Tech question about Ignition advance

[joezek]

I have a 1985 Looper V4 140. I know the mechanical timing advance moves the timer base and changes the timing pickup point, which changes the timing advance. But howcome if the timing is maxed out at say 18 degrees right when it hits the high timing stop screw, the timing will still continue to advance after you start increasing the RPM's??? I can watch it climb right up to 25 degrees or even higher as the rpms go up, while the timing arm is no longer moving. Does the magnetic field change with RPM, hall effect or some other thing come into play...?? As a mechanic I really want to know. I work on cars and airplanes for a living, working on my boat is for fun, but either way I want to know exactly how it works in order to troubleshoot or maintain it.

JOE

 

[Joe Reeves]

Look closer at the top main bearing housing... there you will see an adjustment screw with a rubber cap on the screw head end. That is the full spark advance stop. When the timer base hits that rubber stop... if it's adjusted properly to whatever degree it should be... that is as far as it's going to go. No, I don't remember what the full spark degree setting is on that engine.

What you are seeing continuing to move is the spring loaded vertical throttle arm that from that point on is opening the carburetor throttle butterflies.

 

[ob]

joezek , are you saying that you are using a timing light and watching the degrees of timing dwell advance to 25 degrees while observing the flywheel marks or just a visual of the timer base arm?

 

[WernerF]

What you see is somewhat normal, but 7? advance with rising rpm is unusual. I would have expected 4?. The reason is that the trigger signals will get higher with rising rpm. The power pack triggers at a certain voltage threshold, so the trigger delay will get shorter with rising rpm. This is a desired effect.
Note that the maximum timing advance is to be adjusted at high rpm for that reason.

 

[joezek]

joezek , are you saying that you are using a timing light and watching the degrees of timing dwell advance to 25 degrees while observing the flywheel marks or just a visual of the timer base arm?

The timing arm is seated fully against it's rubber stop, and with a timing light I see the timing just continually advancing with RPM, even though the timer base is no longer advancing.

I don't think it even stops advancing at 7degrees, I think it keeps going. If the trigger delay is expected to get shorter, then is the excessive advancing a sign of a poor ground, defective CDI box, or some other thing? I do have another CDI box to test, but it's a little different. It's from a 1989 110 Evinrude. I believe that one just had a higher rev limiter, but possibly the same thing besides that. I just finished putting this whole engine all back together after it sat in pieces on my hangar floor and on the workbenches. I was very meticulous in making very well sealed connections with anti-corrosion stuff, and clean solid grounds everywhere. I don't think there's a loose wire or ground possible anywhere.

I did see after the first failed sea trial that just above idle the engine would spark knock, and found the timing would go to full advance just barely above idle. I removed the flywheel, looked for loose timer base, checked the keyway in the flywheel and reinstalled (and checked for loose magnets). After that the idle was below zero timing, and full advance was only 8 degrees......after triple checking the zero pointer and restested and the timing was all of a sudden back up to normal, after NO CHANGES. I'm really scratching my head with this.

 

[ob]

When you checked for loose magnets , did you inspect the flywheel center timer base magnet?

 

[joezek]

I honestly don't know, I poked and prodded every magnet I could see, shook everything, used a screwdriver tip to compare all the magnets, tapped and listened for hollow delaminated unglued magnets, this thing feels solid as far as I can tell

 

[ob]

The magnet I refer to is the center hub magnet. It rotates inside the timer base ring. It is specifically indexed to the flywheel with respect to crankshaft position. They come loose sometimes. When that happens your engine timing will be all over the map. It can really screw your engine up too. Some are factory pressed and many are glued with epoxy to a specific timing mark.