I have been reading a lot about jetting on the fourums but still dont quite get it all so here i go with what i think. I have a 86 250r with a k&n, v force reeds, billes pipe and silencer, weisco piston and i run a mix of 40:1 gas oil with premium (91) gas. Here is the problem i before when i was running regular unleaded and 30:1 it seemed to bog in 1st and a little in second and when i would jump when i landed it seemed to not respond right away to the throttle, but now this year after a rebuild and uping the mix and to premium it seems to bog in 1st second and third. the plug was a littile oily. Now the logical thing i think is thats too rich but i thought by putting the exhaust and k&n increasing the air flow would cause it to run lean and also i thought premium would cause it to run lean. So should i go a jet size leaner or is there soemthing else wrong. And does premium cause it to run rich? and how much leaner the stock main jet is 150 (mikuni) and the needle clip at at the 4th position. What changes should i make any help would be greatly appreciated