[kettlekev]

I didnt change the map but CLT and IAT compensations are zero'd.

spark with new coils.PNG (49.37 KiB) Viewed 2649 times

[LPG2CV]

That's backwards ....
An engine needs less advance at idle, and more as rpm increases until about 2500 - 3000 rpm. Then it stays the same.

You now have 24* at idle, where as before you had 24 minus 15.

Cranking advance angle (Spark Settings) should be 8-12* it could be why is more difficult to start.

Cranking rpm (Start up/Idle) is the rpm when the cranking settings transition to the running settings. So its generally, a little above the rpm the starter can achieve if the engine is unable to start.

If you run the engine under load at higher rpm with that little timing, you will lose power, and are liable to melt pistons.

[kettlekev]

I initially thought that as well but being a 2 stroke I was told this is right buy the guys that supplied the original CDI that I am replacing?

Unless our terminology is mixed up... Tuner studio has an ADVANCE table but CDI supplier talks timing RETARD.

So the stock bike starts at 24 deg BTDC at startup and according to the CDI guy it then 'retards 15 ° over the range from 4000 til 10000 rpm',so I took that as it starts at 24 then drops approx 15 degrees by the time it hits 10k and ends up at 24-15 = 8-9 degrees.

What am I missing here? Clearly something fundmental as it makes sense it has to start firing earlier (higher degrees BTDC at higher revs.

So if this is correct. I should start at 24 and end up at 24 + 15 = 39 BTDC?

The more I think about it the more sense it makes, the Belgian/english translation was obviously off and I didn't think it through.....

[LPG2CV]

Perhaps I'm wrong then, as CDI is not something I know about.

[kettlekev]

I've gone round in circles now on this because i remembered the initial research I did on 2 strokes and found the below summary ..........and in summary I do need to reduce the degrees to 10k........

'As you probably know, in four stroke engines, you advance the timing (that is, it happens before the TDC) to impulse the piston at the right moment. As fuel takes time to burn, you start the ignition earlier when the piston is moving faster to kick it at the same moment.

Now, when you retard the ignition in two stroke engines, the ignition occurs closer to the moment when the exhaust "valve" opens. So, you are getting hotter gases into the pipe. Speed of sound is faster in hotter gases. Gases also have more energy, so they leave the pipe faster, giving you more intake from scavenging. This is the first reason.

The second one is a little more complicated. The time to peak pressure (after you ignite the combustible) is smaller with larger RPM because of more turbulence in the mixture. At larger RPM the increase in turbulence is larger, so you have to retard the ignition to keep peak pressure at the same timing.

For example (I really think that you need numbers to get any idea):

C2/C1 = sqrt(T2/T1)

That is, the speed of sound (C) is proportional to the square root of temperature.

So, if you have gases at 400 degrees you have a speed of sound of 520 m/s, give or take.

Let's say that you have a two stroke pipe tuned to 10000 RPM. Now, we have some clever way (retarding ignition anyone?) to add 100 degrees to temperature of exhaust gas. This means that by the empirical relation for CD (crank displacement), which is:

CD = (0.012 * tuned-length * RPM) / speed-of-sound.

The pipe will actually be tuned to 11900 rpm with 100 degrees more (that is, you keep constant the relationship between RPM and speed of sound in the previous equation)'.

[PSIG]

Ignition timing is referenced to TDC, and is adjusted to cylinder peak-pressure crank angle. It varies due to the effective burn-rate, which is affected by everything from the basic fuel blend, to temperatures of everything, to effective airflow, to fuel vaporization, to MAP, to effective compression, to squish and turbulence, to a dozen other things. Any change in any one factor will result in a change to the best timing at that point, in order to achieve peak cylinder pressure at the most efficient and effective crankshaft angle to produce power.

In many engines, this results in a WOT timing curve that continually increases advance versus rpm, while others it may flatten or retard. For example, the changes to burn rate are dramatic around peak-torque or where components reach harmonics (intake, valve timing, exhaust, etc), and why you see so much attention to the timing ramp or hump there. This is why we have to tune it, and account for all of those factors in the result. Speeduino uses a Spark Table, which assumes advance for the majority of users, but also retard for those that require it.

I have shamelessly borrowed a timing graph from dragonfly75's site to show the OEM timing he found on two very similar but specific engines. They begin at just below 10° at idle rpm, and end-up at nearly the same timing at high-rpm. For the reasons above and as stated in @kettlekev's quote, these engines tend to require less advance (retard from peak timing) at higher rpm. Test and tune, and give it what it needs. Roaming his web pages will show how some factory curves are outright horrible. We have the ability to fix that.

Credit: dragonfly75.com