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Old 04-10-2007, 01:48 PM
Big Maico 393's Avatar
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Default Carb setup 101


This is repinted with the permission of David Hopkins from KTM Talk, some of it applies to the KTM line, But a lot of it applies to the Kehein carbs on all the newer bikes!

For Kehein FCR carb
Note: do to the complexity of the accelerator pump I have broken this into two sections, the other being Accelerator Pump Set Up 101

Theory: The function of the carb is a controlled siphon! Air is drawn in by the engine, some of it passes thru one of those “air jets” at the air filter end, then passes the orifices above the jets and it siphons gas up thru those jets. Different jets control different functions, “starting” effects the effectiveness of the choke (really an enrichener), “pilot” effects primarily idle, the rest passes thru the “main jet” however it function is blocked by the “jet needle” until the throttle is lifted all the way up, thus the main effects primarily full throttle. The float level effect how much the siphoning has to lift the fuel.

Different models: The first of these that I worked on was a 98 Yamaha. Those had a steel rod for the accelerator pump. These are very prone to the rod rusting and becoming seized in the bronze guide it slide in and the accelerator pump will not work. All of the KTM’s I have met had an aluminum rod.
00-02 KTM’s 400-520 used a 39mm carb (02-250 had a 34), the accelerator pump rod was exposed to the elements on the right side of the carb body had a rubber boot as it passed from the elements to inside. The boot would fail and water & dirt would get inside, the pump diaphragm would fail.
03-05 KTM’s, The accelerator pump got both a screw that adjusts when the pump starts, a black plastic cover to keep the pump linkage clean and the 250 went to 37mm, 450 & 525SX (SMR what ever year it started) increased to 41mm. The have a tab on the side that is commonly used for the BK Mod (see Accelerator pump 101).
06 KTM’s, That tab for the BK mod disappeared and the rear air horn changed.

Throttle travel: US models should not have it but Euro and others models are likely to have a throttle stop. The is screwed into the top of the throttle slide and sticks up from the slide maybe ¾ inch to stop it from opening all the way. It should be tossed either immediately or after a brief break in period.

Throttle Slide: Avoid pulling this out! On the front/engine side of the slide there is a black plate called the “vacuum release plate”. It has a hole about ¼” near the bottom. This plate can be installed backwards which leaves that hole at the top and not visible. When installed wrong the slide drops normal when the engine is off but when running the slide pauses before it complete closing, bike refuses to idle properly and well it just runs ugly.

Throttle Cable: On throttle cable they come with one for you, to open the throttle and one for the Lawyers and Department of Transportation (DOT) to close it. Personally I like to ride with minimal slack in the cable. When you do this on the Kehein it becomes less easy to twist as the pulley on the carb that the cable runs around is a two pieces of stamped steel design which leaves the base of the pulley with a kind of crotch, the cable gets jammed into the crotch and becomes more difficult to operate.
Dave’s way: Don’t let me talk you into something you don’t want to do but I toss the closing cable, the throttle will work so much easier you will never want a closing cable again. As it will work as a spare opening cable you can save it as a spare or for the long distance rider, leave it in place just disconnect both ends.

Fuel Tank Vent: This hose has a couple underrated functions, the one we commonly think of, as fuel is consumed, the level goes down and air must flow in to replace it. What we may forget is these four strokes have the engine and tank so closely integrated that the tank/gas get hot! This is OK but often the gas expands faster than the engine burns it so early into a ride it is not uncommon that if the tank can not vent out the gas becomes under pressure, this sinks the float, floods the engine and frustrates the rider to no end. Tank & cap must be vented to atmosphere both ways.
Dave’s way: Take your gas cap apart and toss the check ball! None in the hose! And while we are looking at that, take a look at where the hose goes? When that gas expands we do not want overflow dumping on the head pipe. KTM was clever in routing it into the frames down tube, that way it can drool out on the ground and bypass the hot exhaust!
Another good tip in this area, remove the tank and use self sticking aluminum foil under the tank to stop engine heat from penetrating the lower tank and heat the gas./

