I'm writing this from Australia..trying to remedy my grandsons junior carby problem....I used to be on a junior web page but can't remember it..specifically the mikuni 42mm carby has just started to hang up after the finish line and he has to kill the engine to stop...we talked to many other parents but they can't think of any remedy...we rebuilt the carby and tried an extra return spring on the mechanism where the cable sits...still hanging up...in the pits it works perfectly and this only just happened at the last test day we went to...nothing was changed except weight taken out to help decrease ET...were missing something but can't work it out...I know there would be many on here with kids racing juniors so can I get some ideas what making it happen...btw the car is deadly accurate on ET's so whatever it is it's not affecting it's performance
Thanks in advance

Sounds like your carby has gone past centerline at WOT. I know on Holleys there is an adjustment so that won't happen. I had one do that to me on a borrowed 850 years ago, and no matter what I did it wouldn't come off WOT, had to shut it off past the finish line.

the mikuni is just a slide and needle carby..very simple and i make sure that the slide doesn't go past the opening area with the adjustment on the cable

There is a large o-ring on the slide that can swell and cause the slide to hang up. Need to pull the slide and replace. You can generally feel if it is catching by pulling throttle cable.

Don't use carb cleaner on the junior 42mm carbs.