Having had the carbs from my sons 78 CB550 four k ultra sonically cleaned without any improvement in the mid-range popping and banging I stripped the carbs for the 4th time. All the jets and any holes had been prodded and poked to dislodge any solids and then blasted with the airline previously. With the carbs turned over and the float bowls removed next to the main jet is a brass tube which on most carbs has a slot and is threaded for removal. On these carbs it’s a plain tube and is not shown in the W/shop manual. As we had a spare carb I decided to risk giving this tube a twist and a tug with some grips. Sure enough, although tight, it did come out, revealing an emulsion tube and jet with most of the holes blocked. Once these were cleaned out and the housing blasted with air on all four carbs I tapped the tube back into the carb body, bike now runs sweet.
Just as a follow up, I’ve since come across this set up on a Honda XL185 and similarly there’s no reference to it in the workshop manual, only the more normal screw in pilot jet gets a mention.
