Ignition 101

[Nathan S]

It means you really ought to put it on the dyno.
My MX250 felt like it was too retarded - turns out it was significantly too advanced. Picked up a few HP and a fair bit of rideability simply by trying different timing settings.
Further tweaking with the jetting made it even better.

[Graeme M]

Yes, you'd be right that dynoing it plus a few other things would be effective in working out exactly what my bike needs.

But my question is a bit more general than that - I'm really trying to learn more about how this stuff works. The main thing I observe now after figuring out the timing details is that the S is retarded about 6-7 degrees over the M. The A is retarded even more again. So, what I wanted to get a sense of is why is that so? The M and the S are very similar, the differences being porting, pipe, head and carb. I will guess then that the M revs lower and slower and needs that much advance to run efficiently while the S revs higher quicker and runs hotter (or maybe more efficiently) and thus needs retarding to prevent it getting into deto territory at peak revs.

[Graeme M]

On the whole, I've picked up a lot, just out of curiosity I guess. After this thread, and some more reading, I've learned generally what advance/retard means, how ignitions work, how to 'read' a plug (theoretically that is cos in practice probably not really) and so on. I've never really bothered trying to get my head round that stuff before - run what the factory says has usually been my motto.

[mboddy]

Seeing that you just put a hole in your piston and the M and S CDIs are the same,
I would be trying the S timing specs if I was you. 

[Graeme M]

No pretty sure that was jetting. I checked timing and it was set to S spec, so I don't think that's the issue.  The bike previously ran quite fine on the 'between' M/S timing mark so being retarded from that should not be causing any probs... I was worried I'd not done the screws up tight enough and the stator had slipped.  It hadn't. But surprised I didn't notice any pinging. Next time out I'll be starting a further size up!

[Lozza]

A hole underneath the plug indicates too much advance.

[Graeme M]

Just reread this one... I finally got the chance to take the RM125 out for a ride again at the HEAVEN round last week and it didn't run at all. I am assuming an electrical issue but who the Hell knows? Anyhow, I checked all the wiring and connections and so on and all looks OK. So I decided to recheck the timing and that's what led me to reread this thread. The strange thing I've found regarding the RM125M is that there appear to be two stator baseplates. One has a single mark on it, the other has two marks, one either side of where the single mark would have been. Now, in an early Suzuki tech bulletin, the timing is supposed to be right by setting it to the single mark. Yet my service manual, and later tech bulletins all talk of using the two mark type to set for either M or S timing.

So I checked the parts list, and sure enough, there are two baseplates (32100-28300 and 32100-28301). One with two marks to suit the M/S timing, and one with a single mark to suit the M only. What's very strange here is that the single mark type is used in the 74 TM125L also (but not the 75 TM125), yet the TM and the RM-M have different timing figures. TM, RM-M and RM-S all use the same CDI unit.

I have no idea what to make of this all, except that I guess if I use a single mark baseplate I should time it to that, and if I use a two mark type I set it to the M spec. Any Suzuki gurus got any thoughts on all of this?

Anyways, regardless of all this, I shall have to get the tester out and check all the coils and wiring continuity cos I need to get this thing running by next weekend!

[Lozza]

Have you got that timing light yet

[Graeme M]

Well, I wouldn't know what to do with it if I had one. All I can do, as someone pretty ignorant of the intricacies of ignitions, is follow the manual. My problem is that there are some curious discrepancies. Interestingly, my manual states that "the PEI system does not lend itself to conventional stroboscopic checking of ignition timing". So maybe a timing light wouldn't be much help?

[Lozza]

Don't see why not the PEI system works like every other CDI. The reason you don't trust marks on plates is they are inaccurate. Far better to make your own TDC and degree/mmBTC marks and strobe against that.Once you do that moving the stator a small amount either side of the new mark and riding the bike will tell you a great deal. I'm sure you can do that You don't need to know th intracies you just need to know if I put the plate there it makes great low end/mid if I put it there it makes great top end and in between makes a bit of both.

[Graeme M]

It's interesting you say don't trust the marks. The fact there are two separate plates with different marks does make me suspicious, especially as the mark for M timing is different between the two. I did get a dial indicator and find 3.8mm BTDC and that didn't match either of them. I didn't know what to make of that...

[chrisdespo]

Just a thought ive had a couple of these bikes all be it when they were new and i have had a bit of a play with a couple, one of mine i lent to chris oldfield to race at christmas hills it had a bit of work done just jetting and a small amount of port work went ok i think he may have won on it . I always found that if you stick with what suzuki specs were they ran well no trouble. as far as timing marks being inacurate i think not. the Rm-m had a much smaller carb which would work better down low better acceleration the S with its larger carby had much better top end you know more fuel and air going through but i think same top revs . the A if i can remember was a complete different engine still shared some parts maybe but the first of the six speeds and reed valve lighter crank maybe and i think maybe a shorter stroke but i am old and my thing that holds thoughts is going whatever they call that. i would guess that the a would have had a lot of differant specs as it was so different to the m and the s. I pose a question on timeing why do you need advance is it because that when the revs increase that the spark reaches the plug in the same number of miliseconds at any revrange but the piston and fuel charge gets to TDC earlier? dose anyone know what i mean? static timing like on an rh250 you know the ones with points set the timing and watch out when you start it cause i dont think they had any advance from memory.hell those bikes could kickback a lot of advance at low speed.

[Graeme M]

I do see what you are saying but can't comment as I just don't know. It sounds reasonable but I suspect it's more to do with the elapsed time of the actual burn compared to piston speed and hence position at point of combustion.

As far as the RM125M goes, it's in the back of the shed and I have sort of lost interest. The problem a couple of posts above was a dud magneto, since remedied by a good one. But the thing still doesn't run right and I have no idea why. Might try a new coil sometime.

I remain intrigued by the two different stator plates, and the different timing adjustments specified in the early tech bulletin, and the actual owners manual (and a later tech bulletin). Given my plate's timing mark matches that in the earlier bulletin, I would assume this is the better option.

As mentioned I did try setting timing with a dial gauge but the correct figure according to that didn't match any of the marks on any plate!

I have returned to blissful ignorance regarding timing and will only ride bikes set to factory spec. In fact all my problems have been solved by putting the vintage bikes in the back shed and only riding the 05 YZ125...

[Graeme M]

Couple of spark plug related questions, and would appreciate any views.

1. Are iridium plugs worth the extra expense in a 2-stroke?

2. My modern 125 requires an NGK BR9EV plug which I believe is a resistive type. Are these suitable for the vintage 125 (75 RM125) or is it better to avoid these? I've always just used the B9ES plugs for it.

3. Plug heat range. The 75 RM125 runs a B9EV standard, would that be suitable for dirt track (assuming everything else is correct), or is a colder plug worth considering?

[Graeme M]

C'mon, someone must know spark plugs!