Need help, problem with my YZ125

Edited Date/Time 7/10/2017 11:02pm
Went riding on it yesterday and a couple of times when I went to start it, it would fire then bog and stall. I thought that's odd but it started up ok, rode it & it was absolutley fine. I go out for my last ride of the day, again I fire her up, bogs, cuts out. I start it again but it's ok, I rode it for about 10 minutes then it starts popping & farting. Bring it in, pull the plug out & the bit that curls over the electrode, sorry I don't know what its called, as nearly broken off. One more lap and I'm sure it would've broken off. The plug gap must've been a good 5mm wide. I'm thinking, shit that was close. Put a new plug in it and would it fire up? Nope. It's got a spark but I'm not sure if it's a great spark. Tried bumping it, putting some fuel in the barrel, disconnecting the stop switch, still won't go. I'm thinking maybe because the plug gap was so big, it's strained the stator or coil???

Just to let you know the engine has been completely renewed, cases, crankshaft, barrel, piston & reed block. Incidentally when we put the engine back in, I spend about 45 minutes trying to get it start, I bump started it & it fired straight up & it hasn't had any problems until yesterday. The engine was rebuilt 8 hours ago.

Sorry for the long post but I'm a bit baffled at the moment.
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lostboy819
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7/9/2017 10:15am
Check your compression, I bet you cooked the motor. When your plug does that it is more than likely from detonation and running too hot and I bet your piston looks worse. Bogs and popping are a sign of running to lean and I would pull the cylinder to see how bad it is.
7/9/2017 10:19am Edited Date/Time 7/9/2017 10:20am
lostboy819 wrote:
Check your compression, I bet you cooked the motor. When your plug does that it is more than likely from detonation and running too hot and...
Check your compression, I bet you cooked the motor. When your plug does that it is more than likely from detonation and running too hot and I bet your piston looks worse. Bogs and popping are a sign of running to lean and I would pull the cylinder to see how bad it is.
Haven't had the barrel off but I've took the reed block & exhaust off & I cannot see any damage to the piston or barrel.

Compression seems fine.
moore433
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7/9/2017 11:29am
I would bet there is damage to the top of the piston and bottom of the head. I'd pull the head and check.
7/9/2017 11:41am
moore433 wrote:
I would bet there is damage to the top of the piston and bottom of the head. I'd pull the head and check.
Yeah gonna do that tomorrow.

The Shop

Brad460
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7/10/2017 11:19am
Out of curiosity- Was the engine running fine before you pulled it out and rebuilt it?

Leaning toward something electrical- make sure you have all the harness wires plugged in and the coil grounded.

Get the mag and stator re-assembled correctly?

2 stroke are super simple should be too hard to solve.
lostboy819
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7/10/2017 11:21am Edited Date/Time 7/10/2017 11:23am
It is running too lean as there is no wash on the piston, the carbon should not go all the way to the edge of the piston. Did you check compression before pulling the head ? You may have a stuck ring in the piston groove so it is not sealing. when you pull the cylinder you will know for sure. Also you did have detonation at one time by the looks of the outside edge of the head.


7/10/2017 11:24am
Brad460 wrote:
Out of curiosity- Was the engine running fine before you pulled it out and rebuilt it? Leaning toward something electrical- make sure you have all the...
Out of curiosity- Was the engine running fine before you pulled it out and rebuilt it?

Leaning toward something electrical- make sure you have all the harness wires plugged in and the coil grounded.

Get the mag and stator re-assembled correctly?

2 stroke are super simple should be too hard to solve.
It was running sweet before it threw a rod, done 50 hours on it until that point. Pig to start once it was rebuild but has been fine until yesterday. I checked the spark in the garage with the lights out & to me it looks good, big purple blue colour.
7/10/2017 11:28am
lostboy819 wrote:
It is running too lean as there is no wash on the piston, the carbon should not go all the way to the edge of the...
It is running too lean as there is no wash on the piston, the carbon should not go all the way to the edge of the piston. Did you check compression before pulling the head ? You may have a stuck ring in the piston groove so it is not sealing. when you pull the cylinder you will know for sure. Also you did have detonation at one time by the looks of the outside edge of the head.


Compression felt fine but I will pull the barrel as soon as I can to check the ring. Will it just be a case of getting a new piston & sorting the jetting or do you think more damage has occurred?

Thanks for the reply.
lostboy819
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7/10/2017 12:06pm Edited Date/Time 7/10/2017 12:10pm
If its a stuck ring then its just getting a new piston and ring. If you have less than 100 psi of compression It wont start. Also pull your stator cover and make sure your ignition plate is tight and on the mark and not come loose because it if did it can advance the ignition and cause it to run lean and detonate and not start,but you would still have spark just not at the right time.
MAS06
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7/10/2017 1:02pm
Keep in mind electrical issues mimic fuel problems
MAS06
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7/10/2017 1:05pm
also I had a coil go bad on a yz125, Was very hard to figure it out since coils never seem to fail, wasnt until I tested it hot that I figured it out
TJ 755
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7/10/2017 3:30pm
aGREE ON THE ELECTRICAL. sATORS AND COILS DO FAIL. i HAD TO REPLACE THE STATOR ON MY 06
Cygnus
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7/10/2017 6:42pm
lostboy819 wrote:
It is running too lean as there is no wash on the piston, the carbon should not go all the way to the edge of the...
It is running too lean as there is no wash on the piston, the carbon should not go all the way to the edge of the piston. Did you check compression before pulling the head ? You may have a stuck ring in the piston groove so it is not sealing. when you pull the cylinder you will know for sure. Also you did have detonation at one time by the looks of the outside edge of the head.


Yep that piston looks toast.
moore433
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7/10/2017 6:59pm
Really surprised there wasn't any damage to the head or piston top. Maybe it's nothing major then.. Good luck!!
Paw Paw
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7/10/2017 7:56pm
The photo of the head shows some serious detonation in the past. See the pitting on the edge of the squish band. I would recheck the squish.

Paw Paw
babba boey
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7/10/2017 10:03pm
Pull the flywheel and check if the key sheared off. Sometimes the key gets damaged from the motor locking up and shears off later. This will let your flywheel move on the crankshaft and the bike will be out of time.
7/10/2017 11:02pm
Paw Paw wrote:
The photo of the head shows some serious detonation in the past. See the pitting on the edge of the squish band. I would recheck the...
The photo of the head shows some serious detonation in the past. See the pitting on the edge of the squish band. I would recheck the squish.

Paw Paw
Yes, that head took a beating.
7/10/2017 11:22pm
The micro pitting is cause by detonation. A coolant leak "could" cause it but normal it is timing or lean. Triple check that the stator plate it on the factory mark and do a plug test to make sure you are not lean.
7/11/2017 10:19am
Thanks for all the replies, its really appreciated. Speaking to a guy today and he said even if my jetting is spot on, a head that has been damaged by detonation, will still detonate because of the rough texture. How long do you think the head has been damaged like that? The reason I ask that is because I had the engine completely rebuild 8 hours ago by a shop, would've he seen the damage to the head?

My jetting is stock, I use Shell V Power which I believe is 98 octane, it might be 99 but I don't think it is, Rock Oil oil at 40:1.

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