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gringostd 10-03-2012 11:11 PM

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!

warewolf 10-04-2012 02:57 AM

You'd be best to read and/or ask your question in the BST bible.

When I made similar mods, the bike was very stall-prone off-idle. The solution was to move the needle clip back to pos 3. Been fantastic ever since.

supercoyote 10-04-2012 12:15 PM

I've done everything you did in terms of carb jetting, except that i'm 2 and a half turns out, but i'm running the stock supercrapp with 12 discs. My best guess is that your supermoto exhaust is causing your bike to run lean. The fact your bike heats up a lot faster jives with the hotter, leaner exhaust gasses. I'd suggest dropping the needle back to the 3rd slot to richen it up and see if that helps.

If not, send your supermoto exhaust to me and i'll do a test ride to figure it out :evil

jon6.0 10-04-2012 12:20 PM

3rd clip would lean it out. That moves the needle further down.

gringostd 10-04-2012 01:31 PM


Originally Posted by jon6.0 (Post 19747313)
3rd clip would lean it out. That moves the needle further down.

Right. I've read several examples where dropping the needle back down [leaner] will help a stumble off idle. But my initial impression is in line with SuperCoyote's thoughts - that hotter, leaner exhaust gasses are making the bike run hot. Could an over-rich condition make the thing overheat?

bmwktmbill 10-04-2012 08:28 PM

What does the plug look like?
Run your finger around the inside of the exhaust outlet...carbon?

Is the choke cable too tight?
You need some slack there.

Carb screwed on tight.

Valves set correctly.

The idle jet(47.5) is a 'fuel' jet(not an air jet) so you richened it there with the bigger jet at and just off idle and you raised the needle...why did you do both?

Why did you install the 150 main?
That's a big jump too.

I would get the bike running right, then take it out on a flat stretch and see if it will go 100 mph with 16/42 gearing and stock jets. BTW did you enlarge the snorkle hole?

When it pulls cleanly from 2500 rpm and goes 100 mph that's all there is unless you change cams or get the hi flo head.

Be happy, it's faster than anything else in a drag race including the Honda 600XR racer with stock jets. It will run at 15 thousand feet with no jetting changes.

It is a big bike and weighs 400 lbs. It will do close to 50 mpg.

Running hot is a different problem with different reasons.
Check the oil for water contamination right away.


gunnerbuck 10-04-2012 10:05 PM

In my experience the stock 45 pilot fuel jet normally does not need to be changed even with the mods... I have messed around a lot with the needle in the fourth clip and always seem to end up with a hesitation off idle, this gets amplified when your jet needle/needle jet takes on wear... I have used a pop can shim to raise the needle a part position above the 3rd and this helped to get rid of the bog but if I went a full position it would come back... Also the float height will have a noticeable effect if it is not close to ballpark... An incorrect float bowl fuel level really shows its stuff when you ride over choppy bumps or whoops by causing the engine to bog and cut out...

supercoyote 10-05-2012 09:40 AM


Originally Posted by jon6.0 (Post 19747313)
3rd clip would lean it out. That moves the needle further down.

Right, I should have played it safe and said "just richen the needle-thing"
But either way, our bikes are fairly similar and i'm getting really snappy response and strong pull all the way through. Barring a problem somewhere else on the bike, the only difference is that your exhaust is leaning out the carbeuration.

that is unless you live at a vastly different elevation than I do (2300ft more or less)

bmwktmbill 10-06-2012 12:36 AM

Did you check the oil for water contamination and the water level in the radiator?


The stock jets for the low flow head were 45 and 142.5.

If I were you I would install those jets or get as close as you can, bolt on your new muffler and go for a does it run?

Now adjust the fuel screw for fastest idle, then close it 1/4 turn. Reset the idle to 1500rpm.

Go for a ride, how does it run?

Since you have no snorkle next remove the side cover leaving the air cleaner in place and exposed. Go for a ride.
Does it run differently and or better?
If it does consider enlarging the snorkle hole.

Replace the side cover.

By now the engine should be running perfectly and accelerating cleanly.
Idle should be 1500rpm.
If it isn't concentrate of that problem. Consider the needle position and changing it if the engine is bogging.

Keep notes.

Now that the engine runs right if you want to try bigger jets go for it.

Change one jet at a time.


gringostd 10-09-2012 01:55 AM

Thanks! All sorted.
Thanks all for your help.
Spent some more time tonight, and I think I've got it sorted. Here's where I ended up - in case anyone's interested.

The bog off idle was solved with setting the needle jet one notch richer [clip @ 5th position from top]. I tried leaning it out one notch first, but that made it considerably worse. I'm now maxed out for richness without going to a different needle. I may look into that after my ride this weekend - I know it's not ideal to be on the richest setting. This seems to have solved the hot running too - which makes sense. This created a bit of a flat spot at the transition from the needle circuit to the main jet. Richening the air mix screw and adjusting idle position accordingly seemed to help with that. May try out a bigger main jet in the future.

To recap, in case anyone has the same setup and is looking for a starting point - here's my current setup. Running well so far - and HUGE 'get up and go' improvement over stock.

1999 KTM 640 LC4 Enduro
KTM 'Supermoto' slip-on [see photo below]
Stock headers and 'S' section
Airbox snorkel removed and opening slightly enlarged [I'm guessing... by the ragged edge on the hole. This was done by PO]
Drilled carb slide holes for faster throttle response [.125"]
150 main jet
Stock needle @ richest setting [clip at 5th position from top]
47.5 pilot jet
Mix screw ~2.75 turns out

Thanks again.

MORT666 10-09-2012 02:07 AM

I would go 170 main.snorkel out, emission control disable.:norton

gringostd 10-09-2012 02:15 AM


Originally Posted by MORT666 (Post 19778436)
I would go 170 main.snorkel out, emission control disable.:norton

170? You crazy.

I may try a 152.5 though.

gunnerbuck 10-09-2012 02:52 AM

Having it at the 5th position seems a bit strange, are you missing the little metal washer that sits under the needle by chance?

gringostd 10-09-2012 09:22 AM

No, the metal washer is there, as is the thicker plastic one.

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