This is a motor I rebuilt prev owner ran no oil . New rods ,pistons , crankshaft , fresh honed cylinders , recut valves and seats , all new seals and gaskets , welded the window in the cases , fresh oil and filter . Starts and runs well for a second and then starts to puff black smoke and hunt , recleaned carb , drilled out both welch plugs and ran a strand on electrical wire through everything . Noticed the skinny little tube had serious corrosion/staining on it . New welch plugs installed and reasembled same thing . Ordered a new Kawi carb . While waiting got it hot and did a compression test , 150 both holes , plugs are sooty as expected . , rechecked coil distance at .010 ok . ran motor with neon plug tested on it , no misfires or anything weird . Install new Kawi carb and it runs exactly the same . Cannot be the carb so checked the fuel pump looks like fair flow , started it and run it and used a pair of pliers to pinch fuel line to about 1/2 flow , takes a minute and it cleans up and runs fine . Release the pliers and it goes back to hunting and smoking black . What the heck is going on ? Any ideas ? 40 years at this and never ran across this . Its on a Toro zero turn maybe 10-12 years old . Help