coupling disengages.

erskine2017 · Jul 11, 2017

I am still confused. If the impulse HAS to retard the spark, then timing must be advanced after the impulse disengages and the tractor starts to run. I understand how the advance lever works but my question is how much difference between impulse timing and run timing both with the lever retarded.

n9lhm · Jul 11, 2017

I'm not sure what the lag angle of the F-4 mag is, and the I-H magneto book (at least the one I have) doesn't say. I set mine so that the impulse trips at TDC, so the timing at cranking speed CANNOT occur before TDC. I start and run mine with the spark lever full down (advanced) and it starts and runs just fine. When I pull the lever up to shut off the engine, it labors and slows down as I approach the point that the shorting tab kills the spark. So my running advance is whatever the lag angle of the impulse coupling is. It doesn't need much advance, it's only running 1600 RPM max.

rustred · Jul 11, 2017

basically nothing. the impulse speeds up the magneto as if its running at a slow idle. mag. should be set at TDC inretarded position. advance manually as required.

steelfronts · Jul 11, 2017

(quoted from post at 12:12:38 07/11/17) I am still confused. If the impulse HAS to retard the spark, then timing must be advanced after the impulse disengages and the tractor starts to run. I understand how the advance lever works but my question is how much difference between impulse timing and run timing both with the lever retarded.

Ok remember its an impulse Coupling what it does is make a connection between the engine and mag. This is not a direct solid connection. The coupling has a catch that will stop the magneto from turning but allow the engine to continue turning. As this is happening the engine turning is loading a spring in the coupling.When after the engine has turned some degrees and the mag hasent the spring is loaded. A pawl is tripped unloading the spring coupling letting the mag catch up to the engine in that time the mag spins faster due to the spring and has a better chance of generating a spark hope this helped

More F-20 Questions?

erskine2017

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Oldiron29

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SW EM

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James Williams

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teddy52food

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n9lhm

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erskine2017

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n9lhm

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rustred

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steelfronts

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