Why does my CRF250R have a lean bog?
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.
What causes lean bog on Keihin FCR carburetors?
(The Background Story) 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.
When to replace a leak jet on a bike?
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.
How does the accelerator pump work on a Keihin FCR?
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.
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.
Where is the timing screw on a Keihin FCR?
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.
How to fix lean bog on Keihin FCR?
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.
Why does my FCR carburetor lean at lower rpm?
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.
Where is timing screw on Keihin FCR carb?
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.
What does a longer stroke on a FCR mean?
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.
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.
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.
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.