Ignition 101

[Graeme M]

OK, here's my newbie question for today. I know a little about the mechanical side of bikes but have to admit to never really getting to know ignitions. All I've ever done is make sure the timing is right.

But I was checking my manual today for the timing specs on my RM125M and found the following:

RM100A  2.26mm BTDC
RM125M  3.80mm BTDC
RM125S  2.20mm BTDC
RM125A  0.31mm BTDC

Now I don't get that.  That's some big differences right there, esp the 125A. I'm assuming ignition timing is set to work with the porting/pipe combo to fire at the moment when the charge is at some optimal arrangement, but I'd not have thought that there would be THAT much difference between the 125M, S and A.

Also, what exactly is ignition advance? Does that mean firing closer to TDC or earlier? I understand that the CDI manages the ignition curve, but what IS it doing? I used to hear all these stories of guys advancing their ignitions for more power, yet I remember Gordon Jennings claimed many production bikes ran better with a little less advance than stock.

I know I could go read up on it, but thought someone might be able to offer a few quick insights.

[DR]

you think too much Graeme my moto, if it feels right it is right regardless of what the books say advanced ignition might 'seemingly' give a bike a false sense of more power down low but it's quite detrimental up top and detonation will soon follow..nasty trade off I'd say

[DR]

PS, I probably know much less than you so curious George will sit here and wait

[David Lahey]

Advancing means the spark happens earlier in the cycle. Retarding means closer to TDC.
Electronic ignitions with an advance curve measure the RPM and change the timing of the spark relative to the trigger point for different RPMs.
A typical curve advances the spark as the revs rise.
Some old bikes (eg DT175B) have a points type ignition with an advance curve. They use weights working against springs inside the flywheel to rotate the points cam as the revs change.

[JohnnyO]

Graeme is that a genuine Suzuki manual you are using? 3.8mm btdc is a lot of advance and .31 doesn't seem enough. 2.0-2.2mm is a more normal setting.

[Lozza]

Jennings statement was true at the time when flat or straight line ignitions were all the rage. Then the retarding ignition arrived and all that went out the window.
IT is not so much about working with ports and pipes, it is all about getting the pressure peak in the cylinder at the right time and getting the correct amount of heat in the pipe.

[oldyzman]

Lads,

So whats the deal with the timing say at 1/2 rev renge and full noise in general on CDI.

Can someone tell me what is the end effect of detonation?

Just another quicky slightly off topic, if a bike 2 stroke 125 was running flat out down the straight and then lost almost all power due to the botton cylinder front flange mounts being sheared off and ending up with a 5mm split under the exhaust port on the barell. What would cause this? too little squish ie 0.3mm coupled with a bit of piston rock. instaed of the recomended 0.7mm. or could this be caused by detonation
Brett

[montynut]

Graeme,
Not withstanding Lozza's post which is completely correct and modern ignition systems do all sorts of things with timing both advancing and retarding it in very complex graphs or curves. The graph or curve is often influenced by by far more than simple rpm. Often the graph or curve can be 3 dimentional.

Our older types are much more basic and relate only to RPM or sometimes fixed. Increasing the advance moves the spark earlier (further before TDC eg at idle it may be 8deg BTDC at maximum rpm it may be 22deg BTDC) in the cycle. The reason this would happen is due to the fact that the fuel/air burns at a relatively constant rate therefore you need to get the stuff ignited earlier in the cycle so that the fuel burns at the correct point in the cycle. Without this increase in advance the 'best' part of the burn would move further and further after TDC as rpm increased.

Too much advance and the burn process is started too early and you can imagine what that would cause as the piston tried to continue to compress the fuel/air mix when it has already started to burn and therefore expand rapidly

Does that all make any sense. I'm sure that someone can correct me if I'm wrong. This is only the basics and I do not claim to be a tuning expert by any stretch of the imagination

I'm happy if I can just get all my bikes to run at the same time Well not literally at the same time but you know what I mean

[Graeme M]

JohnnyO yes, factory manual. And that's the reason for my question. I checked the timing on my M and it was fine, but I was amazed by the difference between three bikes that I think are relatively similar. Esp the difference in the M and S. From what people are saying, the M has a lot more advance and the A almost none. And from my limited knowledge I couldn't easily see why there would be such differences.

