Mikuni TMX 38 slide removal

I recently bought a 2005 Honda CR-125 for my son that had been ridden about 10 hours and he has about 5 hours on it so far. It's a great bike for him but fouls plugs way too often. The motor seems sluggish and slow to rev but pulls really hard once it's wound up. I had planned to try some different jets but wanted to know what was in it before I started buying new mains, pilots, and needles. The manual calls for 430 Main, 55 Pilot, needle in the 3rd slot, 2 turns out on the air screw. Right now it has a 410 Main, 45 Pilot, 1 1/2 out on the air screw. I took the top off the carb to look at the needle but couldn't figure out how to get the slide loose from the throttle cable. I collapsed the spring up to the carb top and pulled the white plastic collar up out of the way and thought the ball on the end of the cable should come out of an opening on side of the hex nut on top of the slide but it won't even begin to move. The hex nut doesn't want to turn at all and I got on it hard enough I was afraid it would break off. What holds the throttle cable on to the slide?
I had planned on getting one of the jet kits from JDJetting for this bike and see if that would help the way the bike runs but if I can't get the slide off to replace the needle or move it up or down on the slide I'll have to take it somewhere and pay a mechanic to try and clean up the jetting for me. That may be the safest option anyway because it's already quite a bit leaner than stock and I'd hate to seize it up if I got it too lean. Thanks for your help.

Just wanted to give y'all an update. I finally got the cable out of the slide,but I didn't see what was keeping it from coming out in the first place. It's like the lead slug got stuck in the groove in the nut on top of the slide. anyway, it had the stock needle (6 DGY 28-68) in the middle position. I ordered the JD Jetting kit and have installed it, but have to wait for some decent weather to try it out. It's about 20 degrees out now with a wind chill of 10, we don't ride the bikes when it's that cold out so the jetting probably wouldn't be quite right anyway.
The kit jetting recommendations were close to the factory except for the pilot jet which is several sizes leaner and the needle is much different but doesn't have any markings as to size etc. Maybe this will solve the plug fouling. I'll let you know after I ride it.
Thanks for the responses.