300ex sputtering in 5th

my brother has a 93 300ex that sputters when shifted into 5th gear. it never does it at any other gear or rpm. it has good gas in it, the choke is NOT on either. i cleaned the carb, put another carb on it, changed the cdi box, cleaned air filter, changed coil, plug, and the little box on the left side of the front between the cdi and the coil, i am not sure what it is though. changed gas tank and made sure all vents and gas line was open. oh also it is stock jetting, and all the parts that were changed was off another 93 300ex that is in good running condition and still runs good with all those parts on it that were swithed. does anyone have any idea where to go from here? sorry this is so long.

anyone have any suggestions

If it's when he's trying to run wide open in fifth, then it's called a rev-limiter. Get an aftermarket CDI. Sparks racing sells one.

check your stator, mine came loose and made the quad sputter and run horrible.

ill try the stator first and if that doesnt work ill look into a sparks rev-limiter. thanks guys. ill try the stator and post my results
would either of you guys know what that little box is between the cdi and the coil is? it is fastened to the frame by a rubber sleeve

that is where your rev limiter is, its in that cdi. you can send it to gthunder and they can take out the rev limiter. way cheaper than buying a whole new box from sparks. if your sputtering is not at really high rpm's then its not your rev limiter. if it is your rev limiter, your shifting way too late. you shouldnt get near the rev limter with a stock motor as it builds max hp quite a while before the rev limiter kicks in

well the stator didnt do it :(
i go 1st to 4th and its ok, then i shift into 5th at lower rpm's and it sputters i cant even get any high rpm's in 5th gear. now in 1st the 4th i can keep it mashed and its at really high rpm's and it runs fine no sputtering at all. thats why i didnt think it was the rev-limiter.
could it be something internally or within the engine itself?

Try stoating out in like 2nd or 3rd gear to see if it when it is undre a load that it's breaking up. That's just a guess.

i never thought of doing that ill try that tonight