2015 Ktm sxf250 radiator cap?? 2.1 bar??

2.1 should work fine.

Paw Paw

Changing the cap should help. It might have just been a hot day. If it keeps happening then looking into it deeper but I had many KTM 250f's and only overflowed fluid a few times.

If you overfill your radiator it will spit a little out also.

The higher the pressure the higher temperature the coolant will boil at so going to 2.1 bar will mean your bike will have a higher pressure/temperature when it does open the valve in the cap, this in turn will put more stress on your hoses and also internally on your water pump seal.
I am currently building a sidecar to race in a 3 hour long beach race, deep, wet sand running a KX500 motor and have gone from a 1.1 stock to a 1.4 last year ( we still boiled over ) so am going up to a 1.8 this year but I'm replacing the water pump seal and cover gasket, running new silicone hoses on the motor and new heavy duty auto hose from the motor across to the rad that is mounted on the front of the sidecar. I have also fitted an electric fan to assist in cooling during slow running, a lot of factory 250fs run fans in hot weather, you could look into this option.
I have been doing a lot of research in this area the last month or so, hope this helps you out.

If it's only drops of coolant than the 2.1 may be perfect but if you still have issues after that I think KTM sells a high flow waterpump kit (but not sure if for your exact year and model) . It has a larger impeller and a spacer to make room for the extended impeller. I would check into this cause in my opinion a cooler running bike will have less issues and reduce maintenance over time so the 2.1 might just be a bandaid. I've gone form 1.1, to 1.6 to 1.8 but never over that.

A stock KTM cap is a 1.8 bar and a 2 bar will be a little better. Our KTM radiators come with a 2.0 bar cap.