I agree with you by the way Doc. I have never ever played with timing on a bike cos I figure that I know nothing while the factory has a vague idea...   

But I do like to make sure it is right according to the manual.

[Colin Jay]

Montynut, has it pretty well spot on. A lot of the 70's early electronic ignitions do not actually have an advance curve, they have a simple "step advance" where the ingnition timing will electronically shift from one point to another at a preset rpm. This style of electronic ignition rely on multipe pick up coils and at a predetermined rpm will advance the ignition to its full advance position by changing the actual pick up coil being used.

 The next generation of electronic ignitions provide a linear increase in advance. As I don't have an early CDI 2 stroke at present, I can only give examples from one of my 4 strokes, My '83 XT600 Yamaha is notionally timed at 12 degrees BTDC at 1200rpm, starts to advance at about 2000 rpm and should acheive full advance of 36 degrees BTDC at 4000rpm. Being a 4 stroke, the igniton timing actually retards at revs lower than about 800rpm to make the engine easier to start.

With both these types of ignition, as the timing advance increases instantly or quickly over a very short rpm range, it is quite easy to drive one of these engines into detonation. The classic example is pulling out of slow sandy corners in a gear too high. The engine is pulling hard under the load and suddenly the ignition advances and is too much and you get detornation.

A lot of 2 stroke CDI's also retard the ignition timing at high rpm to act as a rev limiter. This can present a problem if you try to hot up the engine, as in most cases the extra power is gained at higer rpm, and having the ignition timing retard kills of the power and makes the engine feel flat.


CJ

[Graeme M]

Thanks guys, very interesting. So can anyone say what's going on with these RM125s? The S at 2.2mm BTDC sounds about right according to JohnnyO (I'll believe him, I have no idea what's 'right'). But my M is significantly more 'advanced' and the A is 'retarded'. Yet I'd have thought these engines are relatively similar to each other. Clearly their CDIs just do this step advance thing, so there's not too much magic there.

[Lozza]

Not quite correct Col, ignitions retard is used on a 2T because they don't need all that advance after the engine is in phase. Advance is added after the power peak to limit rpm.

There is no way the 0.31mm figure is correct maybe for some static timing figure, when running you should have something like 22deg at 7000 and 16deg at 11,000.

[Graeme M]

Yeas that's the static timing numbers. I'll have to have a look and see what the running figures are. But why would two 125s use numbers in the 2-3 mm range for static timing but another be almost at TDC? What would/could the difference in engines be that require such a difference in timing?

[tony27]

Possibly a typo, more likely 2.31 going off the others

[Graeme M]

Just revisiting this one... I did actually sit down and check the figures again using the manual.

The figures are:

RM125M  3.8mm BTDC and 29 degrees at 6000 (I assume this is full advance)
RM125S  2.20mm BTDC and 22 degrees at 6000
RM125A  0.31mm BTDC and 8 degrees at 11000

I'm guessing I can easily convert mm to degrees which gives us

RM125M 14 degrees static and 29 at full advance
RM125S  8 and 22
RM125A  1 and 8

and for interest, the RM125B is 17 degrees at full advance.

Interestingly, I have three stators for my M. One has the timing marks as per the manual, the other two do not - they have a single mark approximately in between the other two. Statically, that transaltes to around 2.8mm BTDC. Yet all 3 baseplates have the same part number on them.

TM125 and RM125M use the same CDI unit, the A a different one and thge B/C a different one again, which fits as per above. I'll assume from that the M and S use the same CDI unit...

What does all that mean? Beats me, but it's curious just the same...