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Posts: 6,270
Join Date: September 7, 2010
February 4, 2011, 10:14 AM

Based on my online searching, there are two Keihin FCR carbs for the DRZ: The old slant body type that used to come with the drz400-E and the newer FCRMX which has a couple more bells and whistles.

Can anybody confirm that this is the slant-body one? Also, anybody know where to get a good manual for it?

Would it be worth upgrading to the current MX version?

Also, what's that open port on the back side of it (to the right, just above the turn in the fuel line hose)? I'm not sure if it's supposed to be like that.
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*Not intended to be a factual statement.

Last edited by Rail; February 4, 2011 at 10:16 AM..
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February 4, 2011, 10:16 AM

1. yes
2. manual, i have no idea. thay are relatively simple.
3. it's a vent, there should be a vent tube on it.
4. the new versions have a much better design.

KTMs and Yamahas
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February 4, 2011, 03:24 PM

Where should the vent tube go... Just hangs down?

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February 4, 2011, 03:29 PM

catch can, if you plan to bring it out to the supermoto days at Summit Point this year.

Did you find any technical info yet? Have you tried thumper talk?
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February 5, 2011, 02:02 PM

Originally Posted by OrangeShirtDude View Post
catch can, if you plan to bring it out to the supermoto days at Summit Point this year.

Did you find any technical info yet? Have you tried thumper talk?

Well, I've found this with regard to basic disassembly and such (the links in the first post):
Jetting the Flat FCR carb, DIY picture tutorial. - ThumperTalk

And also this: The Keihin FCR Carb Rebuild With Zip-Ty Racing - Dr. Dirt - Dirt Rider Magazine

Haven't gone through them yet but it's a start!
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February 5, 2011, 02:16 PM

fcr setup from ktmtalk

(For the Kehein FCR Carb)
Note do to the complexity of the accelerator pump I have broken this into two sections, Carb Set Up 101 and Accelerator Pump Set Up 101

First lets understand the basics, a carburetor works on the principle of air flow siphoning gas as it passes the needle & jets. At idle the air flows thru a very restricted area adjacent to the jets. When you twist the throttle quickly at low RPM the air flow is suddenly moved from right at the jets to spread over a big area, same amount of air spread over a larger area, vacuum decreases and air speed at the jets decreases too the point that fuel siphoning stops, no fuel equals Bwaaaaaa!

To supply fuel from the “twist” until vacuum is restored the manufactures have several options;
1: Smaller carburetor.
This keeps the air flow close to the jets, makes a bike work nice in low speed maneuvering but kills top end power!
2: Fuel injection.
Fuel injection, it does not rely on the siphoning effect. Like it or not its coming as;
A) As they are not vacuum dependant they can use a bigger throttle body thus Rev higher equals more power, and
B) Once the R&D work is done and you turn up production volume it may cost less to produce than a carburetor.
3: Constant Velocity (CV) Carb:
A “CV” carb is an excellent deal, you whack the throttle open (or think you did) but a vacuum controlled deal keeps it only part open until the engine is revved up enough to use it. Common on street bikes, to put one on your KTM you would have to cut a notch out of your knee to get the space and get stronger springs as it would add several pounds. Not suited well for a “race bike”.
4: An accelerator pump:
This is the route most of the state of the art off road bikes have until we have FI, this provides a little shot of fuel to fill in during that moment of lag in vacuum. Problem is, how much gas? Earlier designs squirted gas “while” the throttle was being moved, once the movement of the throttle stopped the squirt stopped. That was a bit to brief and left many a car or MC pre-igniting (Pinging) for a moment after the “twist”. This Pinging can lead to a piston failure! The common setup on the Kehein carb is set to squirt for 3 seconds after the twist. This is not necessarily wrong, just a compromise setting chosen by the manufacturer. What we need the AC pump to do is bridge the gap from the “twist” to the engine being revved up a bit, lets just guess that 3,000RPM will be enough to get the air flowing sufficient to get the siphon effect underway again, while a big twist at low RPM can make the engine “fall on its face” even once past that point a heavy load on the engine the engine could be experiencing a lean spot that can lead to pre-ignition.

We could debate names all day but in my definitions:
A: A “Lean Bog” is when lack of vacuum to maintain the siphon effect has caused the engine to run out of gas before the RPM got up to where the siphon thru the jets can take over. The AC pumps job is to bridge this gap, and
B: A “Throttle Stall” is when too aggressive of an AC pump floods the engine.

The area in between is the Happy Zone we all strive for, the manufacturer does not know how your going to use your bike but they do know, not enough AC pump can cause pinging/piston failure, too much AC pump causes a bog, which is worse? So they have to seek a compromise combo on the AC pump.

