Re: Bought a second hand engine/bike, having some problems
I got a set of micro drill bits and a pin vise from widgetsupply.com, If I remember right the main jet size should be soldered shut and re drilled with a #70 to 73 bit for best performance. The drill bit sizing system is the higher the number the smaller the bit, you can also find out a jet size when it's not stamped on the jet by using the micro drill bits to see which one pushes thru with just a slight resistance. You can use jet size pins if you got them, but the drill bits work the same way to quickly determine the jet's size before drilling.
Personally I wouldn't re drill a jet or change it unless you nkow the previous owner was messing with the jets, the stock jet in these is usually on the rich side which is safe, but can make the engine a bit sluggish, if it's really boggy it could be running lean from a dirty carb or an air leak. It could also be a loss of compression from a bad head gasket or head nuts that came loose. Getting an old engine that's been sitting for some time can have all kinds of problems from something simple like the carb being gummed from fuel that's just sat in there for a few months or longer to blown gaskets or seals that'll let air sneak in or it could have gunked up fuel inside the case or even porosity caused by corrosion if it's been sitting a really long time...