Have you had this set up a while? I have lots of Weber experience and the spitting back of one of the carb throats, intermintently, is fairly normal. The idle jets do get clogged very easy, as the jet hole is VERY small. This will make it idle rough and spit back more often. You can try helping it out by adjusting the idle mixture screws, VERY SLOWLY, untill you get the best idle. SLOWLY lean each cylinder out untill the engine drops that cylinder and SLOWLY back the screw out untill the cylinder comes back. Do this for each cylinder. Try driving it like this. If it still spits back too much, back out, richen, each cylinder 1/8 turn and try it. If it spits back infrequently, be happy and drive it.