Kristifer,

I searched the China Riders forums, and I found the thread I mentioned in my previous post.  The gentleman from Australia installed a Mikuni VM28-418 carburetor in his motorcycle.

http://chinariders.net/modules.php?name=Forums&file=viewtopic&t=9886&highlight=carburetor+zongshen

I see you are registered at China Riders. If you have time, please post a thread in our Introduce Yourself forum; I'm sure the people at China Riders will be very glad to meet you.

Spud

Usually it should only be the main jet but the necessity of a jet changes comes when you add more air (ie a conical filter). Its not always necessary if you only put on a different exhaust. 

You should try these forums

http://chinariders.net/modules.php?name=Forums&file=viewtopic&t=9198

and

http://chinariders.net/modules.php?name=Forums&file=viewtopic&t=9165

It talks about the carbs off of the Dual sport Sierra 200GY2.  It has pretty much the same carb except for a bore difference (28mm sierra vs 30mm 250gs). 

Haven't tried putting in the new jets yet, but for less than 20 bucks for a few jets, I'm gonna try it.

Indeed, join us at China Riders!   I own a Zongshen Sierra 200GY-2 motorcycle, and I am a moderator at the China Riders forums.  I maintain a list of maintenance and modification threads for the Zongshen Sierra 200GY-2 motorcycle, located at the following link.

http://chinariders.net/modules.php?name=Forums&file=viewtopic&t=8673

The bottom half of your ZS250GS engine is virtually identical to the engine in my 200GY-2 motorcycle.  Also, your sportbike’s carburetor is almost identical to the TK MV28 carburetor in my dual sport bike.  You can learn a lot about your bike by reading the threads in both our Street, and Dual Sport Forums; we would love to have you join us.

Spud