OK guys, pull those cubs out of the junk box and check the needle. Many are very blunt. Apparently they did not have the final step that puts taper on the needle. Result is that it goes from slobbering rich to full lean in about half a turn. Using a file or sandpaper, add some (but not too much) taper from about 1/4" to the tip and try again.
I had this problem on several of the "later" versions with the tube in the intake (which is not necessary, by the way). They run fine without the tube. They were probably installed in order to choke some RTF.
Also use at least 15% nitro...these have a low compression ratio.
George