DDK - Die Deutschen Klassiker

Son's 125 Honda has lost compression. What are the chances I will get away with just changing the rings? I guess the more likely explanation is that there is bore scoring? I've found some cylinder/piston kits on ebay for £50 or £100. Hard to tell which is the better item

I guess I need to take apart and message

https://www.ebay.co.uk/itm/224222920771 ... media=COPY

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Take it, it is four stroke?
Might actually just be the valves?
Have you checked the clearances. They may not be seating properly, build up of carbon...................burnt out.

We have reset the clearances twice. Ran better for about a month and then started to lose power and struggle to start

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If it ran better adjusting the valves, then possibly not piston rings?
Could be the valves are carbonised or burnt out. Probably best to take the engine out and take the head off first to see what state they are in. Honda engines are normally pretty good and well built. Check the carburettor as well as ethanol may have caused problems there, with crap in the fuel lines and jets?
Is the bike throwing lots of smoke out to make you think the rings are worn?

Have you done a leak down test to see if it is the rings?

The sequence of events was

Reset valves
Bike ran better for about a month
Son put some fuel injection cleaner in and it started to run crap (not sure if this is really the cause)
I reset valves but no improvement this time
Then the bike just wouldn't start at all and struggled to run

Let you said. I think first take apart and clean/measure

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Have you run a pressure test?

Have you stripped and cleaned the carb? sounds like the miracle clean stuff may have gummed jets up?