its not tuned properly.

get it warmed up, over 200 degrees:

HSN should be so it gets optimum performance at full speed but you still get smoke from the pipe. You shouldnt tune by temp but it is an indicator. Make a couple full speed passes and temp it. somewhere between 230 - 260 is good.

pinch the fuel line 1 inch from the carb. Should take 4-5 full seconds for it to die. If it takes longer, LSN is too lean, if it dies sooner its too rich.

If its not idled up high enough it wont stay running either. I always take the air filter off when I install/reinstall an engine just to be sure that the servos are set correctly that carb just opens all the way up at full throttle and that the idle isnt set so low that hitting the breaks will kill it. Generally, there should be about a 2mm gap in the slide opening in the carb. Check this with the engine off obviously and the servos turned off.