First off it’s not a Ducati issue but I know there are some on here with greater experience in such matters than me. It’s a 2000 Honda Hornet, 600, IL4, the issue being the revs ‘hang’ when the throttle is closed. It’s running too lean on the slow circuit I hear you say. My initial thoughts too. The carbs (Keihin) have been stripped & cleaned & rebuilt (the bike had been standing for some time), the old fuel drained, the tank flushed out with new fuel and drained again. There are no air leaks in the inlet manifolds. According to the workshop manual the standard fuel screw/pilot jet setting is 1 3/4 turns out, I’ve increased this to 2 1/4 turns. According to the workshop manual the std slow circuit jet is 40, that’s indeed what was in there, they were a bit blocked up with old fuel but were ultrasonically cleaned in carb cleaner, then reinstalled. I even put in new 40s just in case. I’ve since increased them to 45s as even the new 40s made no difference. The above actions have improved matters somewhat. When warm the bike starts and ticks over as it should, around, 1200 rpm. However, blip the throttle to say 5-6k and the revs rise and quickly fall back to 3k and then slowly fall down to 2.5k and just hang there. Yes, the throttle valves snap shut with force, there are no restrictions in their movement or that of the throttle cables. I’m at a loss now as to what else might be causing it, so any and all suggestions/advice/thoughts are welcome.
I had a Sunbeam with a very similar issue. I found the throttle slide to be slightly worn allowing too much air past. Fixing that improved matters but didn't cure them. I could only put it down to the poor fuel with lower calorific value we have now so upped the main jet and reduced air through idle screw and it improves again. Very different bike and carb but you are probably on the right track.
Another for an air leak. Tracking it down is usually tedious. Try spraying carb cleaner around the inlets and see if the tickover rises.
There will be various o rings on the carbs, each pilot screw and also on the throttle shaft where it goes through the carb bodies.These are all prone to going hard and wearing which in turn cause various small air leaks which make the revs hang on when closing the throttle. Very well known problem on the old Suz GS1000 engines as I found out when I had the same problem. I just bought a s/h set of carbs that had been refurbed which sorted the problem. Choice is to strip, clean and rebuild with new seals etc or just fit a new set.
Yep , that’s ok Done that, to no avail. No air leaks from outside the carbs that react to that This is where I’m at now. The fuel screws/pilot screws, post throttle valve, have a rubber o-ring on them to keep air out. I suspect, even though they’ve been visually inspected, that one or more of them has perished with age and the fact the bike stood unused for some time. I’ve ordered 4 new ones and will replace and see how things progress
