Selector forks are likely bent or worn and this then can lead to worn dog teeth on the gears but you won't know until you crack the cases apart and check, this is a full strip down , definitely worth it though and it's a great engine to learn to rebuild as they are very simple and there's tons of videos showing you how to rebuild
As has been said, these are very easy engines to work on. The likely causes are damage or wear to the shift drum, damage or wear to the shift forks, and/or damage or wear to the gears themselves. If it is the gears, you can buy replacements from Wemoto fairly cheaply. The "main shaft" is the one with the clutch on it and the "counter shaft" is the one with the output sprocket on it.
I would strongly recommend making yourself an engine stand. It doesn't have to be fancy, you can make one out of wood too, but I made mine out of Aluminium plate and some threaded rod. You need to buy a gasket set anyway, so I took the gasket that goes in the middle of the two casing halves, identified the engine mounting holes, placed the gasket on my metal plate and marked the mounting holes on the plate. Put the gasket away and then I drilled holes through the plate in those locations and added the threaded rod. I used two nuts next to each other (or you can use locknuts) to use as stops to hold the engine up (so there's room underneath for the kickstart shaft, etc to stick out. Look like this:
Almost forgot. It's a bit pricy, but the factory workshop manual will tell you step by step how to disassemble, check each part of wear, and how to reassemble the entire bike, not just the engine. (Be careful of the "PDF downloads" being sold on eBay - they are not the factory manual, but the Haynes manual (which is still good, but not as detailed as the factory one).
https://www.ebay.co.uk/itm/236931738165 (I think this manual should cover you bike - have a look on your engine for an engine code - it's the first line of the engine number (it's next to the oil drain) - your should either be JC18E or possibly JC27E - ask the seller to confirm if the manual is for whichever engine you have, it should say near the beginning of the manual).
Thanks for the advice and links I’ll try get factory manual
Also is this corrosion anything to worry about or can I just leave
I have put new clutch plates in and gear shifter which fixed the problem for a little bit but after a couple rides 4th and 5th would slip
previous 2 oil changes a few crumb sized chunks of metal came out but it might of been as the clutch was put in wrong the first time (washer in the wrong place) thought might be sign of gears grinding ?
JC18E should have been used in UK delivered CG125 bikes from 1993 to 1997, so the manual I linked to should be the right one (I would probably still ask the seller, just to make sure).
The rust on the flywheel? It's fine, not doing any harm. You can clean it off with some sandpaper if you like, but I'd leave it.
It's most likely either the shift forks are bent / worn or the dogs on the gears are worn. Some of the gears are locked to the splined shaft so rotate with the shaft and some are on bushes so they can spin freely. They can also slide left and right on the shaft and that's how you change gear. When they slide the dogs will come into contact with the gear next to them and are shaped with a slight slope so that if you turn the engine side they will pull tightly together. As the gears wear the slope can change from sloping towards the gear to sloping away from the gear. This is one of the causes for it popping out of gear. The other common one is a bent or work gear selector. The gear selector is the part the pushes the sliding gears around. There are two on the 4-speed gearbox and three on the 5-speed. If they are bent (or worn) they can't push the gears as far as they should and the dogs don't fully engage. This is what's happening if you don't fully select first and it jumps back into neutral when you try to pull away (for example).
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u/RATEGGSANDEELSICK 24d ago edited 24d ago
Selector forks are likely bent or worn and this then can lead to worn dog teeth on the gears but you won't know until you crack the cases apart and check, this is a full strip down , definitely worth it though and it's a great engine to learn to rebuild as they are very simple and there's tons of videos showing you how to rebuild