Never seen the inside of one (I'm assuming it is a FS-48), but the parts list (
http://www.osengines.com/parts/xr35800.html ) does not list shims so it is
probably a simple wrench adjustment.
I'd contact the folks below and either get a copy of the manual or at least ask for the valve clearances. Be a good start.
I've lost the manual for my OS engine. How can I replace it?
Unfortunately, we do not stock the manual for your equipment separately. However, we can have our office copy photocopied for you at a charge of $10.00 plus $5.50 shipping (US) or $13.50 shipping (international). If you wish to purchase a copy from us, please fax us at 217-398-7721 with a credit card # for payment (Visa/MasterCard), or mail us a check to:
Hobby Services
3002 N. Apollo Drive, Suite #1
Champaign IL 61822