It can also mean an over-lean condition, I changed props (from 13 x7 to 14 x 6) on an Irvine .72 that has about 5 flights in it and sincve it is in a pipe it is hard to tune by sound (I forgot my tachometer [:'(]) so i put it in the air and although it was sagging a bit when it comes off the pipe it fle well.. after te flight I checked and to my dismay the residue wsa black and smell a little of burnt castor. I use Mach 7 (Omega) fuel 5% Nitro, with the 13 x7 the residue was dark blueish (mach 7 is blue in color) so I re-tuned thte engine from fully open and flew again. mucho more consistent, less bog down coming off the pipe and after a 2nd flight the residue was lighter in color.. still too dark for my liking but the burnt smell was gone... i will tune it a gain but will bring my tach along!
enough ranting