Vent Hoses: My 400 EXC had been fairly trouble free so I had not dwelled on this, recently spent a day on an 03 EXC RFS and we where slopping in mud. The engine had tried to die once in a stream crossing and I had read several posts about vent hoses and was thinking about them. I had also noted the ones on my ’01 ended about the swing arm pivot, on this 03 they hung at least six inches below the bottom of the chassis. My gloves where totally gorped with mud so when we rode into a wide shallow river, not even up to the pegs, I said “good opportunity to wash that glove”, I stopped, idling in neutral and bent down to rinse the glove. The engine died, would not restart, I had to push out of the stream, then waited for the bowl to refill and use the choke to start it. Tip is as it needed the choke to start the bowl was empty, obviously what is happening is not water getting into the carb, rather gas can not get in without pushing displaced air out and the pressure difference is not adequate to push the water aside to let that air out. This is a bit like putting your finger on a soda straw, then pulling the straw out of the soda only in reverse.
Later at home I ran a test, started the bike and let it idle on the stand, I took a cup of water and held it under the bike dipping all of 4 the vent hoses into the water, in a few seconds the engine died. Then I bought on of those $19 aftermarket kits that is supposed to fix this. Anyway they believe the cure is to vent “up” rather than “down”.
Looked at the stock setup of two “T’s” and four vent hoses (plus on drain hose) and determined somebody at Kehein is nuts? (In their defense the reason Kehein likes these tubes hanging down as it reduces the risk of drooling gas on a upside down bike causing a fire.)
I looked at the $19 kit which includes an mini air box & filter to put in the air box, thought is was overkill and if the hoses are just run up I have a concern that if the bike lays on its side the vent hoses could spill gas on a bike tipped more than 90 degrees. After soaking up the info that was in the kit I tossed it. Of the five hoses one is a bowl drain (from the bottom of the carb) which we can ignore, four are vents, come from the upper part of the carb. If I may number these vents;
#1: Comes from the side of a “T” on the left side of the carb and goes down.
#2: Comes from the top of a “T” on the left side of the carb and goes up, over & down the right..
#3: Comes from the side of a “T” on the right side of the carb and goes down.
#4: Comes from the top of a “T” on the right side of the carb and goes up, over & down the left.
Also on the float bowl screws there are two tabs to retain downward hoses.
Dave’s Way;
#1: Is left alone.
#2: Is run over the top but shortened and run into the right side “T” thus negating #4.
#3: Is run up into the air box and with a small piece of tubing spliced on the hose you have left over from #4 and goes to the bottom of the air box. My preference is to pinch it between the air box & the air filter to secure it there!
Then I tossed the right side tab, Hose #1 can run thru the left one.
Now if the bike is in water the bowl will still be vented via the top hose, and if the bike is upside down and you ran the hose to the bottom of the air box it should not let gas run out. If you let it end at the top of the air box gas can run out, get on the exhaust as you stand the bike up creating some risk of fire.
Other options here, I have heard of aftermarket filters and or check valves to put in the hoses, for hose 1 still down in the water that can not be a bad idea,

Idle speed! You must set it faster than your common seat of the pants level! 1800RPM is proper for KTM’s RFS engine, any slower and your “automatic decompressor” is still activated which cuts your power in half (until the RPM exceeds some level such as 1500RPM). Additionally it is more primed & ready for the quick acceleration and as you turn the idle speed up you will find the bike jumps and goes thru whoops flatter rather than nose diving. A low idle makes the nose dive into every hole and the bike becomes a handful to ride.

Float Level: You have fixed a toilet, right?? Fairly simple deal we have fuel in the line, gravity trying to feed it in (toilet has water pressure to feed it) and a valve called the “needle valve” trying to hold it out. Now we need to stop and talk about definitions, the “needle valve” is just that a valve. The “needle” we talk about in the jetting section is totally different, its real name is “jet needle” (but if you say that know body knows what your talking about).
The float has a tab on it that as the fuel raises the float floats up and at some preset point that tab presses the “needle valve” into it seat and stops the fuel flow. This becomes the “regulator” of the fuel level, thus the name “float level” is really addressing “fuel level”. Now a couple points we need to understand about the “fuel level”, as the fuel is siphoned up thru the jets the higher it has to lift it the lesser amount of fuel is going to go thru the jets. Thus you could have jet sizes that where known to be perfect for your model bike, but a higher than standard float level could make it run sloppy rich, a low float level would make it run lean. Thus a given amount of air siphoning from a;
Low float level is going to siphon less fuel/run lean OR a
High float level is going to siphon more fuel/run rich.
If your working with a bike that has a few years on it and especially if hauled on a bouncy trailer that float level may have raised from the wear on the float, its hinge, needle etc.

