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Old 04-10-2007, 12:52 PM
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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 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.
This it does not rely on the siphoning effect. Like it or not it’s 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 get 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 too 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 3,000 RPM 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: “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: “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 old blue” bike, you enter a corner with this MX track like berm, your leaned over maybe 60 degrees and you whack the throttle open anticipating roosting old blue, you anticipate “Rriiiiippp” but hear “Bwaaaaaaaa”, engine dies, 60 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

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. The “Honda piece is the 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, if you buy from;
Honda, you get a couple other pieces with it that you can ignore a bargain at under $20 (Honda allegedly owns Kehein so they can sell parts for less). Honda’s number is 16021-MEB-771, or
Yamaha, price is a bit higher I think close to $30 but there is some in between lengths available
part # 5JG-14940-19-00, (9mm post) would be the same as the Honda (short squirt)
part # 5JG-14940-18-00, (8mm post) a little longer squirt
part # 5JG-14940-76-00, (7.5mm post) a little longer yet
part # 5JG-14940-17-00, (7mm post) a little longer yet
part # 5JG-14940-16-00, (6mm post) I believe same as KTM

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?) JDJetting Carburetor Parts Photo Gallery by James Dean at 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 but that can trash a good shirt.

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, probably just fine but much more money the make one of the simple adjustments above.


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 shortens the duration somewhat like that of mods 1-6 above. BACK TO THAT WATER BOTTLE, ADDING A LEAK JET IS LIKE HAVING A SECOND NOZZLE ON THE OTHER END OF THE BOTTLE (ONE END GOES TO ENGINE, THE OTHER END GOES BACK TO THE BOWL.)
Leak jets can be had thru Yamaha dealers, the size is a percentage of a mm in diameter and as this is not ultra critical I am not opposed to drilling jets when needed.
#40 –s 4JT-1494F-03-00
#50 –s 4JT-1494F-07-00
#60 –s 4JT-1494F-11-00
#70 –s 4JT-1494F-15-00
#80 –s 4JT-1494F-19-00
#90 –s 4JT-1494F-23-00 (there are more but somewhere around here the pump is disabled by the leak)

B: Wire Mod: I am deleting this as it caused confusion and has been superceeded by the O Ring mod below.

C: ORing Mod: 03 & newer only! Credit for this twist goes to JD as a modification of a mod by Redbeard. This involves taking an Oring (JD sells a kit) or an ID about 5/16th inch and OD about 7/16th inch and installing it around the two black arms.
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 & road tested this on a KTM 525 and it brought this bike to life! It worked very well in conjunction with a limited travel diaphragm of mod #1!

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:>) & I will try to find you some old decal.

As we learn more about this the combo that consistently gets good reviews is either of the above methods of restricting volume in conjunction with the O ring! The O ring gives a bit stronger initial pressure then the restriction of volume allows the duration to be short for a crisp throttle.

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.

Bigger bikes are happy with just “#1” (or “#2” if 03-05) and “#C” is worth trying.

Street, Dual Sport who have 100% traction need the more duration then off road, maybe 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 carefully 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
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
Diagrams/parts: Carboretor Needles
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: AP Modifications
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.

Another way to look at this
Think of this as sitting in a very small room with a 400 watt light bulb hanging over your head. You get a headache, right? So:
Quick Shot is like getting a 500 watt bulb, and a decal but its heavily advertised.
Honda Diaphragm is like getting an 80 watt bulb (probably just right and you will probably never think about it again.
BK mod (which only works from 03-05) is like having a dimmer, your probably going to dim to the 80 watt level but you can adjust as you like.

O ring is lets say it is like turning the same light on with a dimmer and going too bright, then instantly bring it back down.

Also see “Carb Set Up 101”.

Dave “DJH” Hopkins
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Old 04-10-2007, 01:48 PM
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Great post, big Thank You to David Hopkins for allowing you to reprint/repost. It takes a lot of time and ambition to illustrate the intricacies for everyone. Good job.
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Old 04-10-2007, 02:01 PM
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I love it!!
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Old 04-10-2007, 03:49 PM
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KB Material for sure.
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Old 04-10-2007, 07:12 PM
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Moving to the KB tomorrow. Watch for the grand opening in the paper.

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