Welcome to the forum decafe.
Your 78 550 would have come with 46C PD carbs, is that what is still on it? That would be my 1st question, as these were jetted lean from the factory. Regarding your the issue with the idle. My 550's 46A PD's currently have one size larger slow jets (40's), and 98 mains (stock 76 sizes), and is very lean above 4K. However, it starts (on choke), idles very nicely, and pulls cleanly. I will be going to 110's, possibly 115's because of my pipes & the filter. So your 105's could be slightly lean at the top end. The fact that the idle problem seems to go away once fully warmed up is interesting. Have you checked that the carbs are closing freely (no cables), and that the cables are in good condition, routed properly, and move smoothly?. Quick, simple test - (cold, without engine running) roll throttle open to 1/2, 3/4 open, then let go of throttle. The carbs should close instantly, with an audible snap.
For a carb sync to be any good, all the other parameters have to be in spec. 1st (timing / valve clearance - the basic 3K tuneup).
As ffj said, the filter kit was designed to alleviate the slow speed / part throttle problems Pods cause by the removal of the stock stacks, while promoting free breathing everywhere else. I do have 550 kits available (blk, or red stacks currently), but will leave it to you to decide if it is for you.
