Tuning The Advance Curve/Spring Rate...

I'm trying to get as linear and smooth an advance from idle to full, that I possibly can.

If the springs are too stiff, the advance curve stays flat longer, then suddenly begins to advance closer to 3000rpm, making the bike "surgy" in the 3500 to 4500 range.

If the springs are too soft, the advance curve rises too quickly, and you get a flat throttle response very early in accelleration, at 2000 to 3000, it just feels very weird.

I have a very precise digital zenon timing light, that you can adjust to .1 degree accuracy.
Using this light, my goal is to find a spring rate that proceeds as follows:

1000rpm, 10 degrees/default advance.
2000rpm, 20 degrees advance.
3000rpm, 30 degrees advance.
4000rpm, 40 degrees Full advance.

No matter what I do, the bike seems to reach full at 4000rpm.

Any suggestions? Are there better curves to be had with different springs? Is 4000 where full should be, or earlier/later? WHAT? :laugh:

PEACE!

When I had to make new springs, I just tried to get my advance to follow the curve in the FSM. The springs are a shade lighter, so the advance comes up a little ahead of the curve. I think it runs better but who knows. The published curve has a fair amount of slop range.

TeK9iNe wrote:

I'm trying to get as linear and smooth an advance from idle to full, that I possibly can.

If the springs are too stiff, the advance curve stays flat longer, then suddenly begins to advance closer to 3000rpm, making the bike "surgy" in the 3500 to 4500 range.

If the springs are too soft, the advance curve rises too quickly, and you get a flat throttle response very early in accelleration, at 2000 to 3000, it just feels very weird.

I have a very precise digital zenon timing light, that you can adjust to .1 degree accuracy.
Using this light, my goal is to find a spring rate that proceeds as follows:

1000rpm, 10 degrees/default advance.
2000rpm, 20 degrees advance.
3000rpm, 30 degrees advance.
4000rpm, 40 degrees Full advance.

No matter what I do, the bike seems to reach full at 4000rpm.

Any suggestions? Are there better curves to be had with different springs? Is 4000 where full should be, or earlier/later? WHAT? :laugh:

PEACE!

Full advance should be at 3300-3400rpm on a 900-1100 and at 3600 on a 550-750

I used to do this on cars, back before many of you guys were born.

In order to change the advance rate, I have found that you have to change one or both springs. A spring with few coils and fairly large wire diameter will hold the advance retarded until RPM is high. On the other end of the spectrum, a spring with many coils and small diameter wire will begin to advance earlier. Try a spring with smaller wire diameter on one side to see what happens. The advance should start sooner and reach full advance a little quicker. If it still does not reach full advance by about 3,400, then change the other spring. Trial and error.

Also, if you use too small a spring, the advance may begin too early - in other words at or below the idle speed. This will make the idle erratic and also may throw off your initial timing setting. Use a slightly stronger spring. Good luck.

WABBMW wrote:

I used to do this on cars, back before many of you guys were born.

Try a spring with smaller wire diameter on one side to see what happens.

Shouldn't both weights have the same spring rate so they pull evenly? Ed

I would think they would provide a combined force against the expansion, so by changing just one you are in essence lower the spring force of the pair, just in smaller intervals.

If the two arms are independent, then yes, I would think the springs would have to match.

The KZ mechanical advancers are a tradeoff. The problem is that mechanical spring/weight systems go from no advance to full advance in a small RPM range. Ideally, you'd want it more gradual, and take up a larger RPM range.

The later ignitions with electronic advance can get more power from the timing by allowing more total advance because the electronic advance can be made to steadily increase with RPM.

If you look at the curves for both types you will see the mechanical ones start out gradual, then suddenly jump up to full, then the advance stays there through redline.

The Electronic ones start out advancing faster, then gradually taper off to full advance as the RPMs near redline.

Because of the behavior of the mechanical advancers, they have to use less total advance otherwise you would get too much knock at middle RPMs, but then you end up with less advance than you'd want at higher RPMs.

A better mechanical advance would have a two-stage curve. One would go from around 1500 RPM to about 3500 RPM, then the next stage would go from 3500 up to 10,000. This is sort of what the electronic ones behave like.

I'm just rambling here, but the point is to get an electronic kaw ignition if you can (what model KZ probably does not matter a whole lot for street use).

Here's a mechanical curve.

Here's an electronic one for comparison.

All very interesting info, but I'm not about to go swapping ignitions as I have just installed a new Dyna S and Coils.

I guess it will come down to trial and error...

Anyone have some magic numbers for inital timing and max advance??? How far can I advance the timing?

Thanks!

DoubleDub wrote:

I would think they would provide a combined force against the expansion, so by changing just one you are in essence lower the spring force of the pair, just in smaller intervals.

If the two arms are independent, then yes, I would think the springs would have to match.

Correct. The reason there are two symmetric weights is so it will be balanced load when the sucker spins up to 10k RPMs.

Hay Lou, you should see if you can incorporate an electronic advance into your GM ignition system. That would be fantastic. Do you think that the older stile (pre 1984) pulsing coils would be interchangeable with the newer stile pulsing coils? Meaning, that just a more modern IC igniter with electronic advance, could be used with the older pulsing coils?

Here are the curves out of the 83-85 GPz1100 manual. The timing curves are quite a bit different. This also utilizes an electronic advance module. I also checked the 750 manual and have found that that has a different advance yet, albeit that it is much closer to the 1100 curve than the 550 curve. Also, did anyone ever notice that until the very end in 2005, the police bikes kept the original stile IC ignition with the original pulsing coils and mechanical advance? Can anyone TRY and explain that?