XR400 carburetor looking from the back, with the choke closed. After silicone, 2002.
Same chokeplate, first inspection 3 years later (2005)! SUCCESS it works. The silicone is a little discolored, but firm.
The problem: There is a flapper valve (C) on the choke plate (A).
Its function is to release the pressure if the engine backfires while the choke is closed.
The flapper valve is spring-loaded in the closed position with a torsion spring (B). Over time, the torsion spring (B) can break. Mine did - the spring tab is gone in this photo (but not easily visible thru the silicone). This tiny piece of spring steel then passes thru your engine, and ideally, out the exhaust. Then the flapper (C) is not held down. Because it now flaps freely in the pulsating intake, eventually the pivot (D) which is designed to rotate only during a backfire, is now flapping thousands of times each minute, and it will wear out quickly. When the hinge wears out the flapper gets sucked into the engine where it could raise a little hell. If you're lucky it blows out the exhaust. In other cases it bangs around until it closes the gap on your spark plug, then you're on foot. Think about this. An XR rider on foot, that's a strange sight!
There is a company that makes a billet choke plate for XRs which eliminates the flapper, replacing its function with two drilled holes and no moving parts.
My spring was broken and the flapper valve was hanging vertically. Here's my solution.
As shown in the photo, I've (thouroughly cleaned, then...) covered the spring and pivot with silicone to prevent movement at the pivot, but the flapper plate still opens a little in case of backfire. My recommendation is to silicone the spring and pivot immediately, before the spring breaks. Silicone will prevent the spring from 'buzzing" and as the theory goes, if it doesn't buzz, then it won't break off.
If you want to get the billet plate, we recommend Baja Designs, (760) 560-BAJA (2252).
Please email me if you've tried this or another solution.