Any lean-out condition will cause it to go crazy. Start at the beginning. Check petcock flow, then fuel line, then float adjustment, jets ect. Could be leaking head or base gasket. There is plenty of thing it could be. If it just started doing it, look for plugged petcock or debris in tank or jets. Old problem is probably floats need set properly. Could also be that the slide is not returning to a closed position. Other real experts would know about the size of carb issue. Good luck with it.