Have you verified that the throttle blades are open the same amount on both ends of the carb.? If not try that, it sounds like your not idling on the idle circuit now. Also, if you can, see what size the idle bleeds are now on all four corners and post that size. You should be able to have the motor idle at 1000 rpm in nuetral and drop around 100 to 200 rpm when you put it in gear. What is the initial timing at now, can you get the motor to idle at 1000 rpm and check the timing? Another easy thing to try to fix the idle is to get some small wire,.020 or smaller and make some short L that will fit into the idle bleeds to see if the current idle bleeds are to big I like to make my carbs idle at or below 800 rpm in gear and only need around 1/4 to 1 full turn out on the idle mixture screws to idle clean and not stink out the exhaust That usually requires custom tuning the idle circuits and the bleed sizes on all four corners