Hi, again,
I'll confess to knowing very little about Surfriders, but I understand that they were just re-badged Bridgestones. So your beast would bea re-badged 90 Mountain, but there is something strange about the serial number for a non-injected BS90. A sertial number starting with "20" indicates a BS100 Sport, and if you take the third character of the serial as a "Q", note a zero, that fits in nicely with the serial sequence for a BS100 Sport, but the 100 Sport was an Oil Injection model. If you are taking your serial number from the plate on the frame, has somebody swapped the frame (or just the serial plate) at some time in the bike's life? If the whole frame has been swapped, you should see evidence of an oil tank mounting on the right hand side of the frame.
The only other possibility is the BS100 SR racer, which used a 100cc motor (thus serials starting "20") but came from the factory without the oil injection system. The 100SR is a bit rare, and I know even less about them myself - there are other members on this site who are founts of knowledge about the BS Racers. Perhaps one of them could add few pointers.
Finally, a picture of the bike might help to establish its provenance.
Graham
