Reviving a CBF125 (Part 3 - Cylinder & Piston inspection)

[@GdaySport]

My grandson's CBF just done this, but only lost one side of the piston skirt below the oil ring and he switched it off immediately. Piston is obviously buggered by I think the bore is OK and so is the head. I think these engines run quite hot for an air cooled - the plug indicated quite a lean running engine...

[@xmoroseguyx]

I reckon they were flying along and pulled the clutch in, Dropped a few gears and just let the clutch out and revved the engine so hard it broke the piston

[@buddysinsight9834]

I had that bike and engine as a teenager and I ceased it up a few times. I would then let it cool down and it would start up instantly.and be totally fine. I used to get 80mph out of it on a big down hill and thats what did it. I don't think it's hit the valves. A new top end and it will be perfect. My bike was a brand new F reg at the time if that helps

[@MrScififan]

Thanks Buddy - impressive you got 80mph out of one of these - I don't think the earlier models of these were fitted with rev limiters so possibly something similar happened to cause the damage - glad there doesn't seem to be any valve damage but I'll inspect those in the next day and see if they're good to go.

[@lordracula2461]

That's because the fuel pump was faulty from factory. Lot of noobs call it "overheating" or "seizure" but it is most definitely a faulty fuel pump, anybody with any mechanical knowledge knows the other reasons are bollocks. Only the 2013 & 2014 year CBFs came with no fuel pump issue from stock

[@buddysinsight9834]

Lordracula My bike seized solid. What this means because I am not sure you understand is the engine wouldn’t crank over it was locked up. Trust me years of motorcycling from a little lad gave me the experience to diagnose the problem. It does make me laugh when someone tells you your wrong on here hahaha.

[@lordracula2461]

@@buddysinsight9834 I've never heard of anything like that in my entire life, the starter just makes a whirring sound?

[@buddysinsight9834]

@@lordracula2461 When a single cylinder bike seizes it doesn't just instantly jam. It slows you up like you have the breaks on hard. I was often thrashing the arse off of it. Then if you put it in 3 gear it will not move or turn over its locked up. Starter doesn't engage and if its a kick starter thats jammed. I am from a trials and motocross back ground so dealing with this is not uncommon.

[@forevercomputing]

I WOULD look at the piston if you kept the camera still. It's injection, therefore will have a limiter. Mine does.