The manufacturers always give you some spec, it may be “level” (carb upside down) or 17mm or whoknowswhat? That made sense to me way back in the dark ages as the “needle” was a solid piece, you lift it up and the fuel shut off at one precise spot. Solid piece needles went away in the early ‘70s as when your bike went thru bumps the float bounces, fuel was on off on off etc the float level went too high and the engine ran rich until you slowed down (the bikes gave us lots of frustrations back then). So then needles started coming with a spring in the middle of them. Float pushes on needle, needle pushes on spring, spring pushes on needle valve, fuel level is much more constant but, setting the float is a bit more weird as when the carb is upside down the weight of the float pushes it down further than the point that it shuts the fuel off!
Dave’s way; I blow thru the fuel line while manipulating the float by hand, find the point where it shuts off the air/fuel flow and record it!

Float Level: Always start with the factory spec here but some double checks are;
If the carb drips while the bike is on the side stand its way to high!
If you burn up the engine with jetting that is within normal levels it may be too low!
On the bottom of the float bowl there is an “overflow hose”!
Bump Test: I remove, set aside this hose, wipe the transmission cases clean and dry, then ride around for a minute or so, very mellow, low gear just rolling over some light bumps, no “G Force” deals, then stop and check the top of the transmission case. My theory, no gas should drool out, if it does the float is too high.
Hose test: Same hose, leave it connected to the carb but pull it loose from the bundle that goes down with the other drains. Bike upright on a stand, on the bottom of the carb there is a drain screw, (4mm allan?) loosen it, so fuel starts running out, now grip the hose and turn it upwards laying it against the side of the left front corner of the carbs float bowl. The fuel level in the bowl should be the same as the level in the hose, you can mark this on the side of the bowl with a felt pen, compare to another bike etc.

Popping: I frequently hear someone say “I have popping in the exhaust under deceleration so my carb must be lean!” Deceleration popping tells us a couple things;
1: Inside the exhaust pipe is hot.
2: Idle circuit is allowing fuel into the engine.
3: Somewhere the fuel in the exhaust pipe (which is not burnt as we are not allowing oxygen into the engine) is coming into a source of oxygen! Thus finding some air so it can explode!
That’s it! It does not mean its lean! You can richen up the idle mixture until the exhaust is no longer hot and the popping may go away? But now you have a cooler combustion chamber and the AC pump is going to have more trouble getting the engine to respond to the twist, you may find yourself going around in circles?
If you want to understand the engines function better? The engine is a big air pump, as long as the engine is turning over it is trying to draw air in. That airflow is blocked by the throttle slide so under deceleration there is very high vacuum on the front side of the slide.
What is happening is a combo of two things:
1: High vacuum at the intake port is sucking fuel from the idle circuit, that tiny orifice forward of the slide so some fresh fuel is being run thru the engine, as the air supply is almost closed off this fuel goes thru the engine mostly unburned. (BTW, several years ago when cars got FI and the mileage improved! The reason the mileage improved is the FI shuts off the fuel when the vehicle is driving the engine).
2: Under deceleration high vacuum as the exhaust valve opens before BDC at the end of the power stroke it draws air from the exhaust back into the engine. The exhaust had momentum going out, suddenly there is suction making it reverse. At that moment the suction at the exhaust valve area is so high that the tinniest leak at the head pipe will draw in fresh, dirty air!
“Fresh” in that it has lots of Oxygen! This ignites the un-burnt fuel out in the exhaust port!
Dirty in that it brings with it dirt that is around the front of the cylinder head, this gets under the valves and causes some minor pitting of the valve. Not catastrophic but when I take a head apart I have a pretty good idea of how good your head pipe seal is! (it has been a growing problem since the Siamesed pipe design that KTM went to in 04).
Dave’s way;
1: Do not use popping as a gauge to lead you to modifying your jets,
2: Remove the pipe, seal its inside with high temp silicone, let it dry a few days before you start the engine.

Idle circuit: The idle circuit is made up of the pilot jet and a mixture screw, the mixture screw on 4 stroke carbs adjusts fuel, thus out is rich. 2 strokes are the other way around, they adjust air and out is lean. The idle circuit is working at all times but as its volume is small it has a very limited effect in the upper ranges. As it has little effect on upper ranges your bike could run fine at full throttle with the idle circuit messed up but starting and response would be a mess.

