It's pretty easy to take it apart - a couple screws in the cover plate... It's been a while since I've done it, but I had that problem with a 450 maybe five years ago. If I recall, there's a nylon (I think) washer and a sort of wavy spring washer to keep the metal valve disc pressed against a rubber valve part with fuel passages in it. I replaced the spring washer with a narrow rubber "O" ring that just matched the perimeter of the valve disc and lubricated it with silicone, then put the metal cover back on with a couple "spacer" washers made from drilled pieces cut from a tin can (because the "O" ring was thicker than the original washers) as a temporary measure. Like a lot of temporary things, it's still put together that way and it still doesn't leak.
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