Just bought a '99 LC4 640, first KTM, nice bike but not familiar with KTM quirks or CV carbs. Airbox open with big gun exhaust, plug sooty. Mikuni BST 40 carb, 142.5 main, float height looked waaay higher than a normal level setting, air filter clogged, over-oiled. Now have 150 main (biggest I have) now plug looks lean, bike runs clean crisp at low RPM, accellerates strong, but sputters at even throttle settings and stumbles badly on decelleration. I have swapped main jets with little effect on overall running. I have seen postings telling to run main jets around 160, please advise as to a starting point for main jets and how to accurately set float height.

Geezer655 said:

Float height:





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Jetting:

Data is for 2003 and later and applies to the 640 Adventure and LC4E equipped with a Mikuni BST40 Carb. This info may be used for the '03 SXC, however, the off-road type muffler may alter the final jetting outcome.
Addendum: Additional information has been added for the benefit of 2002 and earlier LC4 owners. It is highlighted in Blue. All other information applies as well.

1. Main jet.
I believe a 152.5 main is still the stock piece. If you are at 3000 ft. elevation or less, a 160.0 main jet is a good place to start. Above that elevation, drop down a size for every 1000 feet or so.
Drain the float bowl using the float bowl drain screw on the bottom left front of the bowl.
Pay attention to the orientation of the spacer that comes out with the jet. This change will improve performance and throttle response at 1/2 throttle and above.
(2002 and earlier bikes have a 142.5 main jet stock... a good place to start there is a 150.0 to 152.5)

The main jet recommended is a starting point. You may find after riding for a time that you will want to try 1 size larger or smaller.
These jets are the same main jets used in Mikuni HSR carburetors for Harley-Davidsons, which makes then available at just about any H-D dealer or custom shop.

2. Removing the vacuum cap from the carburetor.
Remove the fairing and fuel tank, you don't have to remove the dash, simply tie the dashes "ears" up to the handlebars.
Remove the 2 screws holding the cap on while lightly holding the cap down. Remove the cap, and gently remove the vacuum piston/slide assembly. Remove the spring and push the needle up and out the top of the slide.
Note the position of the parts; refer to your owner's parts catalog for correct position.

2. a. Changing the needle clip.
Remove the clip (do this over a large table, maybe with a white bath towel on it... just in case you "loose" the clip) from the 3rd position from the top and relocate it to the 4th position.
For elevation, this may be an option. You may find that the stock position responds well or that rather than shift the clip one full rich position you could install a shim washer that would move the needle 1/2 a groove spacing.
This change will improve performance and throttle response at 1/4 throttle and above.

2. b. Drilling the slide.
Obtain a drill bit, preferably a longish one, .110" to .125" in diameter (3mm is the prefered drill bit size) and drill out the two vacuum ports in the slide, on either side of the needle hole to the drill diameter. Drilling from the top is slightly better because the surface inside the slide is square to the ports; the outside has a radius that requires you to have a bit of drilling talent. This bit works regardless of elevation... it simply allows the vacuum piston to react to throttle changes more rapidly.
This change will quicken throttle response at 1/8 throttle and above.

A note about cutting springs. A few folks like to promote cutting the vacuum piston return spring. I would explain why this is not such a good idea, but I've done it once and that was more than enough.
If you really think this is something you want to do, do a search and read up on it.

2. c. Pilot fuel and air jets.
You probably have a 42 or 45 pilot fuel jet and a 1.2 pilot air jet, and you may have heard that richening this circuit is a good idea. Personally, I have found no improvement by increasing the fuel or decreasing the air jet size. I live at 500 ft. above sea level.
(some 2002 and earlier bikes, those that are not equipped with the "high flow" cylinder head do seem to benefit from a slightly richer pilot circuit)

Again, if you think this is something you want to do, do a search and read up on it. The jets are cheap, so if it doesn't pan out for your location and riding style... no big deal.



That's all the mods... you can reassemble the carb. You can use your finger to help guide the needle back into the needle jet in the carb body, then insert the spring. Be careful when reinstalling the cap that you do not pinch or distort the vacuum diaphragm. Don't over-tighten the cap screws.

3. The air box is easy... either buy or make a copy of the KTM high flow air box side cover. Do not remove the snorkel on top the air box. Bear in mind that if you clean and re-oil the air filter, you bike will run richer than normal until the excess oil on the filter dissipates.
This mod improves air flow to the carburetor without upsetting the pressure balance required for it to work. Removing the snorkel is a recommendation made only when certain carb and exhaust conditions are met.

