Fixing the Dreaded Lean Bog on Keihin FCR Carburetors

I recently had the pleasure of performing a complete frame down rebuild on a 2007 CRF250R that came to me with an intolerable bog issue when the throttle was stabbed from idle or low RPM (both when riding, and on the stand). After the head was rebuilt, and the engine freshened up with a new piston and rings, the lean bog was still there. There is very little in the Honda service manual regarding the accelerator pump circuit, or leak jet, so I documented the process that I used to solve this problem, and decided to share.

The information presented here is applicable to any brand of motorcycle that uses the ubiquitous Keihin FCR carburetor.

If you are experiencing the dreaded “lean bog” when stabbing the throttle on your Keihin FCR carbureted dirt bike from low RPM’s you don’t need to buy expensive “fixes” such as the Boysen Quick Shot or the R&D Racing Power Bowl… you simply need to tune your carb using the adjustments that are already there… I'm not saying that Boysen or R&D hasn't come up with a valid fix, their products certainly make it much easier to properly adjust the accelerator pump to tune out the bog, my opinion is that in most cases you don't really NEED those items to fix the issue... In the case of this particular bike, it only needed a $10.00 leak jet and the problem was solved.

Please follow along while I present my case :-)

Bigger carbs mean more air/fuel mix can enter the engine, which means more power is available from the engine. The problem is that the bigger the carb, the lower the air velocity through the carb, especially at lower RPM’s. Air velocity is needed to create the vacuum that draws the fuel up from the carb to be mixed with air on its way into the combustion chamber. With a larger carb (within reason), an engine will run fine at all RPM’s, as long as the RPM’s remain fairly constant and acceleration demands remain very mild. At low RPM’s, there is just enough vacuum to keep the engine properly fueled.

Now throw acceleration into the mix. Most of us don’t hold a dirt bike engine at constant RPM’s for very long, nor do we want to accelerate slowly up to speed. So what happens when we suddenly open the throttle at low RPM’s where there’s just enough vacuum to feed the engine and then want to accelerate? There isn’t enough vacuum to suck the needed amount of fuel up from the carb, the engine goes lean, and we encounter the dreaded “bog” that many have experienced with the large diameter carbs used on later model dirt bikes (pre-fuel injection models). In order to keep this from being a problem, carburetor designers added an “accelerator pump” to the system. What it does is to shoot a carefully timed squirt of fuel (see photo) into the intake manifold every time the throttle is opened quickly in order to keep the engine from going lean while the RPM’s build up to the point where there is enough vacuum for the normal carburetor fuel circuits to function properly.

The problem on some bikes, is that the squirt duration isn't enough to allow the engine to build RPM’s before going lean. The duration is tuned using the “leak jet” located in the float bowl (Part #42 on the schematic). The accelerator pump system works as follows… a plunger is moved up and down with opening and closing of the throttle. This plunger pushes against a diaphragm (Part #47 on the schematic) which acts as a pump. The accelerator pump is fed with fuel from the float bowl. Fuel is pumped to two locations… the accelerator nozzle in the venturi, and also back into the float bowl.

Why pump fuel from the float bowl back into the float bowl? The duration of the squirt (and overall volume of fuel) is controlled by adjusting the ratio of fuel that is pumped up into the venturi vs. fuel that is returned back to the float bowl. The “leak jet” is what controls this ratio. A smaller leak jet will restrict the amount of fuel going back into the float bowl, and will force more fuel up into the venturi. Due to the system design, this will cause the squirt time duration to increase. A larger leak jet will allow more fuel to return to the float bowl and less to be sent to the venturi, and will decrease squirt time duration.

In all actuality, the biggest factor that effects the duration of the squirt is the stroke length of the diaphragm. A longer stroke will mean more fuel is pumped to both the venturi and to the float bowl. If the squirt duration cannot be adjusted adequately using the leak jet to control the ratio pumped to the venturi vs the bowl, then the diaphragm stroke can be adjusted. Stroke control is built into the diaphragm, so a new longer stroke diaphragm will be needed.

Why don't we use the diaphragm as our first approach to adjusting squirt duration? The diaphragm makes big changes. It's better to step in smaller increments using different leak jets. Most times, the diaphragm that was selected from factory will be in the correct range, and you will find that simply adjusting the leak jet size will solve the problem. Also, there are many more leak jets available to tune duration than there are diaphragms.

In addition to squirt duration timing, there is also a need to control when the squirt begins. Too soon, and it will hit the back of the throttle slider valve. Too late, and the engine will be starved for fuel.

The first photo is of the timing screw location on this 2007 CRF250R FCR carb.

The second photo shows the timing adjusted too early… Notice the wet stain on the back of the throttle plate.

When setting the timing, I give the throttle a quick zero to 100% to zero crank while taking note of the back of the throttle plate. I then use a screwdriver to advance the accelerator pump (on this carb, turning the timing screw CW advances) until I start to see fuel hitting the back of the throttle slide plate. When this takes place, I then use an air gun to dry the back of the throttle plate, back off the timing screw a small amount, crank the throttle wide open and closed, and observe the throttle plate for signs of fuel. I repeat this process until no fuel stain is present. This ensures that when the throttle is stabbed, the squirt will start at the exact moment the throttle clears the stream of fuel.

The timing needs to be adjusted so that the squirt starts just after the throttle plate gets out of the way. This will need to be done every time the leak jet is changed. Larger leak jets retard the timing, and smaller leak jets advance the timing.

The first picture (yellow) shows the location of the leak jet. The second (red) shows the location of the accelerator pump.

Before you start with the troubleshooting and repair steps below, use carb cleaner and an air gun to clean and blow out every jet, orifice, and passageway in the carb. In order to work correctly, the carb must be gunk free, and not worn out (eg: leaky seals). If in doubt, install a fresh carb kit to ensure everything is sealing up correctly. Here is a link to an excellent pictorial guide on how to refresh your FCR carb before you go and try to adjust a tired out old carb. The instructions are given as you click on each of the pictures. Follow in sequence.

Now that you know what's going on, and are certain that you're adjusting a carb that is in good shape, I have drawn up a flow chart to aid in troubleshooting and fixing the dreaded lean bog without having to buy expensive "fixes". (Please click on the flow chart PDF or JPG image attached).

If your bike has a severe bog issue, and you find that the flow chart is pointing you to replace the leak jet with a smaller one, I would recommend jumping by 10's. For example, if your bike has a #70 leak jet, try a #60. If the bog is present, but not severe, try jumping down in smaller increments until the bog is gone.

In the case of the aforementioned 2007 CRF250R, It had the stock #70 leak jet installed. A #60 helped, but it wasn't gone. I eventually found that a #50 cleared up the issue completely.

One other point to note: This Instructable assumes that you're working not only on a good carb with no leaks or other issues, but also that you have tuned the carb correctly prior to beginning the diagnosis flow chart. You should have the bike running well with proper pilot jet, main jet, and fuel screw settings. If the bog remains, then proceed to troubleshoot the AP circuit.

Here's proof that a modern 4 stroke dirt bike can be tuned to respond crisply to large throttle grabs from low RPM...

I hope you find this information useful!