Most off road riders are using relatively low gears, this together with wheel spin gets the RPM “up” quickly!

Example 1:
You have left camp on asphalt headed for the trails, riding pretty mellow, think your being easy on the bike, exit a corner at say 1,500 RPM in 5th gear, you roll on a little bit of throttle and the engine slowly builds momentum so its 3 or 4 seconds before the engine gets back to 3,000RPM. What would happen here is without squirt of the AC pump the engine would be very lean yet with a fair amount of load on it, several seconds of that “easy treatment” may even be melting away a small bit of the piston. By designing an AC pump that continues squirting for say 3 seconds that lean spot is patched over by this “squirt”!
Example 2:
A bit down the road you turn onto a trail of delicious loamy soil, you whack the throttle open and the rear tire breaks loose, the RPM jumps almost instantly into its happy zone, say to 5,000RPM and while the phun meter heads up yet the engine seams just a kinda hang there a couple seconds, then takes off! What is happening here is when the RPM jumped up the air flow thru the carb drew in all the fuel the engine needed for its proper fuel air mixture thru the jets! Yet at the same time it was being supplemented by the AC pumps squirt. Combined the two fuel supplies and the fuel air mixture was overly rich, that “rich” mixture burns slower than a correct fuel air mixture and the engine feel just a bit lazy.
Example 3:
Your on your shinny new Orange bike and leading your buddy who rides this wrasspy old “faded other color” bike, you enter a corner with this MX track like berm, your leaned over maybe 70 degrees and you whack the throttle open anticipating roosting old blue or whatever, you anticipate “Rriiiiippp” but hear “Bwaaaaaaaa”, engine dies, 70 degree lean angle becomes 90, phun meter collapses. Old blue’s owner gives you some squidly look. What happened here is your AC pump was set for example 1 and basically flooded the engine!

These “High Performance Engines” have a carburetor that is big for its size so they do not maintain vacuum when wide open so it needs that AC pump squirt to fill the gap of the "lean bog". Often the AC pump is set up too aggressive and floods the engine, the “throttle stall”.

Kehein was very clever in this design as it squirts during the twist AND for some predetermined amount of time after! First and foremost your pump needs to be working!
Check for
A: Function! It must squirt when you twist
B: Start of squirt! It must squirt as quickly as the slide starts moving up but should not splatter on the back of the slide!
C: Duration! The duration is the amount of time is continues to squirt after the twist!
Start of the squirt;

On the 03 and newer models there is a screw on the right side of the carb that adjust the start of the squirt, when correct the squirt does not hit the slide, the slide should lift and the squirt follow just under it. The following link gives you a good starting point:
IM001258.JPG photo - James Dean photos at pbase.com

Duration of the squirt:
Most bikes as a compromise are being supplied with the AC pump set to squirt for 3 seconds after you twist. If your riding is strictly off road the bike can be more responsive by shortening that duration! Do Not reduce this squirt duration on any bike that is used in high traction riding such as Dual Sport! You can’t have it all and trying to make one of those bikes respond like a moto crosser would make it deathly lean when you gently rolled on a little throttle on asphalt!

You have to judge how long of squirt time you need! That is tied to how low or high of gears you use, wheel spin, gearing, size of bike etc. If you are a very passive rider, roll on the power gently and seldom spin a tire the factory 3 second setting may be prefect. However most off road riders are geared down for the woods and ride aggressively so the low gearing and wheel spin will have the RPM up in about a second, thus they want a 1 second squirt time.

Pay close attention, test to follow! The way the AC pump works is very clever in that the throttle activates a link pushes a spring that pushes a second link (you gotta look at it) that allows the second arm to be delayed, the delay is caused by the fuel pressure in the pump circuit! To illustrate, visualize yourself draining a hot water bottle full of water, you toss it into the sink, pop the cap off and press on it. What happens parallels when you twist the throttle, the throttle door that controls air is allowed to open right up but the hands pushing on the water bottle takes some period of time for the water to squirt out!
On the carburetor this creates a squirt that in stock setup is for about 3 seconds after you twist, we can reduce the squirt by blocking the pumps movement OR by leaking off some of the fuel pressure! Like draining the water bottle we can vary how long it takes with pressure, or by stopping the push! Stopping the push on the diaphragm is achieved by a mechanical stop, a bit like tossing a rock into that water bottle so you can’t squeeze all the water out:


1: Honda Mod: Kehein makes a series of different AC Pump diaphragms with a post on the bottom to stop the diaphragm just as the rock did in the hot water bottle. Basically the longer rivet thru the middle that stops its travel, the shorter the squirt. My recollection is that there are 4 lengths available? The KTM comes with the shortest post, longest squirt Honda Part #16021-MEB-771 or Yamaha part # 5JG-14940-19, is the opposite. Longest post, shortest squirt. The best part is this works on all years of the Kehein FCR carb, is a simple, no brainer, minimal talent required just change the diaphragm, ignore a couple other pieces that comes with it and a bargain at under $20.

