The carbon comes from the oil burning, not the methanol or nitromethane. Most car fuel has primarily synthetic oil for lube with a minute fraction being castor. Your average synthetics burn off much sooner at a lower temp than castor so it lends to why more carbon will show up. Now if you run richer fuel mixture, you are running more oil through by volume and consequently at a lower operating temperature. Remember, alcohol burns much cooler and slower than gasoline. A gasoline engine will have more carbon because of the fuel AND the oil, glow engines get carbon from the oil only.

I mix my own fuel and use a 50/50 oil blend of castor and klotz in my car engines. Running at peak mixture still results in no carbon because of all that castor. The oil comes from the exhaust a nice dark brown color.

Crappy fuel like Traxxas is a good carbon producer, probably due to the high synthetic oil content. You have to lean it down so much to get nice throttle response that the crazy oil amount just burns.