If its spitting back, I'd question timing.... but you said..... . Ignition and carburation can mimic eachother. It could be a bad condensor if you still run points ignition. Other items to check, plugs (for cracked porcelin) or wires for bad connection. On the flip side is lean condition caused by clogged fuel filter. Check to make sure there is enough fuel in the bowl or temporarily remove the filter to make sure you have good fuel pressure and fuel getting in. Get back after you check those.