2: BK Mod: For ’03-05 only, you drill & tap a hole, install a screw & spring to make an adjustable stop called the “BK mod” (Brian Kenny). The link to its instructions are near the bottom of this page. For the bikes that it works on this is my choice because its almost free and fully adjustable.

2.A: For the ‘00-02 bike you do the same thing by bending a tang on the side of the carb, it is like a small fork and you need to bring the tips closer together, then recheck. For a pic go to JDs web site photo gallery, currently it is page 3 (but that may changes as more photos are put on the site?) http://www.pbase.com/jdjetting1/keihin_fcr...pictures&page=3 look for photo titled Pinched_Linkage.jpg

3: Taffy mod: This involves a wheel spacer from some model airplane that is slipped over the rod of the AC pump, while this is perfectly viable and I believe works only on 03 & newer, I have no further info.

4: I have also heard of gluing a valve adjustment shim into the bottom of the AC pump cover, someone even did this with a button.

5: And threading a hole in the cover and putting in a set screw adjustable stop! I like this with the exception of fear of gas leaks.

6: There is an AC pump cover called a P-38 that also restricts the pump travel.


A: “Leak Jet”: As the AC pump is pumping more fuel than we know what to do with another way to reduce the squirt is via a metered leak back to the float bowl! Most of the bikes other than KTM have a “leak jet” in the float bowl, up thru 05 KTM did not have the hole machined for it, rumor has it that some 06’s do?? If not you can by a bowl from JD that is machined for the jet. Common jet sizes are in the 50 to 80 range. What this “leak” does is reduce the fuel pressure of the squirt! As the duration of the squirt is tied to the volume of fuel pressurized under the diaphragm adding a leak quickens the draining of that chamber. Thus adding a leak has two effects:
1: The duration is shortened somewhat like that of mods 1-5 above, and
2: As the fuel pressure in the AC pump circuit is decreased the initial hit of the squirt is weaker!

B: Wire Mod: Credit for this trick goes to Redbeard. I want to tell you how cool it is but have not tried it so I will leave you in his hands if you want to try this route. His site (below) has a host of photos that you can refer to regardless of what route you elect to take. This involves safety wire to effectively tie the two linkage pieces together. I am still not on the same page as this as now the AC pump is moving the same speed as the throttle. This can only be done with the pump diaphragm that allows full travel AND can only work with a substantial sized “leak jet”. Object is it hits the pump harder then the big leak jet bleeds off excess to get back to some normal duration of squirt. Notes; He shows an optional way of doing the same with an Oring rather than the wire. In this application the Oring is solidly tying the two links together.

C: ORing Mod: Credit for this twist goes to JD as a modification of Redbeards mod. This involves taking the same Oring as shown in Redbeards site above (an ID about 5/16th inch and OD about 7/16th inch) and installing it as shown on his site but setting the linkage and the squirt duration in a conventional manor.
Many paragraphs back I said there would be a test! And described how one arm pushed on another with a spring, well this ties the two pieces together with a bit of elasticity which increases the pressure of the spring, thus increasing the fuel pressure of the pump for its initial hit! This yields a higher volume initial hit, then the pressure tapers off. I personally watched & rode/tested a KTM525 and it brought this bike to life! It worked very well in conjunction with a limited travel diaphragm of mod #1! With this setup as with stock when you open the throttle the first arm gets ahead of the second arm! This is what creates the "duration of squirt!"

D: Quick Shot: This does the opposite, it increases the squirt. Its purpose appears to be for some bike (that I have not met) that needs a stronger squirt than original. For the rest of us I see no value in the piece, I have heard that some have benefited from its instructions on setting the start of the squirt which are free here so just take the $94 you just saved and send to me:>)

Dave’s way: As we learn more about this I am leaning more toward tailoring the squirt to the size of the engine! We are all learning but my thoughts at this time.

250SXF Has trouble using the full volume of fuel of the pump, is easily flooded when stalled and at least in a trail application favors a softer hit of the pump (reduced fuel pressure) AND the reduced duration. Thus my recommendation is BOTH mods “#1” and “A”. Side benefit, this will help if not cure the hard starting when hot problems we see at the MX races.

350-450 is happy with just “#1” or “#2” if 03-05 and “#C” is worth trying.

525 and bigger responds very well to a bit more fuel on the initial twist and less duration, thus I am favoring both mod “#1” or #2 if 03-06 and “#C”!

