I ride a Yamaha yz125 m.y.2003 only on motocross tracks.
In last month it's driving me crazy.
Sometimes it powers off and there's no way to make it runnig but changing the spark.
With new spark it goes on perfectly, then goes off again with wet spark.
When it runs goes on for hours perfectly.
The piston as 6 hours of work, I already changed cabling and ignition coil.
Carburetor is Ok, reed valve is OK.
It seems like the spark looses power, so the gasoline doesn't burn completelly and the spark gets wet.
What can I do more?

I ride a Yamaha yz125 m.y.2003 only on motocross tracks.
In last month it's driving me crazy.
Sometimes it powers off and there's no way to make it runnig but changing the spark.
With new spark it goes on perfectly, then goes off again with wet spark.
When it runs goes on for hours perfectly.
The piston as 6 hours of work, I already changed cabling and ignition coil.
Carburetor is Ok, reed valve is ok.
It seems like the spark looses power, so the gasoline doesn't burn completelly and the spark gets wet.
What can I do more?
After 1.000 attempts and checks I changed gasoline.
Nothing special, I just took it from another pump on the street.
It seems OK now.

I ride a Yamaha yz125 m.y.2003 only on motocross tracks.
In last month it's driving me crazy.
Sometimes it powers off and there's no way to make it runnig but changing the spark.
With new spark it goes on perfectly, then goes off again with wet spark.
When it runs goes on for hours perfectly.
The piston as 6 hours of work, I already changed cabling and ignition coil.
Carburetor is Ok, reed valve is ok.
It seems like the spark looses power, so the gasoline doesn't burn completelly and the spark gets wet.
What can I do more?
Well, the oil and gas mixture isn't right because I had the same problem. Since you have the wrong oil mixture, the plugs won't create the spark. Get a new oil mixture.. Probably 40:1, I have the same bike. So try that.

Wrong spark plug