No compression with new cylinder

Question

Last year I was riding my motorcycle, and the motor turned off while driving. It was a harsh stop, it didn't die slowly like when there is no fuel left. After checking the compression (which didn't exist) and taking apart the motor, I found the issue which was a slight piston jamming.

I purchased a new cylinder kit (with piston and all gaskets) and installed it according to a well documented guide.

Now I have compression, but it is very low. 4 to 5 bar max. I read online that this motor should have around 10 bar.

Nevertheless I installed the motor to my bike. It starts successfully but dies off when opening the throttle. The motor only starts revving when the throttle is opened very gently (like 5%)

I already tried another cylinder head (which also was from a motor that got a piston seizure) but the results were the same with 4-5 bar.

Maybe someone has an idea what i could try next, before i get ripped off by my Yamaha service center.

The bike is a Yamaha WR125R 2013 Model (4 stroke, 1 cylinder)

Answer 1

1. It sounds to me like you need to adjust your carburetor properly. Maybe get it cleaned in an ultrasound bath. that should cure your throttle problems.

2. You need to brake in your new motor. The Piston-rings need to set and the honing of the new cylinder will be broken in. There are different opinions on how to do this, but you should give it some consideration. Best to read up a little bit about it online and then decide for yourself. Change the oil after 50 km and then 1000 km before starting normal driving. Check the compression again, should be higher then.