There should also be what i call the 'support' bar..
Which is wht the cable slides into and its the stopping point.
I will demonstrate
I know how it's supposed to work, i messed around with my stock carb before. The thing is, on this carb for some odd reason, there is nothing to stop the cable from just pulling right out which is why i have to loop it around the spring making it too short. I guess i can try stripping the outside part of the cable down a bit and see how that goes.
We call them China copy carbs 603s even thought there not. The 15mm is on the bottom and my Walbro 19mm HDA48D have No bulb at all. I can spot a real Walbro a mile away cause the inlets and return fittings are black plastic not chrome metal. I have both, plus a 668 and 2 813s.
Sounds like your cable has to much slack. When you crank the throttle and let off and it snaps back, the lead on the end of the cable slips far enough out of the swivel it will slide right out of the slot. Took me along time to get all the bugs out of all my Walbro carb engines. Still happens once in a while on my Lucky 7 semi chopper.
I forgot, try cutting alittle off the top tube on the carb to make it shorter, then re-install the carb and cable. Also where the cable fits in the top of the carb you need a rubber cap or RTV silicon to seal it, or it can suck air into the carb. On the square piston, spring and cable, it's nightmare. I run the cable thru the spring about 1/3 from the top and hook it to the piston. Have to slide it thru the piston, then pick up the cable it should lock in. Might have to do it a few times to get it right, if there is one,lol.