Car knowledge, dirt bike dummy

The pics would help a great deal. As most 250cc bikes are a little under that ie 250 might actually be a 246, 247,248, 249cc. And the 123 could be a 125 actually. But the only 246 that comes to mind right now is the 1965 YDS3 SuperSport. Here is a pic of it orginally.

Well if I must say so, you've come to the right place to learn, no....wait, the best place to learn about dirtbikes. First off, those exact metric displacement numbers you gave us (the 246cc and the 123cc), those we refer to as 125's and 250's. The 123cc being the 125 and the 246 being the 250. It's a way we classify dirtbikes. As far as identifying the two bikes, pictures would be very helpful, there are a lot of people on DRN that could most definately identify the bikes. If you are so inclined to learn every ounce of dirtbike knowledge that you can at this point, Supercross (SX) is on every saturday. The show goes a little in depth on dirtbikes, key word on little, it'll ocassionally show bike setups, explain rules, classes, etc. But for the most part it shows indoor racing. Hope you can get those pics up soon.

If they both have a single shock rear suspension, with a triangular swingarm, they are definitely Yamahas.

Noob! I love it!