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Old 03-27-2009, 05:46 PM   #762
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Joined: Oct 2006
Location: northeast/western mass
Oddometer: 829
Originally Posted by Silas
Howdy folks,

I've got a 2001 640 LC4E with a Mikuni BST 40 and a Supertrapp exhaust and I want to tap into the tribal knowledge to find the "golden combination" of jetting, exhaust plates, and other tuning tips/tricks/procedures. Please tell me what has woked for you or point me in the right direction...

I'm pretty sure it's running lean because the pipe is blue, so I bought the next size up main jet (stock for the Duke), figuring that would help. I'm open to getting a larger one if it's the right thing to do, but I'm not really trying to build a race bike - just something that will allow me to dominate the couple of miles of pavement between my house and campus. Once in a while I do some highway or logging road trips.

I know this is a pretty basic setup so I've been reading threads for a couple of hours looking for enlightenment, but no real luck yet.

Please, if you can, point me in the right direction or chime in about what has worked well for you.

Happy Trails,


What worked for me was about 800 hours of re-jetting and testing.

I don't remember all of my settings, but I did go way up on the main jet to like 170 I think. The bike had 8 disks in the trap stock and I ended up with 12. I added the KTM Hardparts vented side cover, left the snorkle on, and did the exhaust can gutting mod (cored out the mechanical baffle). I also did the carb slide mod. The bike was absolutely transformed into a new animal.

I spent many many hours testing and re-jetting trying to get rid of the off-idle stalling when whacking the throttle open wide. It wasn't a big deal because it didn't do it when riding, but I still wanted to fix it. I think I got it as good as it could be with that carb via the idle jet and needle/needle clip position. Sorry I don't have those settings anymore, but I was back to the stock needle. I never touched the stock air jet. I'm sure I was a size or maybe 2 bigger on the idle jet.

I ran into my bike after I sold it at a local event and several guys that had been riding it were very impressed with the power of the thing. Hope that helps.
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