Street, Dual Sport who have 100% traction need the full 3 seconds. Too short of duration can hurt your engine when riding mellow. If pre 06 you might try Mod #2 as its adjustable and tinker with it out on the road? Listen carfully for pinging and run as long of duration as you can without bogging.
Motard or Ice racers may have the 100% traction but are less likely to gently roll on the throttle, more apt to be revved up all the time? So I suspect a short duration is fine?

Some resources
Setting the start of the squirt IM001258.JPG photo - James Dean photos at pbase.com
Redbeard’s Instructions: http://www.squadpage.com/pandora/Professor...%20AP%20Mod.pdf
BK Mod Instructions: BK Modification, Keihin FCR-MX Carburetor
BK Mod Instructions: http://motoman393.thumpertalk.com/tech/carbpics.html
BK Mod Instructions: 4Strokes.com Redirect - BK Mod
Diagrams/parts: Sudco - Keihin FCR Sidedraft Diagram
Diagrams/parts: http://www.keihin-us.com/needle.htm
Fuel Screws "T"-Handle FuelscrewAir/Fuelscrews
Fuel Screws http://www.slavensracing.com/products_ktm.htm
Fuel Screws Scotts Performance Products
Tech ThumperFaq: AP Modifications
Tech ThumperFaq: Tuning the AP
Slide Plate Vacuum Release Plate (square end down) photo - James Dean photos at pbase.com
Jet kits, float bowls etc. http://www. jdjetting.com

Also see Carb Set Up 101

Dave “DJH” Hopkins

This post has been edited by jeb on Jun 2 2006, 05:11 AM
I have been working on this for a while, maybe help some get their piece right. Anyway it got way too long so I have the accelerator pump section seperate, planning to sugest to the Admin that we pin this but thought before we do that maybe you guys would like to offer some sugestions to add of improve?

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

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 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 ” 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: 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 three inches below the bottom of the chassis. I had one glove that was 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 rise 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. 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. 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, they talk about another problem that I may have experienced and did not know it? That is where you are going fast in bumps and gas bounces into the tube, can not run in out as pressure/vacuum is balancing in the carb and causes the floats to flood?? Not sure I am buying this theory? 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 is 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 (and believing some) 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 go to the bottom of the air box. My preference is to pinch it with 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.

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, Scotts, Slaven’s & Kouba each have high quality brass pieces and I am sure there are some others.

Idle speed! You must set it faster than your common seat of the pants level! 1800 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 1500). 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.

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. Some double

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; Plan on removing the jet and soaking it in lacquer thinner
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 rumored to be 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 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 on 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 throttle: As you open the throttle you transition from idle circuit being the shank of the needle (straight part).
1/3rd to 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,
Fourth Letter “E”: Is the distance from the clip to taper, we need the Kehein Chart
Fifth Letter “M”: Is the shank.

3rd/Taper Kehein gives us two options;
“D” Very little taper, Common opinion, best use is for emissions approval. Not a long way off but on the lean side from to open and when open blocks the main!
“E” Has a lot more taper, as main jet blockage is no longer a factor it needs a smaller main.
5th/Shank: The Shank controls the off idle thru 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 its 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.

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.

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!

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!
Many times I have run this simple test, the float bowl has a overflow tube that runs up from the bottom of the bowl near the center. This tube has a hose that comes out the bottom and on the Kehein it doubles as a drain hose. I remove and set that hose aside, I wipe the top of the cases below the carb clean & dry. Then I ride around for a minute or two, no G force deals just mellow put around with some mild bumps. My theory, no gas should drool out, if it does the float is too high.

Some resources
Setting the start of the squirt IM001258.JPG photo - James Dean photos at pbase.com
BK Mod Instructions: BK Modification, Keihin FCR-MX Carburetor
BK Mod Instructions: http://motoman393.thumpertalk.com/tech/carbpics.html
BK Mod Instructions: 4Strokes.com Redirect - BK Mod
Diagrams/parts: Sudco - Keihin FCR Sidedraft Diagram
Fuel Screws "T"-Handle FuelscrewAir/Fuelscrews
Fuel Screws http://www.slavensracing.com/products_ktm.htm
Fuel Screws Scotts Performance Products
Tech ThumperFaq: Tuning the AP
Slide Plate Vacuum Release Plate (square end down) photo - James Dean photos at pbase.com
Jet kits, float bowls etc. http://www. jdjetting.com
Kehein needles chart http://www.keihin-us.com/needle.htm

Dave “DJH” Hopkins

This post has been edited by jeb on Jul 24 2007, 04:10 PM

KTMs and Yamahas
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February 5, 2011, 03:02 PM

sweetness, thanks again!
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