I am not seeing any spark at the plug, and, it doesn't start.
Without any special Yamaha tools, are there any ohm or voltage readings I could verify before simply replacing parts?
Does timing ever get knocked out, or is that rare? The same condition exists on both of my son's 125s.
I have (of course) tried both a good used and a new spark plug.

I'm not certain on that bike, but there are some good members here that are, they will see this soon I'm sure. One common issues is if the bikes have been sitting with moisture in them for a while, it will cause things to "rust up".

Welcome to the site though.

I am not seeing any spark at the plug, and, it doesn't start.
Without any special Yamaha tools, are there any ohm or voltage readings I could verify before simply replacing parts?
Does timing ever get knocked out, or is that rare? The same condition exists on both of my son's 125s.
I have (of course) tried both a good used and a new spark plug.
Thanks for the welcome! It's always good to be surrounded by those with a common interest. I hope there's an answer or at least a few good ideas. I don't like throwing money at possibilities, but the clock is always ticking ahead. As for rust, I did check for that and the generator is very clean and dry. I'd like to know if I plug a volt/ohm meter into the socket and give it a kick, what I should (or shouldn't) see. I'm hopeful.

I'm not that familiar with two strokes. But, if you have a shop manual have you looked into the wiring diagram and traced back from the plug? It may be as simple as a pinched coil wire, or possibly a coil or rectifier.

I would start tracing backwards from the plug: No spark at the plug, so go to the coil. No spark there, check for electrical feed into the coil with a test light. Nothing going into the coil... etc.

Wish I could help you out more specifically. Unfortunately, it's been way too long since I worked on two strokes. Junior is the master mechanic of all things two stroke. Hopefully he'll see this soon and give you a better response. :thumbup

Thanks fjorn.

Junior, are you out there? I need some insight on this two stroke before I have a stroke myself.

I thought it might be worth a mention to new viewers that one of the sponsors to this site, Mr Cycle, has decent prices on OE parts. My philosophy is, support those that support good causes, even if they are not always the lowest cost provider. I will more than likely be needing those services when the odds of my problem are in a higher percentile.

Best regards,
Dad

You're welcome. I sent Junior a PM about your questions.

BTW, I agree with you on supporting the vendors. :thumbup

hey, sorry I didn't see this till the PM.

should be in the range of 9-16v at the mag, depending on how hard you can kick. I don't have a diagram for the YZ infront of me tho so I don't know which colours to look at. Generally you don't want to see more than 20 ohms in the mag circuit on a 125. I'm not 100% tho, could be that Yamaha just use a 250 mag for the 125 which would be more in the line of 30-40 ohms. Usually with this shit tho it's either in spec or it's no circuit.

Downstream from that check the regulator, it should be outputting 14-15v, assuming your'e kicking it hard enough to actually get to that voltage.

Downstream of that, it's difficult to isolate whether a problem is with a CDI or a coil, but figure if you're not getting atleast 100v off the secondary windings of the ignition coil, you've got a problem. I'm not sure what coils Yamaha used on that motor tho so without having a proper service manual I can't help too much there.

Could be that it's an exciter coil failure in the mag, which would really suck they're a bitch to get to.

Here is the manual for the 2005 YZ250, the 03 should be pretty close as well.

:beer

YZ250 Service Manual (http://www.yamaha-forum.net/manuals/yz250/2005yz250.pdf)

ya 125's generally run lighter electrical systems than 250's tho. Less energy needed to get the combustion going and they need a lighter flywheel.

I didn't realize you had any dirtbike FSM's Rhett. Don't suppose you've got any other 250's? I've got a hardcopy of the KX one but the electronic ones are soooooooo much nicer and more convenient.

whoops I posted the wrong one lmao one sec

ya 125's generally run lighter electrical systems than 250's tho. Less energy needed to get the combustion going and they need a lighter flywheel.

I didn't realize you had any dirtbike FSM's Rhett. Don't suppose you've got any other 250's? I've got a hardcopy of the KX one but the electronic ones are soooooooo much nicer and more convenient.

I don't have many yet, but have been collecting them as they are needed... or I have time to find them. lol I'll look to see if I have any KX ones.

Here is the 125

2005 YZ125 (http://www.yamaha-forum.net/manuals/yz125/2005yz125.pdf)

Thank you men, I really appreciate the insight and advise. I'll get into this ASAP and see what readings it gives me. Even a little light in total darkness can make a big difference. Once I get a better feel for what's happening (or not happening) I'll post the results. Again, many thanks!

