Try a 290 main jet and mess with the airscrews.
I got this off of Banshee Jetting Faq
Start by adjusting the airscrews; on each carb turn them in (clockwise; which is richer) a half turn (180 degrees); adjust both airscrews the same amount. If you have the motor idling allow about 20 seconds for the motor to react to the new setting. If the condition gets worse, try going out (counterclockwise; which is leaner) on the airscrews and see if it improves. If going in on the airscrews helps but the problem persists after you turn the airscrews all the way in, or to within a half turn out from seated (don't torque the airscrews! the tip is pointed and overtightening them can cause damage!), try the next size larger pilot jet, and start over with the airscrews 1.5 turns out; tune the airscrews from there to get a clean idle and off-idle response. Conversely, if going out on the airscrews helps but the problem persists after you get to about 3 turns out from seated, try the next size smaller pilot jet and start over with the airscrews 1.5 turns out; tune the airscrews from there to get a clean idle and off-idle response. Once you acheive a clean idle and crisp off-idle response (no bog or hesitation right off idle), you may need to adjust the idle speed up or down by adjusting the idle screws. If airscrew adjustment does not have any affect on the bog or hesitation, and you are certain the problem is below about 1/4 throttle, check the carb sync, make sure the pilots aren't clogged (it only takes a speck of dirt to block the tiny passage), and insure that both airscrews are set the same number of turns out from seated.