i resently had new valves, gaskets and a cylender hone (cant afford to bore it). broke it in and had no trouble with it then i got to half throttle and it cut out, (throttle cable running into carb) adjustment was outta wack and turnd the nut up and spun my carb 2 times to sink the cable further into carb, so it moved to 3/4 throttle, so called a mechanic shop and he said move ut needle up one clip si i did and adjusted it to regular settings (idol scerw and throttle play) and it sounds like throttle is stuck at alittle under 1/4 throttle but is still hi.
adjusted idol screw-nothing, adjusted cable that goes into carb-nothing
cant figure whats going on, cable is fine just checked it and is smooth sliding whole way, throttle box is fine and thumb lever goes in smooth.
need some help here

 

When you moved the needle one clip you made it get more fuel is the clip should be in the middle that's what I was always told.

 

or it could be your intake boot is ripped?

 

so the slide works correctly? sounds like it is installed correctly anyways.
check the intake boot for rips as already stated-vacuum leaks. then, if equipped, check the boot to carb connection and see if it has an O-Ring on the flange. make sure it's installed correctly if you have one at all. not too familiar with the 300ex, i just got the 250ex.