Let my 1984 xs400 twin sit for a couple of months because of the rain in the Pacific Northwest. I recently started it up and let is run for several minutes while I went inside. When I came out, the bike had stopped running and I haven't been able to start it since. I pulled the plugs (real new) and they were black and oily looking. I cleaned them and the bike still won't start. The starter button won't begin to turn the engine over, so I am charging the battery now. Are there any other ideas out there that might help me get this thing started? Something to put in the gas tank in case the gas is bad, etc. Thanks for any help I get on this matter.

Being a native from the PNW.....I can relate oh too well to these symptoms

Also owned a coupla XS400's in the past doesn't hurt either.

Definitely drain the tank & refill with fresh petrol....then with clean plugs & a charged battery, have another go @ starting it. If the results are still the same....you might need to clean those carbs out.....especially the pilot jets which are notorious for clogging after sitting for a spell.

It's not that difficult & only requires a screwdriver & a can of carb cleaner to sort that problem.

Hope this helps