What year is the 550? If the float bowls are just held on with a clip, drop them and pull the jets out for a cleaning. It’s tight but doable. If the idle jets are plugged up nothing in a can will help.
It's a '74. Float bowls each have 4 screws. Kind of a pain to remove the middle two carbs, but 1 and 4 are easier. Still, I'm hoping for something I can just pour in to help. I pulled the bowls and 1 and 4 already and the jets didn't seem that bad, but the floats and float needles were sticky for sure.
What did the bowls look like when you pulled 1 and 4 off? Still liquid gas or gummy brown or green?
I haven't tried the STP cleaner, just Seafoam and only in Goldwings.
How long did the bike set with the old gas?
Did you empty the tank of the old gas and shine a good bright flash light in the tank to look for rust? Even light surface rust can clog carbs causing bad running.
How long did you ride the bike after you put the STP cleaner in, and how long has it been since you put it in?
Did you put the full bottle of cleaner in the tank or how much?
Depending upon how long the bike sat with the old gas in the carbs will determine if a cleaner might work or not, and how long it will take for the cleaner to work.
The gas we get these days starts to go bad in just a few weeks of setting in the carbs, and can start clogging the emulsion tubes in the idle circuits in 6 months to a year. It could take 24 hrs setting in the carbs idle circuits for the cleaner to do it's job. Then after 24 hrs you need to ride the bike to burn out the gas in the tank in 1 ride then refill with fresh gas.
If it's been setting in them for years, you will have to pull the carbs out to properly clean the emulsion tubes.