Ignition 101

[TT5 Matt]

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.

[Lozza]

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

[Graeme M]

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?

[Lozza]

The spark plug and the piston crown have all the answers. Retarding the ignition is better than over jetting.

[TT5 Matt]

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

[Lozza]

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).

[TT5 Matt]

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

[Graeme M]

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.

[yamaico]

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.

[PCMAX]

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

[John Orchard]

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.

[Lozza]

Did you strobe at 10,000? Did you mark the full advance point?

[John Orchard]

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.

[John Orchard]

The maximum advance, at around the mid rpm range, was 2.40mm.

[Lozza]

Has nowhere near enough in the mid and to much up top. Set the high rpm advance to 1.5mm