Well, it wouldn’t be the first time a bad plug has caused a strange problem…Does this engine have a thermostat? If not, you may want to use the champion J6C instead. OK, you loosened the fuel cap and ran without the engine cover, good simple things to do for sure. And, the choke linkage is not causing any issues….
Have you tried a different fuel tank/hose? Where is the fuel filter(s) on this engine? OK, was afraid you were going to report that the engine has never run correctly for you since you purchased it…There must be some hidden ghost here, hopefully a simple one…
Try running the engine at WOT for a few minutes, then shut it down and inspect the plugs….
Does this engine have an integral tank as well as the separate tank? Post some pictures if you can….