"584.06.003.200 LC4 (Small Airbox) Better airflow rate"



4. Modifying the stock muffler.
The best part about this is that KTM uses a SuperTrapp USFS diffuser disc spark arrestor system on this bike... easy to tune once you remove a bit of "restriction" downstream.
Remove the Supertrapp spark arrestor discs and end cap. Obtain a 1.5" hole saw, find a wooden dowel that fits tightly in the center hole of the mechanical baffle... you need this to help center the drill.
Core out the baffle tube with the hole saw. A tube of about 6 inches in length comes out when the hole saw cut thru.
Reinstall the discs... the bike probably comes with 10 to 12 discs stock... you can add up to around 16 to suit your personal sound meter, but as I recall, 10 to 12 is more than enough for most folks.
This mod improves air flow thru the muffler, improves performance and makes the bike louder. However, the air and sound output of the muffler can be controlled by subtraction or addition of diffuser discs.

Most aftermarket mufflers are 5 to 7 pounds lighter than the stocker. Most will improve power over the stocker, but few will be a noticeable improvement over the modified stocker... unless you want to spend $1000.
I have a Supertrapp IDS2 with a "quiet core". I did this for three reasons... tuneability, weight and cost.

5. Fine Tuning
All of this is based on the likelihood that you do not have a $100 Motion Pro 90° carb screwdriver, and trying to do this hot and keep track of the number of turns you have on the idle mixture screw is a very difficult thing to do.
If you have asbestos covered hands and tiny fingers, you can disregard all this and tune to your hearts content.

Ride the bike at least 10 minutes to get it up to operating temperature; set the hot idle speed at 1800-2000 rpm. Now allow the bike to cool.

Locate the idle mixture screw at the front bottom of the carb body, carefully screw it in until it lightly bottoms... you will need an exceptionally short screwdriver for this.
Turn the screw out two and one quarter turns (2.25) and ride the bike, note the throttle response and idle speed.
Turn the screw to one and three quarter turns out (1.5) or 3/4ths (.75) in from the previous position and ride the bike, note the throttle response and idle speed.
Of the two positions, one will have slightly better throttle response and faster idle... this is the range you want to work in.

The idea here is to use the screw to optimize the transition from the pilot circuit to the needle circuit... not get a perfect idle mixture. That's why your rpm is temporarily set in that transitional range.
Do you want a spiffy idle... or, do you want the best off-idle throttle response you can get?

In a perfect world, you would be able to adjust the screw while the engine is running and locate an optimum position... but as I said, unless you have the tool I have, you have to do it trial and error.
Once you have the low speed mixture where you want it, reset the idle to 1600-1650 rpm.

Loadedagain has made a quantity of extended screws (like the ZipTy FCR ones) for the BST40. PM him if you want one as he only made maybe 50 of them in a CNC run.
It's handy to be able to optimize the idle mix when you have dramatic changes in elevation... especially for the off-road, just off idle, tight single track stuff.

Last thing... the jetting info in this post is for an '03 to present 640 Adventure (and pre '03 if you read closely). Not an 640 SM, not a Duke, not a '96 400 with a BST... not even a 640 with a BST and a complete Akrapovic system... OK?

Have fun,
Creep

Seal the exhaust pipe at all junctions , try a 180 main jet , worked great in mine with the bst 40.

Thanks Creep, appreciate the help. The 152.5 with needle in middle position is pretty close for this 1999. I will not cut the spring and will see if the previous owner has drilled the slide. Geezer 655

Geezer655 said:

Everything that I've covered should be considered a functional baseline. Most folks bikes run great using the jetting refered to, while some work better going up or down a bit.
It's not rocket science... just trying to find the correct air/fuel ratio for your average elevation and temperature, and intake/exhaust mods.
If you have access to a dyno with an A/F sensor, you'll find that BST equipped LC4s make the smoothest power and have the best throttle response when the A/F is between 13.4:1 and 14.2:1.
They'll run OK in the high 12s and mid 14s... but you tend to waste fuel in one direction and run a bit hot in the other.

C

Quote

i own a 640 adventure 2006 - i replaced the full exhaust system with akropovic system including factory kit for jetting.when i reach speeds at 170km/hr the engine stops as if there is not enough fuel flowing in engine.what can i do ?

Took my 640 to Ronin in Sylvan Lake Alberta. He has a dyne and is not afraid to use it. Very happy with work he did. 15 percent better fuel economy and more power. Ran the livin hell out of it. Worth every penny.