Check the operation of the flapper valve (bi-metallic spring) which deflects heat away from the bottom of the intake once the engine heats up.
What did the spark plugs look like before you cleaned them? Where they covered in soot as though it was running very rich?
YF carbs have a diaphragm for an accelerator pump which are compromised by the blend of fuel available today. They start to develop a pinhole which causes a rich condition. As they fail further, it becomes extremely rich. I have had this happen 2 times with mine. I was able to drive the truck home by pulling the throttle knob out to keep the throttle plate open far enough to burn the excess fuel so that it would continue to run.
Carl
