Jetting on my 02 rm250

If you don't want to change your jetting than you pretty much either have to deal with it or stick to the pipe it works best with. The only adjustment you can make without changing the jetting would be to your air screw and that is only going to effect mixture from 0 turn to 1/8 turn throttle which probably will not help you at all. I don't know why your using 2 pipes but you could try widening the spark plug gap or using a step colder spark plug when using the pipe it fouls with if you have to change between pipes. The reason it's running richer on the one pipe is because that pipe is more efficient at returning the unburned fuel to the cylinder and giving the engine a better return charge. If I was you I would keep that pipe on and rejet the bike for that one.

As said, if you won't change the jetting to suit the pipe, you can only make minimal changes to the low speed mixture with the air screw. With minimally more effort, the needle clip position can be changed as needed as well.

If it is dialed in for the one pipe, why not write down the settings that work for that pipe, and then dial it in and record the jetting for the new pipe. From there it is 5 minute job to dial it in for each pipe. If you won't do that, then you are either going to have to deal with the rich jetting with the new pipe, or sell it and stick with the one you have dialed in. You aren't going to be able to have jetting that works perfectly for both pipes, as the flow is obviously significantly different between them, hence the different air/fuel needs when using them.

I don't understand why you would want to keep switching between pipes. You really can't fix this without changing the jetting somehow. It's just not possible. You can try with some little changes as these guys have covered, but apart from that, you can't tune your bike for both pipes. These two basically covered everything. Let me just say one more thing though...I would suggest you make sure your air filter is clean. That would really help, and some people, suprisingly enough, actually forget about it completely.