Water In Tank

What are the ways water can get into a fuel tank?

I ride a Street Twin, it has had issues since I got it but today was very apparent. I left the house this morning without incident, but shortly after there was a torrential downpour of rain that lasted several hours while parked. When I went back to my bike, it started but then barely ran - sputtering, stalling, backfiring/popping, very rough running, erratic idle and throttle response.

I have had this issue before last year and had it in with the mechanics, they found water in my tank. I have since replaced the fuel cap, and it seems to make a good seal. There was no water that I saw when taking it off. I cannot see any damage on the tank. I can't see any damaged/cracked hoses. I ran a wire down the hole near the top of the fill port, but I don't understand how that hole works.

I am very confused as to what is causing the problem and would appreciate any help offered. It runs generally fine again after 5-10mins of riding.

 

I'm not certain yet, I have ordered a little pump to drain the tank (I'm not confident enough to take the tank off and drain it that way). I am just going on an assumption of what happened last time, and that it rained so heavily while parked and it was fine that morning you know.

The sparks seem OK, I didn't see any arcing and they look in good condition. Every time I've had it in for maintenance/repair they've come back to say the sparks are OK.

 

That's what I thought too, surely if the tank is leaking THAT much that this much water could get in (it still runs terribly days later now), surely I would smell fuel or be leaking it.

 

Yeah I think it is a breather. I've looked at the cap meticulously, I can't see how it would leak that badly you know.

I've got E0 in it at the moment, but it's possible that it's the fuel station, I've considered it. I filled up my tank maybe 4 days beforehand - is it possible that the issue is just less pronounced with a full tank? As in, the fuel to water ratio was higher so was OK, but now that I've dipped lower the water is more of a problem? Bike is used practically every day, I generally use the same fuel station as it's in a convenient location, but in the past the issue has manifested soon after filling up. I remember thinking at the time, what a coincidence.

 

Thanks for your replies, I appreciate it.

I have tried something a little...unconventional. As last time I had water in the tank, I have put a hole in one side of a re-sealable plastic bag and pushed my tank cap through it, so it's fully covered and sitting against the lip of the fill port. The bag fully covers the cap and surrounding area.

I have had no issues since even though I've had the bike absolutely soaked with rain, and it's actually running better than ever before. I am going to keep the bag on for now to see if the issue returns, but it seems like my cap just has a leak - I really still don't know how.

Is it possible that a leaky cap can also impact performance due to the EVAP system? I've noticed that my idle revs are much much stronger and I no longer have them dip stupidly low when pulling the clutch coming to a stop.

What would be the best way to improve the tank seal? I've already replaced the cap once, but I suppose it's still not sealing properly.