YZ125 Wont Crank Up

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6/2/2021 12:24 PM

Hello Everyone,

I am seeking some advice for my 1999 YZ125 that i picked up a few months ago that is giving me a hard time.

Bought it knowing it needed a top end and thus, got a good deal on it. Previous owner rebuilt the bike, it ran for an hour then seized the ring around the piston, chopped up the piston pretty good with grooves.

Before i get into the building of the bike i would like to Put it out there that i am fairly mechanically inclined and know my way around a Yamaha dirt bike, this is my 3rd. However, This is my first two stroke.

I've put the bike back together with the following:

- Refurbished Cylinder
- Brand new piston and ring (Gapped Correctly, Standard Bore)
- Brand New BR9EG NGK plug (Gapped at 0.024)
- Fresh tank of 32:1
- Cleaned the carb and set the float height, (Looks to be stock jetting; 350 Main Etc.)
- Brand new rubber intake clamp
- Brand New twin air filter

I have done my research and people always say look for the big 3 which is common sense if you know how an engine works. These have been checked.

Bike has lots of compression (Brand New Ring and piston), the new plug is always wet with fuel every time i pull it and the spark plug has spark as i tested it quickly while touching the block.

So my question is: Is there such thing as too weak of a spark for these bikes?

Any help and response is greatly appreciated as i don't have much direction currently in this project.

I almost want to take it to the dealer but if i can learn instead of paying out of my pocket, I'm all in and a bag of chips.

Thanks again,


6/2/2021 12:48 PM

What compression numbers are you getting? Have you check reeds and cage? I would check to make sure your crankcase isn't full of fuel also. Should have bright blue spark.


6/2/2021 4:17 PM

Make sure the piston isn't installed backward. The arrow should point out of the exhaust.

You mentioned the rubber intake clamp... did you check the reeds? They should not be chipped or bent.

Is the exhaust clear of all debris, rags, etc.?

Choke circuit clean? You mentioned cleaning the carb and setting the fuel height, but...


Braaapin' aint easy.

6/2/2021 4:59 PM

Is the spark a bright Blue is color? If not you have a spark issue.

Paw Paw


6/3/2021 12:51 PM

Thanks for the replies,

While i was cleaning the Carb i did in fact pull the reeds and the cage to take a look at them. If I'm not mistaken i was looking for cracks and damage but everything seemed to be fine.

One thing i did notice is that the reeds on this bike don't appear to be carbon fiber like i have seen on some toehr bikes rather looks like a plastic material. However, with them out I could see light through them but though this was normal.

As for spark, The old Br9 that i took out was making a yellowish spark, the new one is making a blue spark but still feel like it is weak. I am unable to see the spark in daylight, all the lights have to be off in the shop and then it can be seen.

I have the bike stripped again and will be looking into the ignition system further tonight.

Thanks again for the replies and if any one has any other suggestions, fire away.


6/4/2021 2:23 PM

pull flywheel an check timing marks of stator plate

take spark plug boot off and clip some off for fresh end


6/4/2021 6:47 PM

Clean all the ignition coil mounts and connections.



6/17/2021 1:48 AM

i ve seen people rebuilding these engines using the wrong crank seals- these have been changed through the years as the crank diameter has changed- so check if the seals are the correct size- otherwhise you might sieze your bike up again.