The Honda PCX / Forza / ADV150 / SH Forums

Hi all! Just had my Yuminashi kit fail for some reason. I suspect a ring stack failure but overall damage are bent valves, damaged piston, stuck ring stack and scratched cylinder. Had 700km on an oil change. Oil is dark.

My setup started life as a 125 and I still have the cylinder and piston and rings. I do not have the 125 head.

Will the 150 head fit on my 125 cylinder?

I just want/need to get it running asap, can't wait for new Yuminashi parts.

Photos attached.

My guess would be the rings gapped wrong. Why did it bend a valve ? That's a different problem altogether. Or did the valve drop causing the ring and cylinder damage

Hello, this must have happened between two and one... either the final engine stopped working, or the cylinder is lower and caused the valves to hit the piston. Did you measure the squish? what are you using for an injector?

Mechanic that tore down and rebuilt was not very forthcoming on the cause. I got the cylinder and the damaged piston but the damaged valves were not to be found. I see no indication on the piston that there was a valve strike so I am also confused on that issue. I DO suspect that there was an issue with the rings though. The mechanic said I had low oil (in the oil catch pan above) which leads me to believe the rings were messed up by this mechanic when he solved a coolant leak problem. I had put only 700km on the scoot since the last oil change. I will have a different mechanic review the damaged piston in a couple of months.

Scooter is running on original stock 125 cylinder & piston with 150 head, Yuminashi 8.6 cam & upgraded lifters and springs and 140cc injector. I will attempt to hone out the scratch in the cylinder as it is the Yuminashi flat deck.

Am still evaluating my next step though.

(1) I can leave as it sits now,
(2) Go back to the 164cc Yuminashi bore up by replacing the damaged 60cc piston & rings and reinstalling the 170cc injector or
(3) Do a further upgrade to 180cc by installing a spacer, 3mm offset crank pin and longer head bolts and timing chain. (And a couple of other necessary small parts).

Any comments and thoughts? I will start another thread about the Yuminashi 180cc upgrade.

An additional update. I removed and inspected the rings from the damaged piston and discovered that they are not the rings that came with the piston (Yuminashi rings are marked, these were not). I suspect that the mechanic replaced the rings when he worked on the bike to correct a coolant loss issue earlier. There might be an indication that there was a valve strike on the edge of the valve pocket on the piston. The piston skirts show that they contacted the cylinder and the coating is worn off. Never saw the valve seals but know they leaked when tested. Don't know about the valve guides.

My premise is that due to the substandard rings the piston fluttered on heavy acceleration (when it happened) and caused a strike on the piston pocket edge.

Have completed honing the scratches out of the cylinder. Using a caliper (no bore gauge) cylinder size has increased by about .02-.03 diameter. Thinking I should be good to go with a new Yuminashi 60mm high compression piston & rings set.

Since I would need to rebuild again I am getting a wild hair up my butt. Considering installing a 3mm offset crank pin and forged connecting rod which would bring displacement to 180cc. Parts cost probably around $100 US (pin, connecting rod, spacer, gaskets, timing chain). I read on one pin ad that the pin could add 2.5 HP.

Thoughts anyone?