Barton,

Thanks for taking the time to research and post the information.

The stock #90-105 leak jets give a VERY short pump duration and must be nearly .2-.25 seconds. In some cases this may not be enough and switching out the leak jet is a good option. A fast idle per the Yamaha manual also helps along with setting the pump timing as the manual describes.

To explain, the leak jets are far bigger than the nozzle where the fuel squirts into the engine. The nozzle is only .3mm versus a .9mm leak jet, making the fuel mostly return to the float bowl from the diaphram chamber.

With a stock #90 leak jet-
(.9x.9)/(.3x.3) = 9, or approximately 9 times the flow (90%) returns to the bowl compared to what reaches the nozzle (10%).
(It's more complicated to be precise, but you get the idea)

Changing to a #40 leak jet-
(.4x.4)/(.3x.3) = 1.78, and roughly 65% leaks off and 35% is forced to the accelerator pump nozzle.
1/(1+1.78) = .35

Compare 35% to 10% and the results should be noticeable, or .8 seconds versus .25 seconds

The bottom line-- Using a leak jet in the range of #40-#50 looks like a good choice to compare if you have problems with bogging.

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The diaphram lengths are measured from the top (where the pump rod contacts) down to the bottom of the rivet. The stock '03WR250F is 7.5mm.

James

barton,

The squirt times were just an estimate with a stop watch. They aren't nearly as accurate as your methods. The closed leak jet ran-on a little longer on my bike(2+ seconds), which would more likely mean my bike is the one with possible clogging. The longer time means the flow volume was either greater from the pump or the passages to exit were more restrictive. (Or the spring tension is lighter- another real possibility for different model years)

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Cleaning the accelerator pump passages is difficult and risks damage to the seal in the carb. I don't recommend it except as a last resort. Try spraying cleaner thru the passages first.

--As a last option, the carb mid-body can be split by removing the torx and/or allen head screws with the anti-tamper paint on them. The carb will come apart to expose the mid-body seal which is NOT AVAILABLE FOR REPLACEMENT. (Don't damage it ) The internal passages become exposed and the bottom of the brass nozzle has what looks like a flat blade screw head. This is a press-fit brass piece that should be flushed with solvent. It can be removed by pushing down from above and gently turning the flat blade slot head back and forth. The secondary, internal check valve should also be flushed with solvent to see that it is clear of debris too. It is pressed into a blind hole and cannot be removed.
Reassemble without sealer to prevent clogging any passages from excess flow.

Other than the check valve, nearly every passage in the FCR carburetor is available to clean by this method.

James