Was over at my fiance's uncles house looking at his Brute, I had previously asked him if he had the stumble when he bought it and he said yes. So I started it up and was slowly opening up the choke as it warmed up, I had it partially closed and was revving it a bit and it felt and sounded awesome. I opened it all the way and revved it and the stumble showed up in full force. The one I test drove last summer had the stumble. I told him I would put in a jet kit if he buys it. I also am going to let him try my Dyna CDI. Anyone try puting a CDI on a stock Brute that had(s) the stumble? Was the stumble any different? etc.

 

I did, it amplifies the problem. the stumble becomes a full blown miss.

 

It may actually be choke related. The choke plunger according to my dealer has a very tight fit and many times it will not fully disengage which will cause the bike to run too rich at open throttle. At low throttle, the bike tends to run lean so it's a lean / rich condition potentially at the same time.

First thing to do is make sure he has the carb needle update. If that doesn't completely cure it then perhaps dial the carbs in a little richer via the pilot screw.

Have the choke looked at though, most of them have this problem.

 

To get rid of the stumble try this. Install a NFKL needle in the front carb put one washer underneath each needle and turn out your air screws to about 3 turns this will work well up to 1000 ft elevation.

Kawi has a kit from factory to get rid of the stumble they have the front carb to lean do some research from your dealer this has been covered on this forum a number of times, replace the front needle and the stumble will disappear.