I've recently bought a 93 RMX 250.
Have a few problems that I'd like to solve.
Main problem is when i really open her up it kind of splutters or holds back, then kicks in.
I changed the spark plug and it rides better but still same problem. I've noticed that it's probably time to re pack the muffler and also change or clean up my air filter. But would these be answers to my problem?
Before you get into jetting, start with the basics. Definitely get the air filter cleaned and properly re-oiled, this should be a regular occurrence after every few rides or even every ride if the conditions are bad. Take the carb apart and give it a thorough cleaning, making sure all the jets have no blockages. If the bike was stored for a period of time with fuel in the carb it can gum up the works and the bike may be having trouble getting fuel when you open it up. I bought an old RM125 that seemed to run fine but stumbled just like you mentioned after cleaning the carb it was better but still not right so I replaced the main jet and it ran perfect after that.
I have the exact same problem with my kx I replaced the air filter, jets, packing and it still does it. I am going to rebuild the engine becuase that might be the problem with mine, my compression is 70 and i dont think it should be that low!