Hi Speedy
I am not going to be much help, but I would say your problem is something to do with the ready start auto choke system.
As far as I know, the 550ex has this feature; which is a bimetallic spring that opens the choke as the exhaust heats it up
I can't say I have done any of these carbs (except on a masport that had been tipped over and filled the bowl with oil).
Others like AVB are probably significantly more adept with this carby and the issues it has.
This bloke on youtube tends to be really good with precise info.
This is how he says to clean them.
I have been saying the same thing about these mowers - picking these up at tips and off the curb in 5-10 years will be great haha.
Best of luck with the EL.
