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Right jet a KX250F for FMX

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Hi Guys,

I have one KX250F 2008 using it for hitting ramps. I know well how to jet 2 Strokes. But the 4 stroke dows not run as good as my 2 Stroke.

Most problem I have is that on immidately opening the gas the bikes goes uuuumpf and enginge turns off. You know on well jetten 2 strokes the eninge will turn high fast when fast opening the throttle.

I played a littlebit with the jetting on the F, but cannot see that much chances. Also I do not want to go that much lean as the 4 strokers can overheat fast isint it?

Currently the main jet is 172 (original 178), needle clip 4, pilot jet 40.

Can you advice please?



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Setting up the AP

The carb on your bike is a sophisticated, well engineered device. It breaks down the various requirements and controls them with 'circuits'. You have the following circuits:


Choke (actually an enricher)


Hot Start


AP (Accelerator Pump)

On a thumper, it takes a breath in, digests it and spits it out and repeats. Your bike is a very high performance machine, capable of running between 1,800 rpm and 12,000 rpm. To be able to accommodate the high rpm, the carb has to be large enough to easily inhale. The carb has to also suck in fuel, it does this by using Bernoulli's Principle (wish you had paid attention now, don't you). The rushing air thorough the venturi (the carb throat) causes an negative pressure, which sucks fuel up past the needle.

But you decide to roost someone and nail the throttle. The engine is still taking little breathes but no the slide is suddenly up. The venturi is now much larger and the air velocity plummets. As everyone knows, if the air velocity drops, the negative pressure at the venturi drops. It cannot suck any fuel up the needle emulsion tube. The bike bogs, the person your were going to roost, passes you, filling your mouth with mud.

To deal with this, a pump was added, much like a squirt gun. It's job is to add enough fuel to compensate for the vacuum loss at the venturi, but just enough. It has to begin to squirt only when the throttle is rapidly opened.

There are four main parts of the AP system.

The Diaphragm

The Spring and the Link

The Leak Jet

The Timing Screw

Almost all bikes these days use the MX variant of the FCR carb and that is the one we will discuss. If you are unsure of which one your bike has,it is easy to tell. If you look at the right side of the carb, is all the Rube Goldberg parts inside or outside of the carb? Inside means it is the MX variant.Every year, the carb has gotten a little more refined and the tweaks get easier.

First, ensure you have a strong squirt. Wearing safety glasses, fuel in the tank, airboot removed, looking in the carb throat, engine off, nail the throttle. You should get a very strong super soaker stream. If not, you need to service the AP before proceeding

These days, nothing has to be done regarding the Diaphragm except replace it if it is damaged. One down.

The spring and the link are easy to deal with. You can get a stiffer AP spring (Merge Racing sells it for about $16) or you can oring the link. The replacement spring is a one time deal, the oring, needs to be replaced from time to time. To install the spring, you really have to pull the carb. The oring, can easily be installed on the bike. Orings cost about $.20 each (#78 in the plumbing section at Lowes). Some wire the links together. Wiring lasts forever, costs about $.03 however, you must ensure there is no bind at WOT or you are going to be replacing that black plastic cam. Some grind the rivet on the Diaphragm and test, grind and test until there is no bind. I have had orings last several years. To each their own. Two down

Now that you have removed the slop from the system, it is time to select a leak jet. A leak jet does just what it sounds like, it 'leaks' fuel out of the AP chamber back into the bowl. The is is what controls the sensitivity and volume of the AP squirt. There is no one size. What leak jet you use depends on how, where and how well you ride your bike. The stock leak jets were selected for "A" racers, those guys that never drop below 7,000 rpm and do cross ups between trees in the woods at 40 mph. Almost all of us are not that fast nor that good. So we need more from the AP. We need it to respond sooner and with more fuel. To do that, we put in a smaller leak jet. But we do not want too small a jet as that will waste fuel and can make jetting difficult if the AP squirts every time your hand pulses because of your heartbeat. What we want is the largest leak jet that will not cause a bog for the type of riding you do.

So.... I recommend starting out with a small leak jet, then increasing in size until the bog returns. Sadly, this can be a pain to do as the float bowl must be removed to swap the jet unless you get an AP cover that has an adjustable leak jet built in. These are pricey, from $125.00 to close to $200. For most riders, these are a waste of money unless you want carb jewelery (they come with stickers), for some that do jet for the day's ride, they are terrific as you can make an adjustment in seconds, just like with a fuel screw. For everyone else, I suggest install a #40 Leak jet if you do a lot of slow single track and are not an aggressive rider (be honest). If you tend to run fire roads or the desert and/or are a faster rider, a #55 or #60 may be a better starting point. Again, once the bog is gone, re-visit the Leak jet selection and try a leaner one. Three down

The AP timing screw. This controls when the squirt starts in relation to the slide position. We want the squirt to shoot down the slide, not slap it in the face nor do we want it to squirt on Thursday when we crack the throttle on Tuesday. In your manual, there are instructions how to set it. Most of these entail slipping a drill blank of a certain size in under the slide and adjusting the screw to have zero or a certain amount of clearance to the black plastic cam. This should be checked and set, if needed. This will be the initial adjustment point. 99.9% of the time, the squirt occurs a little late. Get your bike fully heated up at your riding place. Test ride. If you still have a bog, turn the AP timing screw in (CCW) 1/2 of a turn and test again by riding normally. Do not force a bog. I can make any carb bog if I want to. Do not test revving the bike in your garage. Test it by normal riding. At most, you will have to turn it 2 turns. Often none to 1/2 is plenty. Four down!

Now that the bog is gone, re-visit the Leak Jet and try a larger one. If the bog returns, you can adjust the Timing screw again but you do not want to turn it more than 2 turns from the initial factory setting.

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Thank you very much for the detailed answer, I can work with that and I am sure I can figure that out quickly.

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