Idle Mixture Screw: These are a bit of a pain to reach so a common cure is to replace it with an aftermarket extended one. I do not care how catchy its name is or how fast the rider associated with them is do not buy the aluminum one! If you have one toss it as it is capable of trashing your carb!
Dave’s way: KTM hard parts is Stainless, Scotts, Slaven’s & Kouba each have high quality brass pieces and I am sure there are some others.
On any of them care is required when you remove them! There is a spring, metal washer and an O Ring on them and they don’t always come out with the screw, fall out later and get lost etc. There is a siphon going on here so if the O Ring is missing it will siphon air rather than gas, thus run lean!
Assemble by holding the screw upright, fit the spring over the screw, then the washer, and lastly the O Ring.
The screw should be around 1 to 2.5 turns out from bottomed. If it wants to be further in go one size smaller on the pilot, if it wants to be further out go bigger on the pilot as if you get out 3+ turns the springs tension no longer holds it and it may just fall out on you!

Varnish: If a bike sits for months un-run the gas in the carb can evaporate which leaves a varnish reside inside. Don’t discount the possibility of this problem on a new bike. The gas run thru them at the factory has some additive in it to prevent this but its not uncommon that the dealer puts gas in it, runs it, then puts it in the showroom and the bike sits for months. Now the bike does not want to start and if started runs miserable! Two things can be effected;
A: The Pilot jet becomes plugged! This jet is so small that the varnish coating can plug it solid!. Avoid poking it with wire, torch tip cleaners are a no no, the size on this piece is way to critical too be poking stuff in it as it will be bigger afterwards.
Dave’s way;
A: Plan on removing the jet and soaking it in lacquer thinner to clean up the pilot. Lets not clean “all the jets”! First its pointless and second you likely to cause more damage then good, and please don’t take the slide out!!
B: This varnish can not only plug the pilot jet but the AC pump has two tiny check valves in its system. Either of those can be gummed up rending the AC pump useless. One check valve is in the bowl, will look like a ball bearing in a hole in the bowls bottom, the other is up inside the body of the carb and un-serviceable. Either of both of these can become stuck with this varnish and the AC pump will either not work or not work up to par. It may free up in time but the rider may hate waiting!
Dave’s way; Plan on removing the bowl, removing the AC pump diaphragm (careful of the small parts) and set it aside. For the lower check valve, soaking the bowl in Lacquer thinner for a few hours, take your partners tooth brush and scrub the green stuff out of the inside, then with a small blunt ended punch and with the bowl submerged stroke the check valve ball up and down and the lower check valve should be good to go. For the upper, there is a passage from the bowl to the carb body near the rear of the carb body. Additionally its possible that the AC pump squirt nozzle plugging, rare but possible. Plan on getting the carb upside down and pouring some lacquer thinner up that hole.
When re-assembled before installing, fill the bowl with gas, hook up the throttle and give it several twists to purge the air out of the system, then twist the throttle to check the squirt, it should start promptly with the twist and while a small stream should continue steady for some pre-determined period. See Accelerator Pump Set Up 101 for modifying the squirt for specific applications.

Water: Occasionally in winter conditions we get water in the gas. You have seen how water mounds up on the drinking fountain. This is from what is called “surface tension”, do to this surface tension, the tiny size of a pilot and the small amount of vacuum of the siphon effect at the jets water will not flow thru a pilot jet! Its plugged yet clear when you remove it for examination.
Dave’s way; In the winter, or anytime a plugged pilot is suspected, toss a couple ounces of rubbing alcohol into the gas, this and water are each is heavier that gas so they go to the bottom of the float bowl, the alcohol breaks the surface tension of the water and both pass harmlessly thru the jets and is burned away.

Off idle thru ¼ throttle: As you open the throttle you transition from idle circuit being the shank of the needle (straight part).
1/3rd to ¾ throttle: This range is controlled buy the taper of the needle and its relative location controlled by a clip at the top of the needle.
Full throttle: In a perfect world this is controlled by the main jet. The reason I said “in a perfect world” is in order to be imported into many counties KTM had to fit a needle that has very little taper. Due to this low taper angle the needle blocks the main jet!