Glad to try and help, as you said try and post up what was wrong so it can help other members in the future with the same issue. :beer

Once I pulled the tank and got acclimated with what wires went where, I pulled the coil and 'bingo'. The ground eyelet had pulled off the wire under the bolt (behind the radiator tank). I thought I had it licked when I saw the plug produce a nice bright spark.
But no, my luck isn't that good. There's another culprit in the mix, and I'm back to a no start condition (good grief).
The plug appears to be a little wet from fuel. Opening the throttle, WD40 in the bore, choke on/choke off, wearing out my patience on the kick starter proved fruitless. This thing is really peeing on my wheaties guys. I have to step back and reassess the situation. How do you know if there's not enough compression? Am I looking in the wrong direction? Compression, spark & fuel, right?

Do you know how many hours are on the engines? I'm not saying this is it, but components such as reed valves and pistons/rings wear out after a certain amount of time.

Once I pulled the tank and got acclimated with what wires went where, I pulled the coil and 'bingo'. The ground eyelet had pulled off the wire under the bolt (behind the radiator tank). I thought I had it licked when I saw the plug produce a nice bright spark.
But no, my luck isn't that good. There's another culprit in the mix, and I'm back to a no start condition (good grief).
The plug appears to be a little wet from fuel. Opening the throttle, WD40 in the bore, choke on/choke off, wearing out my patience on the kick starter proved fruitless. This thing is really peeing on my wheaties guys. I have to step back and reassess the situation. How do you know if there's not enough compression? Am I looking in the wrong direction? Compression, spark & fuel, right?

Yes that's right, spark, fuel, compression, and o2, and you will have fire.

My first guess is it's flooded with fuel after all your attempts to start without spark. I would let it sit overnight and try again, make sure you have fresh fuel as well. First try with no choke, then add it if it doesn't start.

If you still have no luck pull the plug and see if it's wet, if it is then hold your finger over the hole while you have someone kick it over gently. it should blow your finger off the hole. (compression)

If not then you have another issue, maybe a stuck ring or some other issue causing this.

If it's a low mileage bike and no compression, and it has been sitting for a while I would soak the plug hole with WD-40, out a lot in there and let it sit overnight and try again.

If you still have no compression then it's time to pull the cylinder to see why.

Thanks again for the insight on the newest dilemma. Yes, filtered O2 for sure.
My procedure was to (hand) kick it over with a squirt of WD40 and no plug, then I put my thumb over the plug hole and rapidly pushed the kicker. Not a lot of blow, some were a little better. I tried a compression tester, but I heard it's hard tell on a 2-stroke (I saw 40 lbs.). The engine has been ridden hard and my son (the all knowing) used my Quaker State chain saw oil and 87 octane because "he had to ride". Doing it once or twice may be OK in a pinch, but he went through 8 to 10 gallons of that brew (along with a 2/3 plugged air filter). I normally use Yamalube R1 (3-1/2oz. per gal) and 93 octane with a shot of booster and never had a problem. I'm sure it's time for a rebuild. I dread that, as I have no special tools (just a shop manual). I'm an old fuel injection technician and ex-machinist, so I have some smarts, tools and hands on mechanical ability, but this makes me nervous. Do I forfeit or go for the challenge?

I'm not sure exactly on that bike but I would think it should be more then 40lbs... that might also be due to a stuck ring or those 8-10 gallons of bad oil... :lol

As far as tools, you shouldn't need anything major at all for simple hone and ring job, or even a bore and new piston & rings.

Good luck and keep us posted.

(I saw 40 lbs.). The engine has been ridden hard and my son (the all knowing) used my Quaker State chain saw oil and 87 octane because "he had to ride"

120-150 would be a lot more reasonable. you're in for an egg and rings forsure, possibly a jug.


As far as tools, you shouldn't need anything major at all for simple hone and ring job, or even a bore and new piston & rings.


Jug will need Nic if it's scored, which is something you'll have to ship it out for. I've had the best luck with Millennium Tech (http://www.mt-llc.com/), and they sponsor a race team so they do give back to the sport some.

Other suppliers for Nic are:
US Chrome (http://www.usnicom.com/)
Atelier Adrienne Bernard (http://www.cvtech-aab.com/contenu/index_ang.cfm)

2-strokes are mondo easy to rebuild, if you've got any mechanical ability at all it's less than 2 hours work. Most guys that do 'em regularly can punch it out in under an hour.

All of your collective suggestions are appreciated. I may tackle this after all. I will be back with questions eventually and share any pointers for those that will come after me.
Now, where's my 54mm telescoping gauge... :^)

harbour freight, cheap cheap.

Those little jugs are a bastard to port, 54mm bore is entirely too small for a human being of reasonable size to work on.