Can also be very simple issue at fuel inlet to Qjet. Sometimes when you tighten the fuel line there can be some "wind up" in the line. With heat cycling it springs back becoming slightly loose. It only takes a couple drops of fuel to stink. The hot engine makes it vanish as soon as it leaks so no evidence is ever found. Worn primary throttle shaft bushings can cause this as well. During moderate throttle fuel wash can seep out the shaft seals. But when you look at idle that point is now a vacuum leak and everything looks tidy as fuel has vaporized and no evidence. A drop of lube where the flair nut contacts the fuel line should stop the wind up. Slide back the flair nut a bit, apply and re snug with flair nut wrench. Rebushing the Qjet can save it if worn.