OzVMX Forum
Clubroom => Tech Talk => Topic started by: Graeme M on February 12, 2011, 08:11:34 pm
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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.
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you think too much Graeme ;) my moto, if it feels right it is right regardless of what the books say ;D 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 :o
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PS, I probably know much less than you so curious George will sit here and wait ;)
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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.
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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.
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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.
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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
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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 ;D
I'm happy if I can just get all my bikes to run at the same time ;D ;D :D Well not literally at the same time but you know what I mean
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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.
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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
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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.
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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.
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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?
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Possibly a typo, more likely 2.31 going off the others
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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...
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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.
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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.
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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.
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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.
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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!
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A hole underneath the plug indicates too much advance.
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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!
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Have you got that timing light yet :o
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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?
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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 ;D 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.
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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...
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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.
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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...
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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?
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C'mon, someone must know spark plugs!
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try the colder plug and see how it goes,on my 79 cr250rz on morgan mile long track id go up one heat range as it was on wot for 4 miles but on the mx track i needed the hotter plug.as to the iridium plugs Lozza reconds their the go and increase power so try one.
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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 the circuit from the coil to plug must have 5kOhm resistance, sometimes it's in the plug other times the cap but NEVER both.Best is you have a range of plugs 9,9.5,10,10.5 and (for the Spinal Tap fans) 11. What plug you need is defined by your state of tune a GP125/250 only ever run on 10-11 plugs my 400 runs a 9.5. Basicaly if your performance drops the longer the engine is running you need a colder plug.
Matt proper race plugs like NGK R series and Denso 01 series give a nice kick ;)
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Thanks guys. I'll see how the little RM goes next weekend. It's pretty much stock but I did wonder if it might need a colder plug given it'll be screaming its tits off. The modern YZ125 on the other hand takes to it like a duck to water, even at my rather ummm mature pace. Although, it is jetted slightly rich but still a trace of detonation. I have since read on the interwebs that the 05 YZ125s in particular suffer from detonation due to the ignition. They run a 410 main standard and in the UK they claim to have to jet them to 480 to avoid killer rattle. Never heard that before... Oh well, who knows?
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The spark plug and the piston crown have all the answers. Retarding the ignition is better than over jetting.
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hows it go on fuel with more octane then watered down shite pump fuel?tried mixing in some octane booster to see what happens,tried changing brands of fuel as i remember freaky saying something about that
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Nearly all commercial octane boosters do diddly squat to octane. You can make good HP on pump fuel you just need to tune the engine to that fuel(same for any fuel).
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ok didnt know octane boosters didnt work but then again ive never tried them either as im not running hi comp engines any more other then in my lpg powered 265 val ute which doesnt like todays fuels with its extra 4 degrees of static advance for lpg but only rattles if i tramp it too hard towiwing a big 10 by 6 trailer up hills ;D ;D
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I read an article a while back in an Aussie car mag that compared all the leading octane boosters. Most did nothing much and some actually lowered the rating... And a couple did make a positive difference. But on the whole, waste of money it seems.
In the bike I use Caltex 98 cos I once read its about the most consistent of the pump fuels. Don't know if that is true or not but I followed the advice.
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The spark plug and the piston crown have all the answers. Retarding the ignition is better than over jetting.
Don't forget the underside of the piston crown - an indelible indicator.
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Graeme,
Not sure that the conversion from MM to degree of crank rotation you mentioned back on P1 of this thread is that straightforward as you need to account for lack of travel of the piston as the crank rotates through TDC & BDC.
When I was musing this very same issue for my 81 RM125 I found this online calculator which was very helpful.
http://www.torqsoft.net/piston-position.html
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Thanks PCMAX for that link, very helpful & interesting.
Well I am at a loss to work out my RM125B ignition timing as Graeme was/is.
I picked up a used & unlooked after 125B from the wreckers, thought I'd freshen the engine before riding it, upon tearing down the engine I find the stator set at full advance, when I reassembled the engine I set the stator at the mid point in the stator backing plate slot, as per the manual.
Well the bike would not start, an occassional spit-back thru the aircleaner; ahh I think "let's advance the stator fully again", the bike starts but does not want to idle, so I gets out the rat-tail file and go a hacking, I ended up slotting the stator backing plate a further 10mm! With each mm I filed out the bike would run smoother & crisper.
So I gets a timing light and drop the vernier down the plug hole; I find with the stator plate slotted the extra 10mm the timing light shows the timing to be 1.90mm BTDC. According to any specs I can find that for safety I should be setting it at 1.50mm (17 dgrees @ 10,000rpm).
So I guess, as JohnnyO advised me another thread, I may have a rotor from another model? Without sitting them side-by-side I guess I'll not know. I don't know if this issue could have related to Graeme's dilemma?
Next time I ride the bike I will try .5mm each way to see how it changes the power.
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Did you strobe at 10,000? Did you mark the full advance point?
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Although I didn't have a tachometer attached, the amount of advance reduced as it got high in the rpm's and stayed there as it sailed to maximum rpm; it was at that point I marked, and later checked, being the 1.90mm piston position before TDC.
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The maximum advance, at around the mid rpm range, was 2.40mm.
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Has nowhere near enough in the mid and to much up top. Set the high rpm advance to 1.5mm
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Hmmmm, will do. Do you think I could try some CDI units off other models to get more midrange advance?
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Probably, but I really dunno. You have a good process for checking so go for it
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Just incase anyone was interested, the following pic is with my stator plated slotted & adjusted to perfect timing with the timing light. The bike now starts first kick, hot or cold (with choke), and idles & responds beautifully :-)
(http://i1093.photobucket.com/albums/i423/JohnnyRacer89/Retimedstator1-2.jpg)