A friend has a 1998 ZRT 600, about 3,200 miles on the original emgine, all is stock. It has a miss around mid the 3/4 throttle. it doesn't backfire, it just seems like it has a rev limiter kicking. Once it runs past that point it's fine. new plugs gapped correctly, carbs cleaned and fresh gas in it. What could it be?

lets start with the clutch, what kinda condition is it in, look for cracks on the sheaves, spring tension, weights,etc...also there are so many things that could cause this, carb. syncro.?? all 3 have to be perfectly timed to each other, fuel pump, rebuild, condition of the reed pedals, drive belt?, throttle switches on carbs, try by-passin the switches, see how it goes, hope this little bit helps let us know... SWRules

I'd be willing to bet it is in the throttle safety switch,same exact symptoms i had on my zrt, bypassed it and now she flies.
these things are nothing but a pain in the but on cats,my old polaris has one and never had a problem with it yet.

I'm sure it's been mentioned before but what's the proper procedure for by-passing the Throttle Switches? The "jumper plug" that comes in the toolkit doesn't bypass that circuit does it?

cattracks hit it,,,,I would bet ya,,,,,same thing happened to me, and mine were also bypassed,,,,no problems now,,,,at least with the carbs !!!

YES, That little plug will do the fix,down by the base of your handlebars you'll find the connector,unplug and install jumper on the lead going down to motor and try it.
you won't have a killswitch no more so be careful.

It was the throttle safety switch........Thanks

There is an up dated plastic (white) bushing that the throttle pin runs in, cured my problem by keeping the lever traveling straight.