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Craze got it right. Adjusting the air screw in 1/4 to 1/2 a turn should probably clear it up. If it doesn't keep trying a 1/4 turn at a time. If it makes it better but not perfect you will need to go down another size on the pilot and start over. Sounds confusing but if you have any questions...

Sounds like you got a plugged carb and pilot jet. Pull the bowl off and clean out the pilot jet (jet in front of the main jet) and make sure you can see light through it. Also, if you can take an air nozzle and spray through the pilot jet passage way in the carb. Let me know if that...

Im thinking that maybe the countershaft bearing is going out? Under acceleration the noise starts to go away?

Are you sure that the chain is not too loose? Sounds like the chain may just be a little loose?

Also, if it did start for a minute, how did it run? Was the choke on? Without knowing the entire story it sounds like you have a plugged pilot and gummed up carb.

Something is not making sense. I thought you said that you started it before you changed the oil? Do you mean that you started it a month ago, changed the oil and now it wont start? If its been sitting for a month the fuel is going to most likely be bad and the carb is going to be gummed...

The bore and stroke are the same on both year groups you listed. They changed it for a reason though so I am guessing maybe the location of the ports could be a little different? You would need to either see them side by side, or talk to someone who knows the RM125 very well. I am guessing that you...

x3 for the gas back on? If you did, check spark. try a new plug. You MAY have fouled a plug. Let us know.

If it is not where the bucket "rides" then I would file away the small bur just so you don't gouge up the bucket when putting it in and end up doing more damage.

Harbor Freight has some cheap brushes. Usually come in a 3 pack or more. They are wire brushes and work very well for getting into the little corners and pockets.