Sounds like classic tank vent issue,
Here's my write-up on that, also check the tank drain line, located at the tank, nipple with hose attached. Make sure with an air compressor that it's clear both ways. When blowing from the nipple into the tank, be sure the cap is open, place a rag over the fuel opening just in case some crap blows out, don't need that in the tank.
If it's blocked, it won't allow water to drain during a rain, and/or washing and it goes into the tank itself, I've had this issue with my EX500.
Sounds like it could be a venting issue, no worries it's a free and easy fix,
Quote:
You need you to disassemble the gas cap and remove the two small red diaphragms, assuming this is your only problem.
The diaphragm gets hard and losses it's flexibility and gets stuck, and doesn't allow proper venting.
IMHO this modification should be done regardless to prevent future problems whatsoever.
They are ment to vent the tank, but in the case of an accident not allow gas to leak out, but more often cause problems then they fix.
Here are some pictures to help you.
The plastic piece houses the two small red diaphragm pieces, this one being mine, they are already gone, and you can leave the plastic piece out all together, as I'm going to do now.
Some things to watch out for are, there are 5 small springs under the round collar, and a small round collar as well on the locking mechanism, don't lose any of them.
Good luck.
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As far as your poor mileage goes, any changes to the carburetors leads to changes in mileage, good or bad depending on the changes.
If you've done something that resulted in a negative effect, then reverse it, and see if that's the true cause.
For this very reason is why when I do make changes, I do one at a time if possible. This way I know what caused what to happen, cuts down on the guessing game.