XR 250 diaphragm

[coeshow]

J.D,

Are you at all knowledgable on late model XR 250 carbs?? There is a diphragm located on the left side of these carbs that is connected to a number if air ports and into the pilot jet fuel supply. It "appears" to be possibly an enrichment circuit of some sort. What can you tell me about it ??

[James_Dean]

I'm not familiar with the XR250 carburetor. It is probably a richening circuit as you suggested. Anyone else?

James

[XR250rdr]

That it is. Its the air cutoff valve. It is supposed to reduce/eliminate popping on decel.

[coeshow]

I have studied the circuit at length, and it "appears" that it is a lean inducing system. From what I can deduct, at high vacuum,(throttle closed,deceleration any RPM) the diaphragm's needle unloads a small shuttle valve that allows a designated airway to bypass the pilot circuit. Thereby not allowing fuel to be siphoned into the air/fuel inlet stream through the pilot jet system on deceleration. Does this sound correct to you, or have I missed something as far as proper function?? I also thought it possible to work in the reverse manner, possibly changing the vacuum signal at nearly wide open throttle to add more signal to the pilot circuit to "richen" the hi rpm mixture, becuase of where the "signal" port is located in the bore of the carb. The "backfire" reduction seems to be the logical design. Thanks for your input.

[James_Dean]

Not exactly-

The air bypass lets extra air to the pilot circuit in most conditions. Under hard closed throttle deceleration, the extra air is shut off (as the diaphram lifts) and richens the pilot circuit to reduce popping.

James

[coeshow]

Thats why I am inquiring !! I know that backfiring can also be caused by excess fuel(rich) The newer bikes pop a little when they are lean(XR250 included). The older style carbs did not have this diaphragm system. When decelerating, the engine would backfire because the large vacuum placed on the pilot circuit would draw alot more fuel into the intake than the circuit would normally flow during idle conditions. I'm not talking "popping" I'm talking full on BACKfires!! with flames and all. In the old days, you could even tell your jetting was close by how much it backfired.......

[XR250rdr]

What do you mean by older bikes not having it? Mine does and it was continued on the single carb models after it.

[coeshow]

By older I mean 1980. I installed a Honda "power-up" kit for my 1980 XR500. The kit included a bigger carb. The "kit" carb had no diphragm, and the engine would backfire on deceleration.