Timing Question

leoofc

Member
Where would the zero timing mark be on a stock cast iron flywheel? Center of the magnet? Leading edge? Trailing edge? Also where would be the exact trigger on the coil? Also if the flywheel has a circumference of about 21ins ,would that put 30 degrees btdc about 1.75 in away? 21 divided by 12? Just thinking out loud.
 
360÷21= degrees per inch or parts of an inch .
Stock timing is about 23*
Coil fires as the magnets first line is just outside the right coil leg .
Wrap a sewing tape or string around the flywheel you need a accurate measurment .
 
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Here's how I do it:

The release point of the ignition is when the left (trailing) edge of the magnet passes the right hand side of the pick-up module (located to the left of the coil windings.) This will be the firing point @ 4000 rpm. Mark your right hand coil leg with a timing mark (soap stone, white paint, other,) for reference with a timing light.

Caution, DO NOT stand directly in front of a running engine to check the timing.

I set the piston in the hole (cylinder) a specific depth at this point of ignition to set my ignition advance.
Consider also that all stock coils retard as rpm increase (some more than others.)


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I set the piston in the hole (cylinder) a specific depth at this point of ignition to set my ignition advance.
Consider also that all stock coils retard as rpm increase (some more than others.)
I've heard that before, that the Briggs timing retards, and it made me wonder, would the same thing happen on a Yamaha KT100? It doesn't!
We did a test, on my dyno, where we put a very expensive timing light (powered) on my friends KT100 and found that, even at 15,000 RPM, the timing does not retard. I had heard stories that the KT100's timing would retard at low RPMs to help starting, but if it does, we didn't see it. Not that it doesn't, it's just that you can't run the engine at that low of RPM and see it.
I'm just wondering about the discrepancies.
 
I've heard that before, that the Briggs timing retards, and it made me wonder, would the same thing happen on a Yamaha KT100? It doesn't!
We did a test, on my dyno, where we put a very expensive timing light (powered) on my friends KT100 and found that, even at 15,000 RPM, the timing does not retard. I had heard stories that the KT100's timing would retard at low RPMs to help starting, but if it does, we didn't see it. Not that it doesn't, it's just that you can't run the engine at that low of RPM and see it.
I'm just wondering about the discrepancies.


I'm not all that familiar with the Yamahas, Al. Back when I ran them, I had two of the better engine builders of the day build them for me, and I've only refreshed a handful over all my years. I believe they use a cdi ignition, right? I would think that is similar to the pvl used on some Briggs engines today that is programmable. Most people don't realize that the Briggs PVL system has different timing at very low rpm to help start as well, then it's fixed from that point up. The low rpm timing is so low, that I think you don't see it on the dyno because it's generally below the idle rpm.

On the Briggs flathead, the ignition is not digital, it uses the old wound coil. The number of windings, size of wire used, heat, etc affect resistance. They absolutely retard timing as rpm increases. Some do so much more than others. It is very evident with a timing light.
 
Thanks for the flathead timing info. With the piston tdc I could see the stock 23 degree advance using the mentioned reference points. I could also see my 1.75 away from zero timing mark is also about correct resulting in an about 30 degree advance with the piston .212 btdc as Brian Carlson's posted chart. Im more used to using my timing light on outboard engines where everthing is marked out for you.
 
In the 2 cycle world, where you can adjust the timing, we always used an indicator, first to find top dead center, then to find the BTDC timing point. I have a spreadsheet that gives you very precise piston travel to distance from top dead center. All I need to know is the rod lengt (C – C) and the stroke. I've had that spreadsheet since 1982 when I first purchased a computer for business. A spreadsheets a wonderful thing.
 
I'm not all that familiar with the Yamahas, Al. Back when I ran them, I had two of the better engine builders of the day build them for me, and I've only refreshed a handful over all my years. I believe they use a cdi ignition, right? I would think that is similar to the pvl used on some Briggs engines today that is programmable. Most people don't realize that the Briggs PVL system has different timing at very low rpm to help start as well, then it's fixed from that point up. The low rpm timing is so low, that I think you don't see it on the dyno because it's generally below the idle rpm.

On the Briggs flathead, the ignition is not digital, it uses the old wound coil. The number of windings, size of wire used, heat, etc affect resistance. They absolutely retard timing as rpm increases. Some do so much more than others. It is very evident with a timing light.
So what does this do as it retards? Is it good or bad and how can one find one with the least amount of change?
 
Thanks for the flathead timing info. With the piston tdc I could see the stock 23 degree advance using the mentioned reference points. I could also see my 1.75 away from zero timing mark is also about correct resulting in an about 30 degree advance with the piston .212 btdc as Brian Carlson's posted chart. Im more used to using my timing light on outboard engines where everthing is marked out for you.
I have an Excel spreadsheet that will tell you exactly how many inches of piston movement for each 10th of a degree of engine rotation. More or less if desired.

Just enter the rod length (center to center), the stroke and your desired increment. You get the numbers from TDC all the way to 360°.
Email
alvinnunley62@gmail.com for your free copy.
 
So what does this do as it retards? Is it good or bad and how can one find one with the least amount of change?
It's neither good or bad. It's just something that you need to take into account. If you set your ignition timing static on the bench, you had better believe it makes a big difference if the coil retards 2* or 4* at peak torque (around 4000 rpm on our WKA stock flathead engines.)
You can easily check them with a timing light. SnapOn (and I believe Mac) have variable timing lights that make this easier to see because you can dial in the number of degrees of advance rather than just seeing the chalk line behind the strobe. You will see that all legal coils retard. Some much more than others. I prefer ones that retard less, as I think they perform better across the entire power band. Some folks are just concerned about peak numbers, and you can make the same peak number with any coil, just by choosing the correct static timing to account for that coil's retard.
 
It's neither good or bad. It's just something that you need to take into account. If you set your ignition timing static on the bench, you had better believe it makes a big difference if the coil retards 2* or 4* at peak torque (around 4000 rpm on our WKA stock flathead engines.)
You can easily check them with a timing light. SnapOn (and I believe Mac) have variable timing lights that make this easier to see because you can dial in the number of degrees of advance rather than just seeing the chalk line behind the strobe. You will see that all legal coils retard. Some much more than others. I prefer ones that retard less, as I think they perform better across the entire power band. Some folks are just concerned about peak numbers, and you can make the same peak number with any coil, just by choosing the correct static timing to account for that coil's retard.
Is it possible the variance could change with temperature like ohms do?
 
Absolutely it changes with temperature, but that's not the reason for the difference between coils - it's the number of windings and size of wire used...finer wire + more wraps around the coil = higher ohm resistance. The retard is due to the design of the coil itself. This is why the PVL (digital/electronic) ignition is preferred on the animal engines (and more recently clones too.)
 
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