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How to eliminate the 'Bog'

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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.

Resetting the AP timing to 'stock'

Method #1 - Most accurate

Turn the idle speed screw so the slide is completely closed (count the turns and note it) .

Turn the throttle wheel, raise the slide, slip a .8mm drill blank under the slide, release the throttle wheel, pinning the blank.

Turn the AP timing screw so the play just barely goes away.

Remove the drill blank, reset the idle speed to where it was.

Fine adjust the AP timing screw by test riding, turning in 1/2 turn increments to where the bog is just gone. Should be less than 2 turns, if at all.

Method #2 Not as accurate

Back out the idle speed knob (count the turns and note it) until the screw end of the idle speed cable is no longer touching the theottle wheel - Throttle cables may have to be fully lossened.

Ensure the slide has bttomed out on the carb bore'

Set the timing gap between the cam and the screw to be .010"

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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Early Slant Style (External Linkage) FCR only (Not FCR-MX)

Here is the jist of the eddy mod:

With the carb removed, you'll first want to locate your accelerator pump linkage. If you were sitting on the bike, it's on the riders right hand side of the carb. The linkage in question looks like this:


Give the throttle a few twists (manually since the carb is off the bike) and you'll notice that it moves this linkage. The problem over time is that this linkage wears, and to begin with it was outputting way too much.

The solution:


See what is circled in red? Those two pieces are actually separate. What you need to do is WIRE THEM TOGETHER using safety wire. Here is some work another person did to safety wire there's together:


I personally did mine a little bit better (PICS TO COME). I took a very small drill bit, just big enough for my safety wire to fit through, and drilled through both of the parts needing to be safety wired together. I left enough room around the hole of course that their strength wasn't compromised. I then, using my safety wire tool, tied them up properly with safety wire, not just a loose knot like here.

That alone however is not enough, that is only half of the job! Now the accelerator pump is tuned right in with the throttle, which yes is helpful. However you now get a longer stroke on the accelerator pump diaphragm. The two pieces you just wired together, well when you turn the throttle it pushes down on a rad that goes into the bottom of your carb. THAT is the accelerator pump, and inside of there is a diaphragm. It looks like this when taken apart:


Now there are two diaphragms in that picture. Only one is in your accelerator pump. The left diaphragm (it's black, far left piece is the cover for the accelerator pump) is shown with the top facing up. That is absolutely fine, leave it alone! The one on the right is the bottom, and it is NOT fine! That's a stopper, and what'll happen is if you leave it stock, and open your throttle, the linkage will push down on the diaphragm, and it will hit the stopper before you've reached full throttle. This will cause binding, and it will eventually damage something if you don't fix the issue!

The solution to the binding linkage issue: GRIND OFF ALL OF THE STOPPER ON THE BOTTOM OF THE DIAPHRAGM (protrusion sticking up on the right side diaphragm). I ground mine off with a dremel no problem, and in no way did I nick the diaphragm. It's very easy.

With that removed, you now get full action on your accelerator pump, a reduced squirt amount and time (which was the initial cause of the bog) and all should be well!

There is one other thing to check still though!

It would appear that more often than not this doesn't require tuning, but it has been documented so I want to share. This is the archaic fashion in which you adjust accelerator pump on the FCR pumper slant carb.

Put the carb back together, and put it on your bike, with the throttle cables hooked up. Don't put the airbox back on yet, and only place the gas tank on loosely. We simply need to be able to get gas to the carb. Fill the float with gas, and now you'll want to give the throttle a quick twist while looking into the carb. Use a flashlight if you aren't in a bright location.

You should notice that the slide moves up, and almost in sync with the slide raising, gasoline is squirted past where the slide used to be. That is your accelerator pump in action.

Now, if your squirt is HITTING the slide as it raises, it is squirting too early. The fix for this is to take the plastic arm in the top most picture (you just wired it to the linkage from the throttle) and BEND IT CLOSER TOGETHER. Yeah... it's old school I know and not precise, but just keep bending it closer together until you can finally thwack open the throttle, and squirt gas past the slide, not directly onto it.

From my research having to do the above seems pretty uncommon. I didn't have to do this. It is documented however, so I will share this with you!

That's the jist of the modification. This is currently just all the pictures and information from other threads assembled however, based upon my experience performing this modification myself.

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