Spark plug broken!!!

Hi, I just bought one kz400 ltd (four cylinder)
While I was changing the spark plugs the number 4 snapped and left the threaded part and porcelain in place. How can I remove the porcelain and then the rest of the plug without having the engine taken apart. Thanks

Any pics of the broken plug?

Not sure if it good quality picture I am away from the bike right now

I've had the same problem, had to pull the head and needed to have an insert installed in that hole as most of the threads in the head stripped too.

That's a bummer.
A long shot but maybe a nut could be welded to the edges of the plug. You might have to drill out the threads in the nut to match the diameter of the spark plug threads. Heat from the welding will help break down the corrosion bond holding the old plug in place.
Another possibility is to drill out the plug and then use a tap to clean the threads. A ton of swarf will go into the combustion chamber but you should be able to create a vacuum tube to suck that out.

I also vote for getting a professional to weld a pipe or nut to the plug threads.

Maybe use a small punch and break up the porcelain in there, leaving the threaded section hollow and use an easy-out.

You don't want to punch the porcelain all the way through in small pieces. That stuff is hard.

If you can get a grip on the center electrode with some pliers the porcelain should be able to be pulled out.
You can then use an easy out or similar to unscrew the plug thread.
You will probably need some heat and penetrating fluid to aid removal.

Run a shop vac with a small attachment to keep as much stuff as possible from falling in to the cylinder while you work. A smart phone inspection scope would work great for seeing where pieces are that fell in. Work with the piston down. It will be easier to see all the way around the piston and there will be less of an angle to work at in retrieving small pieces.

I managed to get the porcelain out with a small pick and a strong vacuum with a small nozzle . I believe everything came out because the parts came out easily, now I wait for the extractor tool to remove the rest of it, thanks everyone for the help!!

Patience is needed, use a magnet and the vacuum, had this happen on Z1A I have, when I woke it up after 35 years, had ez out hooked to small battery impact driver, w/8point socket, as said before make sure piston is down as tool will hit it, lube and heat, nothing worked, but the ez out thinned the core of the plug, used a small tipped screw driver, tapped around top edges, until i was able to work the core out, finally split it and rolled it out with needle nose, took a while, over a few days with lube soaking, cleaned with the magnet and vacuum, small tip needed, ran a thread chase through it, had nothing to lose, either this worked or take the head off, plugs now installed with anti-sneeze compound, lots of way to do it pick one, good luck