I own a Honda CRF 150 F, 2006 model. The motorcycle starts well but after it warms up (2 to 3 minutes) once I rev it, it wants to die instead of the rpm going up. I checked the spart plug and it has black deposits but not so much. Air filter is also clean and engine oil has been changed. A friend told me that I have to warm it up much longer (for 5 to 8 minutes with the chock on at the middle and then it should be fine. What do you think?