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Old 10-04-2012, 12:11 AM   #1
gringostd OP
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Joined: Jun 2008
Location: Yay Area
Oddometer: 313
KTM LC4 640 jetting - please help

Hi all,

Yes, I've done tons of reading and have pored over the LC4 thread index. Thank you so much for the wealth of information here! I'm still having a bit of trouble with my jetting. Please help.

Here's the situation...
I have a 1999 640 LC4 Enduro. I rode a friend's same bike with some modifications as well as another buddy's Duke - and it became obvious to me that I could be getting more snap and power out of mine. Thus began the quest...

I swapped out the Supertrapp for a KTM Supermoto slip-on. The rest of the exhaust run is stock. This pipe here:


Then, I got into the carb [original Mikuni BST]
- Drilled the slide [.125"]
- Put in a 47.5 pilot jet [it was at 45]
- Put in a 150 main jet [it was at 145]
- Set the needle clip to the 4th position from top [was at 3rd]
- I have played with the mix screw in positions all over the spectrum. So far, I'm getting best performance at around 3 turns out.

The previous owner of the bike removed the airbox snorkel, so for now, that's what I'm working with. I have not put in the easier-breathing airbox side cover yet - though I did run the bike briefly with the cover off entirely, to see if it would solve my problems. It did not.

The bike is making loads more power and is definitely much snappier. Thing is, I can't pull the throttle hard from idle, or it'll 'bog'. I'm using quotation marks, because I'm not sure that's the right term. This is my first time tuning a carburetor. Basically, instead of cracking to life right away, the engine makes a 'chug' sound, and then eventually fires up, after a delay and further along in the throttle turn. No amount of playing with the mix screw seems to help, so far.

The other issue is that the bike is getting considerably hotter than normal when I'm doing city riding [stop lights, idling, moving slowly]. The fan almost never kicked on for me before - now, it's raging all the time, and the bike even started to die on me when I let it idle for long periods. Seemed like it was on a path to overheating.

The other thing to note is that a reputable shop adjusted the float height for me not that long ago. I was having trouble with the bike stalling on hard, downhill deceleration. It's not doing that anymore - no reason to think the float is out of adjustment.


Whaddaya think? Lean? Rich? How to fix?

Thanks in advance for your help and suggestions!
Steve
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gringostd screwed with this post 10-04-2012 at 12:21 AM
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