I'm guessing its a DJ 160 main? Thats usually run with an after market pipe. It could be a 160 Mikuni which would be very fat. I would use a DJ 155 or a Mikuni 145 main jet with a cut air box and stock exhaust. If your getting 30 mpg, something else is wrong though. Your only on the main jet when the throttle is well open. The needle controls the economy the most. With the stock exhaust, I would try the needle in the middle or 3 down from the top. If your at 4, there should be two open notches below the E-clip. I always like the 4th position though. The carb is leaking some where, I would just replace the float o-rings with OEM parts, check the float needle. Does it seal when you blow on the fuel line? If its original, I would just replace it. Put a fresh OEM stock pilot jet in, I think thats a 42.5. www.cheapcycleparts.com has the parts, I usually get my jets through them. Motolab in the vendors section sells the correct o-rings and jets too. If I go into a carb, I just freshen every thing up. It seems like the o-rings last about 3 years now with the ethanol blends. My bowl seal just failed this year, it was like hard plastic after 4 years. The ethanol eats up the fuel hose too, so I tend to replace rubber stuff at least before three years. The float needle is rubber tipped also. I've been running marine stabil for the last two years off and on and it does help greatly to keep things lubed. I always get 47 mpg or better.