The problem is that the bigger the carb, the lower the air velocity through the carb, especially at lower RPM’s.

Please follow along while I present my case :-)īigger carbs mean more air/fuel mix can enter the engine, which means more power is available from the engine.

In the case of this particular bike, it only needed a $10.00 leak jet and the problem was solved. 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. The information presented here is applicable to any brand of motorcycle that uses the ubiquitous Keihin FCR carburetor. 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. After the head was rebuilt, and the engine freshened up with a new piston and rings, the lean bog was still there. 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).

Air screw adjustment keihin carb