it is probably because you are going slow, i had this similar prob with my os 21 rz vo1b, it would idle at about 180, when i putted it around it would jump up to about 260-270, if i drove it fast it would only go to about 280, also add some oil to your fuel, comes in little packets. it could also be because it is new, still not totaly broken in, an engine is not totaly broken in until after about 10 tanks!