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Mag timing most foul

LettersFromFlyoverCountry

LettersFromFlyoverCountry

Well Known Member
I need someone to look over my shoulder and tell me what I'm doing wrong.

Just got the Slick 4371 mag (with impulse coupling) back from inspection. Set the engine to 25 degrees BTC and, with timing pin in, attached said mag, and reinstalled hold down nuts loosely.

Attached light in which open points are indicated by LED going out and beeping starting.

Adjusted mag til I found the sweet spot and tightened (removed timing pin before doing so). Then swung the prop around and the next ti!me around on the #1 compression stroke, the points didn't open until almost TDC.

I repeated this process several times with the same results. What am I doing wrong?

Video here.


https://youtu.be/hcsaQp9Iz-I
 
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At low rpm (like turning by hand) the timing is retarded to avoid kick back on starts. Repeat, but this time turn prop to TDC or more - you want to hear the retard points snap - then turn backwards 35 degrees or so, then forward. Your lights should go out at 25 deg before TDC.
 
mag timing

I believe you have done it all correct. The points are opening near TDC because you are winding up the impulse coupling. If you let the impulse coupling snap and then back up the prop (flywheel) to before 25 degrees BTDC, then slowly move the prop till the beeping starts. I think the timing light will start beeping right at 25 degrees.
 
Yep I just went through the same thing. Right around TDC you should hear the impulse coupling snap. Then rotate backwards till just before 25* BTDC and check from there.

Chris
 
A quick word of advice - don't have your mag timing light hooked up to the P-lead connection when the impulse coupler snaps. There's enough energy on the P-lead to fry the cheaper mag timers (that's how our EAA chapter's mag timing light got fried, or so the story goes...)
 
Era ah number 3 Bob?

Just watched your video and you referred to the cylinder as number 3. On a Lycoming that is number 1 which is the correct cylinder to time. Lycoming number from front to back with number one on copilot side. Continentals are numbered from back starting on copilot side.

The timing difference you are seeing is the "retard for starting" due to the impulse coupling being engaged. The other posts are correct on how to time.

That is a fancy way of finding TDC. Your thumb over the spark plug hole will find compression and then look for the timing marks either on the front of ring gear or the back.

Good free book FAA-H-8083-32-AMT power plant vol 1 chapter 4 starting at page 22. Available online .pdf

Tyler N174MT
A&P
 
Exactly as others have said, if set at 25 btdc, the impulse coupling will snap right at tdc, which can be read on the back of the ring gear lined up with the case split and the front of the ring gear lined up with the mark on the starter. When timing, you always want to pull the prop past tdc then back to just before the desired timing advance. If you are ever wondering if your mag(s) has(have) lost their timing setting, you can use the impulse click to see if it's off tdc. You can only do both this way if both have impulse couplings.
 
One thing to also be aware of when doing this is that when the impulse coupling snaps you will get a spark. One of the reasons you do this with the plugs removed so the engine does not kick over.
 
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