Ignition timing for IC mag

I thought I knew the answer to this, but all of a sudden I've gone brain dead!


Lycoming O-360
90 SMOH
Slick 4373 Left, Slick 4370 Right
Usually started on 1st blade!

I'm now waiting for my starter to come back from being overhauled due to a rather severe kickback. So, start to look for the likely culprits.

First check that the P-leads are intact on the mags mag? Yup

Next, check that ignition switch is killing the right mag on starting? Yup

Next, check the timing to verify it's close to where it was? Yup

Next, check that the IC is tripping at TDC? Yup

OK, never paying attention to it much before, I am getting a timing buzz at 25 BTDC on the IC mag (Left one) BEFORE I trip the IC! I've never paid attention to this before, since standard technique is to trip the IC, then move back to check the timing.

Need a reality check PLEASE! Should I get a timing buzz before the IC trips?

If not, I've solved my problem, at least until I pull the mag!

If so, where do I look next for the source of the kick back?

Help!

Do you crank the engine with both mags 'on'? The 4370 does not retard so it will fire at 25 degrees and possible kickback. If it were mine I would crank it with only the left mag hot (4373).

I'm thinking it should not buzz @ 25 degrees unless the couple has kicked over but I'm not 100% sure. After it has kicked over where does it buzz?

Sid Lambert said:

Do you crank the engine with both mags 'on'? The 4370 does not retard so it will fire at 25 degrees and possible kickback. If it were mine I would crank it with only the left mag hot (4373).

Click to expand...

The ACS ignition switch disables right mag during starting to reduce the likelihood of kickback. Checked and functioning properly.

I'm thinking it should not buzz @ 25 degrees unless the couple has kicked over but I'm not 100% sure. After it has kicked over where does it buzz?

Click to expand...

My thought also, but never really paid attention to it under normal circumstances. It buzzes at 25 BTDC with the IC tripped or not!

Does the...

...Impulse Coupler "click" when you pull it past TDC?

The noise should be very audible.

IC mag timing

I'd pull the prop thru at least 1 full cycle, then stop at where IC trips near TDC on #1 cylinder, then back up until points close (sych light on). Come forward again, and points should open (synch light off) at +-25 b4 TDC. If latter does not happen, something is wrong. This is how mine works.

Usually kickback would be from too much timing advance, i.e. 25 BTDC, at low rpm, not too little, i.e. TDC.

az_gila said:

...Impulse Coupler "click" when you pull it past TDC?

The noise should be very audible.

Click to expand...

Impulse coupler trips at TDC just as it is supposed to.

However, I am seeing my points triggering at 25 BTDC BEFORE the impulse coupler trips, as well as after!

I don't believe I should get a points trigger at 25 BTDC until after the IC trips, but I've never checked.

ddurakovich said:

However, I am seeing my points triggering at 25 BTDC BEFORE the impulse coupler trips, as well as after!

Click to expand...

NO! You are correct. The points should NOT trip at 25 BTDC before the impulse trips. That would totally defeat the purpose of the impulse coupling.

Thanks Mel, I needed that reality check.

It's a little like the light in the refrigerator...... I know it goes out when the door closes, but I've never really checked!

Good catch! That would be a tough one to get.

So what is his problem! Is he really getting a spark off his IC 4373 mag before the IC pops? How can that be, if the mag is not turning it can?t through any fire? The B&C guy told me something that might apply when he was rebuilding my starter from kick back damage; he said that the key switch can occasionally fail to ground the non IC mag causing occasional kick backs, this could be your problem but it is not mine, I have two 4373 IC mags and both are set to be hot at start, mine only ever kicked wile hot, I never solved this but after having some other mag problems and changing the mags it has not happened again? It was a very rare occurrence like once or twice in 700 hours.