This is an OLD motorcycle racer tuning trick - an EMERGENCY method used in order to have the correct size jet when you just cannot obtain one.
To increase the jet size, you can drill out a jet with the next size larger NUMBER drill.
(If you EVER drill out a jet, mutilate the number stamped on it so that no one will later confuse it with an unmodified jet.)
But what to you do if a jet is too large?
Get some flexible insulated stranded copper wire. Poke one or more strands of wire through the jet until you get the engine running right. Secure one end of the wire by wrapping it around the top of the body of the jet (or whatever else is in the area may be handy) and then poke the loose end through the jet. Not all stranded wire has the same size strands, so find wire of a size where each strand of wire will reduce the cross-sectional area by only one percent or so. (Measure, and then use a little math here.)
I know that this sounds like a REALLY funky idea, but I have successfully used it many times.
(I have occasionally used it as a "semi-permanent" solution - holding the wire strands in place by soldering them to the top of the jet after finding the correct number of strands to use.)