Keihin Cr Special Carburetor Tuning Manual Transfer

Keihin FCR tuning manual. Using dynamometers and tuning carburetor. Move the point of transfer from straight section to tapered section.

This a a short video on how to properly tune a Keihin Butterfly carb or Mikuni Butterfly style carburetor. Break In 2 Electric Boogaloo Download Skype. The butterfly carbs made my Keihin and Mikuni are almost identical.

The following tips will help to eliminate carb flooding, carb running rich, and carb running lean The following carb tuning tips apply to both brands. Step 1 clean your carburetor. Step 2 set your float level to within manufacturers specs. Step 3 set your pilot air fuel mix screw air 1 1/4 turns from gently seated. Step 4 warm your engine to normal running temps.

Step 5 set your idle speed screw so your idle is just above 2000 rpm. Step 6 slowly rotate air fuel mixture screw COUNTERCLOCKWISE leaning out the mixture until highest idle is achieved. Step 7 Screw your idle speed screw counterclockwise to obtain desired idle speed.(Yamaha Bravo 250=1100 rpm) Step 8 RIDE IT LIKE YOU STOLE IT;).

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. Nokia Map License Generator Sap more. 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.