Hello!

First time here, non english, so forgive me for any typos etc. 

My YBR 125 engine (on Derbi Senda frame) started to make weird noise. Before that engine ran smoothly. Now power is lower than usual but it runs and shifts like it should. 

Link to video: 

https://www.youtube.com/watch?v=blPZ5RVSqEQ

What it could be? 

I've opened the engine up and no major flaws are seen. Im changing 125cc to 150cc anyway.

Edited September 16, 20195 yr by Olgerz

Best guess, hole in the piston or busted piston rings!

Or a burnt valve,  if you"ve been running without the air filter ,  its running too well in vid,, for a holed  Piston ,

Thank you for replying. Piston is OK, no holes, rings seems to be also OK - don't know how to measure them. Will replace all parts anyway.

Valves are opening and closing properly in my opinion - should I replace the valves and other parts?

What about that periodic humming sound? Could it be bearing or something? Don't want to open both crankcases for nothing..

Edited September 19, 20195 yr by Olgerz

If you are fitting the 150 I think you would be making a mistake not taking the engine right down. 

Then when built back up you know it will handle the extra. If you have stripped the top end down. Splitting the cases is really not much more. 

At the moment you could be fitting a lot of expensive parts to, maybe a bad crank bearing?

 

Is it OK to fit chinese 150ccm or Airsal cylinder is the only choice? Not gonna live forever with my YBR, prob selling it.