Nuttered now a stumble

Hello everyone I've been gone for a while since I sold my CJ about a year ago but i just bought another Jeep! Its an 88 YJ with a 4.2 with weber carb and a 5 speed! Somewhere around 160k miles. Anyway it ran decent before but i did the Nutter bypass on it and set the timing to 8* BTDC. It seemed to idle smoother but upon acceleration at low rpm it would stumble. It has a new distributor in it so i figured that was alright. I replaced the plugs with autolites. the old ones were in rough shape so that helped some but didn't fix it. I replaced the fuel filter also thinking that would be it. Anyway I played with the timing some more and advanced it and it got better and better. Now it runs better than it ever has except from low rpm, say in 4th gear trying to accelerate from 1300 rpm up until about 2000 rpm it'll stumble and sputter. Any ideas what else could be causing this problem? Thanks in advance!!