All those specs: Jet sizes are metric, a #42 pilot is 42% of a mm (about .016”). a 170 Main jet is thus 1.7mm or about .068”.
Needles: There is more info on the Sudco web site but in a nutshell, the needles made by Kehein have five letters. OBDTM is an example
First Letter “O”: is going to apply to all that we use (no need to dwell on it)
Second Letter “B”: is the material, (no need to dwell on it)
Third Letter “D”: Is taper angle, Kehein gives us two reasonable options “D” & “E”; I call these “D series taper” etc.
“D series” Very little taper, its best use is for emissions approval, import certification. Should be tossed as the needle is so blunt towards the tip that it blocks the flow of the main jet! This has been standard in all KTMs from 00 up to the new twin cam 250’s which have an “E series” I can not stress enough the “D series” piece has no place in your bike!
“E series” Has a lot more taper, as main jet blockage is no longer a factor it needs a smaller main.
Fourth Letter “T”: It is more complex but a simplified version is the distance from the clip to taper, we need the Kehein Chart to compare.
Fifth Letter “M”: Is the shank. The Shank controls the off idle thru ¼ throttle range, if the range is not happy nobody is happy! The basics are, each letter down the alphabet is one step (1/10th of a mm) leaner than the letter before it. Exception, they skip “O” (oh) as it is too easy to confuse with “0” (zero). Thus each of these is one step leaner then the preceding one, “M”, “N”, “P”, “Q”, “R”, “S” is the range we are concerned with.
Dave’s way; Two options here:
1: Is the JD needles, they are a bit more complex as it changes the taper angle mid way and are between the two Kehein needles. They are tailored for the bike rather than just picking thru what Kehein makes and performance is the goal rather than emissions or picking thru what is available. With this you get JD’s development work on the specific model, dyno, exhaust analyzer etc. and quite a bit of seat time development, BTW he rides a 450XC. One word of caution, his kit assumes he is fixing the combo from stock. Say your dealer swapped from a #42 pilot to a #48 to try & fix the factory “lean” mixture. JD combo may be planning on retaining the #42 and fixing the lean condition with the needle. If in doubt, consult your manual, compare what was originally specked out and what you have, in the above example you would want to order that #42 pilot separate from the kit.

2: Sudco is the importer of Kehein parts, your on your own but for the KTM 400-525 a OCEMP with a 162 main is a pretty workable combo. Do to the step taper this combo is richer in the ½ to ¾ throttle range then the JD. If you spend a lot of time flogging in sand dunes consider this combo!

The Transition: This is when we first do the twist. As we ask the carburetor to go from idle mixture to needle. You will have a better throttle response with a lean idle to rich needle transition then you can with a rich idle to lean needle. This is part of why I discourage trying to fix the factory lean fuel mixture with a big pilot jet!

Some resources
Some resources
Setting the start of the squirt IM001258.JPG photo - James Dean photos at
BK Mod Instructions: BK Modification, Keihin FCR-MX Carburetor
BK Mod Instructions:
BK Mod Instructions: KTM Tech: Crockett's BK Mod - By Rob Crockett
Diagrams/parts: Sudco - Keihin FCR Sidedraft Diagram
Fuel Screws "T"-Handle FuelscrewAir/Fuelscrews
Fuel Screws Slavens Racing (719) 475-2624 - Aftermarket motorcycle products for KTM
Fuel Screws Scotts Performance Products
Tech ThumperFaq: Tuning the AP
Slide plate Vacuum Release Plate (square end down) photo - James Dean photos at
Jet kits, float bowls etc. http://www.
Kehein needles chart

Also read AC Pump 101

Dave “DJH” Hopkins
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Old 04-10-2007, 01:54 PM
Florida 393's Avatar
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Usflag Re: Carb setup 101

Nice work H, these are going into the knowledge base after a bit of exposure here.

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Old 04-10-2007, 02:03 PM
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Default Re: Carb setup 101


I have some more to load, but it's more specific to KTM, so I'll put it in the KTM section.
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Old 04-10-2007, 02:08 PM
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Default Re: Carb setup 101

That is very good. Useful. Are you going to KB that?
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Old 04-10-2007, 02:10 PM
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Default Re: Carb setup 101


I'm glad you put at the start of this post that this information came from someone else. I knew you wouldn't be able to remember, let alone type out, all that info.

Good work "H".
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Old 04-10-2007, 02:28 PM
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Default Re: Carb setup 101

If I read other's posts I would have known,,I must have been stuck reading it when those went up. Thanks!
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Old 04-10-2007, 07:58 PM
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Default Re: Carb setup 101

Originally Posted by b-bike393 View Post

I'm glad you put at the start of this post that this information came from someone else. I knew you wouldn't be able to remember, let alone type out, all that info.

Good work "H".


I was taught to give credit where credit is due & Dave did a lot of research & is one of the best when it come to working on KTM's & Huskies, you should read his setup on valves & head work.
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